The world's largest particle accelerator is preparing to launch. Linear charged particle accelerators. How particle accelerators work. Why do we need particle accelerators? What is a particle accelerator for?

CHARGED PARTICLE ACCELERATORS
Accelerators

Charged particle accelerators - Installations for accelerating charged particles to energies at which they can be used for physical research, industry and medicine. At relatively low energies, accelerated particles are used, for example, to obtain an image on a TV screen or electron microscope, generate X-rays (cathode-ray tubes), destroy cancer cells, and kill bacteria. When charged particles are accelerated to energies exceeding 1 megaelectronvolt (MeV), they are used to study the structure of micro-objects (for example, atomic nuclei) and the nature of fundamental forces. In this case, charged particle accelerators act as sources of test particles that probe the object under study.

The role of the accelerator in a modern physics experiment is illustrated in the figure. A collimated beam of test particles from the accelerator is directed to a thin target under study containing, for example, nuclei of any chemical element, and test particles scattered by the target or other products of their interaction with target nuclei are recorded by a detector or a system of detectors. Analysis of the experimental results provides information about the nature of the interaction and the structure of the object under study.
The need to use accelerators for the study of such micro-objects as atomic nuclei and elementary particles is due to the following. First, atomic nuclei and elementary particles occupy small regions of space (R< 10 -12 см), и проникновение в эти области требует высокой разрешающей способности (а значит и энергии) зондирующего пучка, обеспечивающей взаимодействие отдельной пробной частицы с отдельным микрообъектом. Во-вторых, чем меньше микрообъект, тем он прочнее и проведение экспериментов с перестройкой или разрушением внутренней структуры такого объекта также требует всё большей энергии.
Knowing the size of the object under study, it is easy to estimate the energy of test particles required to study it. Particles have wave properties. The wavelength of a particle depends on its momentum p and is given by the de Broglie formula

Here h is Planck's constant, and 1 fm = 10 -13 cm. The above formula also gives a relationship between the wavelength of a relativistic particle and its kinetic energy E in megaelectronvolts.
In a scattering experiment, the structure of an object becomes “visible” (through, for example, diffraction of de Broglie waves) if the de Broglie wavelength is comparable to or less than the size (radius) of the object R, i.e. at λ < R. When electrons are used as probing particles, it is possible to "look" inside the nucleus if the electron energy exceeds 100 MeV. To observe the structure of a nucleon, the energy of an electron must already be calculated in gigaelectronvolts (1 GeV = 10 9 eV).
Accelerators differ in the type of accelerated particles, beam characteristics (energy, intensity, etc.), and also in design. The most common are electron and proton accelerators, since these particle beams are the easiest to prepare. In modern accelerators designed to study elementary particles, antiparticles (positrons, antiprotons) can be accelerated, and to increase the efficiency of using the energy of particles, their beams in a number of installations, called colliders, collide after the completion of the accelerating cycle (colliding beams).
Any accelerator structurally consists of three parts - a system where accelerated particles (injector) are “manufactured”, an accelerator system, where low-energy particles from an injector (usually formed in the form of bunches localized in space) increase energy in a high vacuum to the design one, and a transportation system ( extraction) of the beam to the experimental setup.
Conventionally, from the point of view of the trajectory along which the particles move in the process of acceleration, accelerators can be divided into two classes - linear (and direct action) and cyclic. In linear accelerators, particles in the process of acceleration move rectilinearly, and in cyclic ones - either along the same closed trajectory, repeatedly passing the same accelerating gaps (synchrotrons), or along a trajectory resembling an unwinding spiral (cyclotrons, microtrons, phasotrons).

The content of the article

PARTICLE ACCELERATOR, an installation in which directed beams of electrons, protons, ions and other charged particles with an energy significantly exceeding thermal energy are obtained with the help of electric and magnetic fields. In the process of acceleration, the particle velocities increase, and often to values ​​close to the speed of light. Currently, numerous small accelerators are used in medicine (radiation therapy) and also in industry (for example, for ion implantation in semiconductors). Large accelerators are used mainly for scientific purposes - to study subnuclear processes and properties of elementary particles.

According to quantum mechanics, a beam of particles, like a light beam, is characterized by a specific wavelength. The higher the particle energy, the shorter this wavelength. And the shorter the wavelength, the smaller the objects that can be studied, but the larger the size of the accelerators and the more complex they are. The development of research into the microworld required more and more energy of the probing beam. Natural radioactive substances were the first sources of high-energy radiation. But they only gave researchers a limited set of particles, intensities, and energies. In the 1930s, scientists began to work on creating facilities that could produce more diverse beams. Currently, there are accelerators that make it possible to obtain any kind of high-energy radiation. If, for example, X-ray or gamma radiation is required, then the electrons are subjected to acceleration, which then emit photons in the processes of bremsstrahlung or synchrotron radiation. Neutrons are generated by bombarding a suitable target with an intense beam of protons or deuterons.

The energy of nuclear particles is measured in electron volts (eV). Electronvolt is the energy acquired by a charged particle carrying one elementary charge (electron charge) when moving in an electric field between two points with a potential difference of 1 V. (1 eV »1.60219Ч 10 -19 J.) energies in the range from thousands to several trillion (10 12) electron volts - at the world's largest accelerator.

To detect rare processes in the experiment, it is necessary to increase the signal-to-noise ratio. This requires more and more intense radiation sources. The cutting edge of modern accelerator technology is determined by two main parameters - the energy and intensity of the particle beam.

Numerous and diverse types of technology are used in modern accelerators: high-frequency generators, high-speed electronics and automatic control systems, sophisticated diagnostic and control devices, ultra-high-vacuum equipment, powerful precision magnets (both "conventional" and cryogenic) and complex adjustment and mounting systems.

BASIC PRINCIPLES

The main scheme of particle acceleration provides for three stages: 1) beam formation and injection, 2) beam acceleration, and 3) beam extraction to the target or collision of colliding beams in the accelerator itself.

Beam formation and its injection.

The initial element of any accelerator is an injector, which contains a source of a directed flux of low-energy particles (electrons, protons, or other ions) and high-voltage electrodes and magnets that extract the beam from the source and form it. In the sources of protons of the first accelerators, gaseous hydrogen was passed through the region of an electric discharge or near a red-hot filament. Under such conditions, hydrogen atoms lose their electrons and only nuclei - protons - remain. This method (and similar to other gases) in an improved form is still used to obtain beams of protons (and heavy ions).

The source forms a beam of particles, which is characterized by the average initial energy, beam current, its transverse dimensions, and average angular divergence. An indicator of the quality of the injected beam is its emittance, i.e. the product of the beam radius by its angular divergence. The lower the emittance, the higher the quality of the final high-energy particle beam. By analogy with optics, the particle current divided by the emittance (which corresponds to the particle density divided by the angular divergence) is called the beam brightness. Many applications of modern accelerators require the highest possible beam brightness.

Beam acceleration.

The beam is formed in chambers or injected into one or several chambers of the accelerator, in which the electric field increases the speed and, consequently, the energy of the particles. In the first, simplest accelerators, the particle energy was increased in a strong electrostatic field created inside a high-vacuum chamber. The maximum energy that could be achieved in this case was determined by the electrical strength of the accelerator insulators. In many modern accelerators, electrostatic accelerators of electrons and ions (up to uranium ions) with energies from 30 keV to 1 MeV are still used as injectors.

Getting high voltage remains a complex technical problem even today. It can be obtained by charging a group of capacitors connected in parallel and then connecting them in series to a series of accelerating tubes. In this way in 1932 J. Cockroft and E. Walton obtained voltages up to 1 MV. A significant practical disadvantage of this method is that high voltage appears on the external elements of the system, which is dangerous for experimenters.

Another method of obtaining high voltage was invented in 1931 by R. Van de Graaf. In a Van de Graaff generator (Fig. 1), a dielectric tape transfers electrical charges from a voltage source at ground potential to a high-voltage electrode, thereby increasing its potential relative to ground. A single-stage Van de Graaff generator allows for voltages up to 10 MV. On multistage high-voltage accelerators, protons with energies up to 30 MeV were obtained.

If not a continuous beam is required, but a short pulse of high-energy particles, then you can take advantage of the fact that for a short time (less than a microsecond) insulators are able to withstand much higher voltages. Pulse diodes provide voltages up to 15 MV per stage in very low impedance circuits. This makes it possible to obtain beam currents of several tens of kiloamperes, rather than tens of milliamperes, as in electrostatic accelerators.

The usual method for generating high voltage is based on a Marx pulse generator circuit in which a capacitor bank is first charged in parallel and then connected in series and discharged through one discharge gap. The high-voltage pulse of the generator enters a long line, which forms the pulse, setting its rise time. The line is loaded with electrodes that accelerate the beam.

At a high-frequency accelerating voltage, the structure of the accelerator withstands much stronger electric fields without breakdown than at a constant voltage. However, the use of high-frequency fields to accelerate particles is complicated by the fact that the sign of the field changes rapidly and the field turns out to be either accelerating or decelerating. In the late 1920s, two methods were proposed to overcome this difficulty, which are now used in most accelerators.

LINEAR ACCELERATORS

The possibility of using high-frequency electric fields in long multistage accelerators is based on the fact that such a field changes not only in time, but also in space. At any moment of time, the field strength changes sinusoidally depending on the position in space, i.e. the distribution of the field in space has the form of a wave. And at any point in space, it changes sinusoidally in time. Therefore, the field maxima move in space with the so-called phase velocity. Consequently, the particles can move so that the local field accelerates them all the time.

In linear accelerating systems, high-frequency fields were first used in 1929, when the Norwegian engineer R. Videroe accelerated ions in a short system of coupled high-frequency resonators. If the resonators are designed so that the phase velocity of the field is always equal to the velocity of the particles, then during its motion in the accelerator the beam is continuously accelerated. The movement of particles in this case is like a surfer gliding on the crest of a wave. In this case, the velocities of protons or ions during acceleration can greatly increase. Accordingly, the phase velocity of the wave should also increase v phases. If electrons can be injected into the accelerator at a speed close to the speed of light with, then in this mode the phase velocity is practically constant: v phases = c.

Another approach, which makes it possible to exclude the effect of the decelerating phase of a high-frequency electric field, is based on the use of a metal structure that screens the beam from the field during this half-period. For the first time this method was applied by E. Lawrence in the cyclotron ( see below); it is also used in the Alvarez linear accelerator. The latter is a long vacuum tube containing a series of metal drift tubes. Each tube is connected in series with a high-frequency generator through a long line, along which an accelerating voltage wave travels at a speed close to the speed of light (Fig. 2). Thus, all tubes in turn are at high voltage. A charged particle escaping from the injector at a suitable moment of time is accelerated in the direction of the first tube, acquiring a certain energy. Inside this tube, the particle drifts - moves at a constant speed. If the length of the tube is correctly selected, then it will come out of it at the moment when the accelerating voltage has advanced one wavelength. In this case, the voltage on the second tube will also be accelerating and amounts to hundreds of thousands of volts. This process is repeated many times, and at each stage the particle receives additional energy. In order for the movement of particles to be synchronous with the change in the field, the length of the tubes must increase in accordance with an increase in their speed. Eventually the particle's speed will reach a speed very close to the speed of light, and the limiting length of the tubes will be constant.

Spatial changes in the field impose restrictions on the temporal structure of the beam. The accelerating field varies within a bunch of particles of any finite length. Consequently, the length of the bunch of particles should be small in comparison with the wavelength of the accelerating high-frequency field. Otherwise, the particles will be accelerated in different ways within the bunch. Too large spread of energy in the beam not only increases the difficulty of focusing the beam due to the presence of chromatic aberration in magnetic lenses, but also limits the possibilities of using the beam in specific problems. The spread of energies can also lead to smearing of the bunch of beam particles in the axial direction.

Consider a bunch of nonrelativistic ions moving with an initial velocity v 0. Longitudinal electric forces due to space charge accelerate the head of the beam and decelerate the tail. By appropriately synchronizing the motion of the bunch with the high-frequency field, it is possible to achieve a greater acceleration of the tail of the bunch than of the head. With such a phase matching of the accelerating voltage and the beam, it is possible to phase the beam - to compensate for the dephasing effect of the space charge and the energy spread. As a result, in a certain range of values ​​of the central phase of the bunch, centering and oscillations of particles with respect to a certain phase of stable motion are observed. This phenomenon, called autophasing, is extremely important for linear ion accelerators and modern cyclic electron and ion accelerators. Unfortunately, autophasing is achieved at the cost of reducing the fill factor of the accelerator to values ​​much less than unity.

In the process of acceleration, almost all beams exhibit a tendency to an increase in radius for two reasons: due to mutual electrostatic repulsion of particles and due to the spread of transverse (thermal) velocities. The first tendency weakens with increasing beam velocity, since the magnetic field created by the beam current compresses the beam and, in the case of relativistic beams, almost compensates for the defocusing effect of the space charge in the radial direction. Therefore, this effect is very important in the case of ion accelerators, but is almost insignificant for electron accelerators, in which the beam is injected at relativistic velocities. The second effect, related to the beam emittance, is important for all accelerators.

It is possible to keep particles close to the axis using quadrupole magnets. True, a single quadrupole magnet, focusing particles in one of the planes, defocuses them in the other. But here the principle of "strong focusing", discovered by E. Kurant, S. Livingston and H. Snyder, helps: a system of two quadrupole magnets separated by a flight gap, with alternating focusing and defocusing planes, ultimately ensures focusing in all planes.

Drift tubes are still used in proton linear accelerators, where the beam energy increases from a few megaelectronvolts to about 100 MeV. The first electron linear accelerators, such as the 1 GeV accelerator built at Stanford University (USA), also used constant-length drift tubes, since the beam was injected at an energy of about 1 MeV. More modern electron linear accelerators, the largest of which is the 50 GeV accelerator 3.2 km long, built at the Stanford Linear Accelerator Center, uses the principle of "surfing electrons" on an electromagnetic wave, which makes it possible to accelerate a beam with an energy increment of almost 20 MeV per meter of the accelerating system. In this accelerator, high-frequency power at a frequency of about 3 GHz is generated by large electrovacuum devices - klystrons.

The highest energy proton linear accelerator was built at Losalamos National Laboratory in the US. New Mexico (USA) as a "meson factory" for producing intense beams of pions and muons. Its copper resonators create an accelerating field of the order of 2 MeV / m, due to which it produces up to 1 mA of 800 MeV protons in a pulsed beam.

Superconducting high-frequency systems have been developed to accelerate not only protons but also heavy ions. The largest superconducting proton linear accelerator serves as an injector for the colliding beam accelerator HERA at the laboratory of the German Electron Synchrotron (DESI) in Hamburg (Germany).

CYCLIC ACCELERATORS

Proton cyclotron.

There is a very elegant and economical way to accelerate a beam by repeatedly imparting small portions of energy to it. To do this, using a strong magnetic field, the beam is forced to move in a circular orbit and many times pass the same accelerating gap. This method was first implemented in 1930 by E. Lawrence and S. Livingston in the cyclotron they invented. As in a linear accelerator with drift tubes, the beam is screened from the action of the electric field in the half-period when it acts as a decelerating one. Charged particle with mass m and charge q moving at speed v in a magnetic field H, directed perpendicular to its velocity, describes in this field a circle with a radius R = mv/qH... As acceleration increases speed v, the radius also increases R... Thus, protons and heavy ions move along a spiraling spiral of ever increasing radius. At each revolution along the orbit, the beam passes through the gap between the dees - high-voltage hollow D-shaped electrodes, where a high-frequency electric field acts on it (Fig. 3). Lawrence realized that the time between the passes of the beam through the gap in the case of nonrelativistic particles remains constant, since the increase in their speed is compensated by the increase in the radius. During that part of the orbital period when the high-frequency field has an unsuitable phase, the beam is outside the gap. The frequency of reference is given by the expression

where f- frequency of alternating voltage in MHz, N Is the magnetic field strength in T, and mc 2 - particle mass in MeV. If the value H is constant in the region where acceleration occurs, then the frequency f obviously does not depend on the radius.

To accelerate ions to high energies, it is only necessary that the magnetic field and the frequency of the high-voltage voltage meet the resonance condition; then the particles will pass through the gap between the dees twice per revolution at the right time. To accelerate the beam to an energy of 50 MeV at an accelerating voltage of 10 keV, 2500 revolutions are required. The operating frequency of the proton cyclotron can be 20 MHz, so the acceleration time is on the order of 1 ms.

As in linear accelerators, particles in the process of acceleration in a cyclotron must be focused in the transverse direction, otherwise all of them, except for those injected with velocities parallel to the pole pieces of the magnet, will fall out of the acceleration cycle. In a cyclotron, the possibility of accelerating particles with a finite angular spread is provided by imparting a special configuration to the magnetic field, in which the forces that return them to this plane act on the particles emerging from the orbital plane.

Unfortunately, according to the requirements for the stability of the bunch of accelerated particles, the focusing component of the magnetic field should decrease with increasing radius. This contradicts the resonance condition and leads to effects limiting the beam intensity. Another significant factor that reduces the capabilities of a simple cyclotron is the relativistic increase in mass, as a necessary consequence of an increase in particle energy:

In the case of acceleration of protons, the synchronism will be violated due to the relativistic increase in mass at about 10 MeV. One way to maintain synchronism is to modulate the frequency of the accelerating voltage so that it decreases as the orbital radius increases and the particle velocity increases. The frequency must change according to the law

Such a synchrocyclotron can accelerate protons to energies of several hundred megaelectrovolts. For example, if the magnetic field strength is 2 T, then the frequency should decrease from about 32 MHz at the time of injection to 19 MHz or less when the particles reach an energy of 400 MeV. Such a change in the frequency of the accelerating voltage should occur over a period of several milliseconds. After the particles reach the highest energy and are removed from the accelerator, the frequency returns to its original value and a new bunch of particles is introduced into the accelerator.

But even with the optimal magnet design and the best RF power delivery system, the cyclotron's capabilities are limited by practical considerations: extremely large magnets are needed to keep accelerated high-energy particles in orbit. Thus, the mass of a 600 MeV cyclotron magnet, built at the TRIUMPH laboratory in Canada, exceeds 2000 tons, and it consumes electricity on the order of several megawatts. The cost of building a synchrocyclotron is approximately proportional to the cube of the magnet's radius. Therefore, to achieve higher energies at a practically acceptable cost, new acceleration principles are required.

Proton synchrotron.

The high cost of cyclic accelerators is associated with the large radius of the magnet. But you can keep the particles in a constant radius orbit by increasing the strength of the magnetic field as their energy increases. A linear accelerator injects a beam of particles of relatively low energy into this orbit. Since the confining field is only needed in a narrow region near the beam orbit, there is no need for magnets covering the entire orbital area. The magnets are located only along the annular vacuum chamber, which provides huge cost savings.

This approach has been implemented in the proton synchrotron. The first accelerator of this type was the 3 GeV Cosmotron (Fig. 4), which began operating at the Brookhaven National Laboratory in 1952 in the United States; it was soon followed by the 6 GeV Bevatron, built at the Lawrence University of California at Berkeley (USA). Built specifically for antiproton detection, it has been in operation for 39 years, demonstrating the durability and reliability of particle accelerators.

In the first generation synchrotrons built in the USA, Great Britain, France and the USSR, focusing was weak. Therefore, the amplitude of the radial oscillations of the particles was large in the process of their acceleration. The width of the vacuum chambers was about 30 cm, and in this still large volume it was necessary to carefully control the configuration of the magnetic field.

In 1952, a discovery was made that made it possible to sharply reduce the oscillations of the beam, and, consequently, the dimensions of the vacuum chamber. It was the principle of strong, or hard, focus. In modern proton synchrotrons with superconducting quadrupole magnets arranged in a strong focusing scheme, the vacuum chamber can be less than 10 cm in diameter, which leads to a significant reduction in the size, cost, and power consumption of focusing and deflecting magnets.

The first synchrotron based on this principle was the 30 GeV Variable Gradient Synchrotron at Brookhaven. A similar facility was built at the laboratory of the European Organization for Nuclear Research (CERN) in Geneva. In the mid-1990s, both accelerators were still in operation. The aperture of the Variable Gradient Synchrotron was about 25 times smaller than that of the Kosmatron. The power consumed by the magnet at an energy of 30 GeV approximately corresponded to the power consumed by the Kosmotron magnet at 3 GeV. The "synchrotron with variable gradient" accelerated 6 × 10 13 protons per pulse, which corresponded to the highest intensity among installations of this class. Focusing in this accelerator was carried out with the same magnets that deflected the beam; this was achieved by shaping the poles of the magnet as shown in Fig. 5. In modern accelerators, separate magnets are usually used to deflect and focus the beam.

Thus, in experiments with a target at rest on the Tevatron, the useful energy is only 43 GeV.

The desire to use the highest possible energies in particle research led to the creation at CERN and the Laboratory. E. Fermi proton-antiproton colliders, as well as a large number of installations in different countries with colliding electron-positron beams. In the first proton collider, collisions of 26 GeV protons and antiprotons took place in a ring with a circumference of 1.6 km (Fig. 6). In a few days, it was possible to accumulate beams with currents up to 50 A.

At present, the collider with the highest energy is the Tevatron, where experiments are carried out in the collision of a beam of 1 TeV protons with a colliding beam of antiprotons of the same energy. For such experiments, antiprotons are needed, which can be obtained by bombarding a metal target with a high-energy proton beam from the "Main Ring". The antiprotons generated in these collisions accumulate in a separate ring at an energy of 8 GeV. When enough antiprotons have been accumulated, they are injected into the Main Ring, accelerated to 150 GeV, and then injected into the Tevatron. Here protons and antiprotons are simultaneously accelerated to full energy, and then they collide. The total momentum of the colliding particles is zero, so that all the energy 2 E turns out to be useful. In the case of the Tevatron, it reaches almost 2 TeV.

The highest energy among electron-positron colliders was achieved at the Large Electron-Positron Storage Ring at CERN, where the energy of colliding beams at the first stage was 50 GeV per beam, and then was increased to 100 GeV per beam. The HERA collider was built in DESY, in which collisions of electrons with protons take place.

This huge energy gain comes at the cost of a significant reduction in the probability of collisions between particles of colliding low-density beams. The collision rate is determined by the luminosity, i.e. the number of collisions per second, accompanied by a reaction of a given type, having a certain cross section. The luminosity is linearly dependent on the energy and current of the beam and is inversely proportional to its radius. The energy of the collider beam is selected in accordance with the energy scale of the studied physical processes.

To ensure the highest luminosity, it is necessary to achieve the maximum possible density of the beams at the point of their meeting. Therefore, the main technical problem in the design of colliders is to focus the beams at the point of their meeting into a very small spot and to increase the beam current. To achieve the desired luminosity, currents of more than 1 A may be required.

Another extremely difficult technical problem is associated with the need to provide an ultrahigh vacuum in the collider chamber. Since collisions between beam particles are relatively rare, collisions with residual gas molecules can significantly weaken the beams, reducing the probability of the interactions under study. In addition, the scattering of beams by the residual gas produces an undesirable background in the detector, which can mask the physical process under study. The vacuum in the collider chamber should be within 10 –9 –10 –7 Pa (10 –11 –10 –9 mm Hg) depending on the luminosity.

At lower energies, more intense electron beams can be accelerated, which makes it possible to study rare decays V- and TO-mesons due to electroweak interactions. A number of such installations, sometimes called "fragrance factories", are currently under construction in the United States, Japan and Italy. Such installations have two storage rings - for electrons and for positrons, which intersect at one or two points - interaction regions. Each ring contains many bunches of particles with a total current of more than 1 A. V- or TO-mesons. The design of these facilities is based on an electron synchrotron and storage rings.

Linear colliders.

The energies of cyclic electron-positron colliders are limited by intense synchrotron radiation emitted by beams of accelerated particles ( see below). This disadvantage is absent in linear colliders, in which synchrotron radiation does not affect the acceleration process. The Linear Collider consists of two high-energy linear accelerators, the high-intensity beams of which - electron and positron - are directed towards each other. The beams meet and collide only once, after which they are removed to the absorbers.

The first linear collider is the Stanford Linear Collider, which uses a 3.2 km long Stanford linear accelerator operating at 50 GeV. In the system of this collider, bunches of electrons and positrons are accelerated in the same linear accelerator and are separated when the beams reach their full energy. Then the electron and positron bunches are transported along separate arcs, the shape of which resembles the tubes of a medical stethoscope, and focused to a diameter of about 2 microns in the interaction region.

New technologies.

The search for more economical acceleration methods led to the creation of new accelerator systems and high-frequency high-power generators operating in the frequency range from 10 to 35 GHz. The luminosity of electron-positron colliders should be extremely high, since the cross section of the processes decreases as the square of the particle energy. Accordingly, the beam densities must be extremely high. In a linear collider with an energy of the order of 1 TeV, the beam sizes can reach 10 nm, which is much smaller than the beam at the Stanford Linear Collider (2 μm). With such small beam sizes, very powerful stable magnets with sophisticated electronic automatic controls are needed to accurately match the focusing elements. When the electron and positron beams pass through each other, their electrical interaction is neutralized, and the magnetic one is amplified. As a result, magnetic fields can reach 10,000 Tesla. Such giant fields are capable of strongly deforming the beams and leading to a large energy spread due to the generation of synchrotron radiation. These effects, together with the economic considerations associated with building more and more extended machines, will set a limit to the energy achievable in electron-positron colliders.

ELECTRONIC STORES

Electronic synchrotrons are based on the same principles as proton synchrotrons. However, due to one important feature, they are technically simpler. The small mass of the electron makes it possible to inject the beam at speeds close to the speed of light. Therefore, a further increase in energy is not associated with a noticeable increase in velocity, and electron synchrotrons can operate at a fixed frequency of the accelerating voltage if the beam is injected with an energy of about 10 MeV.

However, this advantage is nullified by another consequence of the smallness of the electron mass. Since the electron moves in a circular orbit, it moves with acceleration (centripetal), and therefore emits photons - radiation, which is called synchrotron radiation. Power R synchrotron radiation is proportional to the fourth power of the beam energy E and current I, and also inversely proportional to the radius of the ring R, so that it is proportional to the value ( E/m) 4 IR-1 . This energy, lost at each revolution of the electron beam in its orbit, must be compensated for by the high-frequency voltage applied to the accelerating gaps. In "aroma factories" designed for high intensities, such power losses can reach tens of megawatts.

Cyclic accelerators such as electron synchrotrons can also be used as accumulators of large circulating currents with constant high energy. Such storage rings have two main applications: 1) in studies of the nucleus and elementary particles by the method of colliding beams, as discussed above, and 2) as sources of synchrotron radiation used in atomic physics, materials science, chemistry, biology, and medicine.

The average photon energy of synchrotron radiation is proportional to ( E/m) 3 R-1 . Thus, electrons with energies of the order of 1 GeV circulating in the storage ring emit intense synchrotron radiation in the ultraviolet and X-ray ranges. Most of the photons are emitted within a narrow vertical angle of the order of m/E... Since the radius of electron beams in modern storage rings for energies of the order of 1 GeV is measured in tens of micrometers, the beams of X-ray radiation emitted by them are characterized by high brightness, and therefore can serve as a powerful tool for studying the structure of matter. Radiation is emitted tangentially to the curved path of the electrons. Consequently, each deflection magnet of the electron storage ring, when a bunch of electrons passes through it, creates an unfolding "spotlight" of radiation. It is discharged through long vacuum channels tangential to the main vacuum chamber of the storage device. The slits and collimators located along these channels form narrow beams, from which the required range of X-ray energies is then extracted with the help of monochromators.

The first sources of synchrotron radiation were installations originally built to solve problems in high-energy physics. An example is the Stanford 3 GeV positron-electron storage ring at the Stanford Synchrotron Radiation Laboratory. At one time, this facility was used to discover "charmed" mesons.

Early synchrotron light sources were not flexible enough to meet the diverse needs of hundreds of users. The rapid growth in the demand for high flux and high beam intensity synchrotron radiation has given rise to second generation sources designed with the needs of all possible users in mind. In particular, magnet systems were chosen to reduce the emittance of the electron beam. Low emittance means smaller beam sizes and therefore higher brightness of the radiation source. Typical representatives of this generation were storage rings at Brookhaven, which served as sources of X-ray radiation and radiation of the vacuum ultraviolet region of the spectrum.

The brightness of the radiation can also be increased by forcing the beam to move along a sinusoidal path in a periodic magnetic structure and then combining the radiation that occurs at each bend. Undulators - magnetic structures that provide such a movement, are a series of magnetic dipoles deflecting the beam at a small angle, located in a straight line on the beam axis. The brightness of the radiation of such an undulator can be hundreds of times higher than the brightness of the radiation arising in the deflecting magnets.

In the mid-1980s, the creation of third-generation synchrotron radiation sources with a large number of such undulators began. Among the first sources of the third generation are the "Advanced Light Source" with an energy of 1.5 GeV at Berkeley, generating soft X-rays, as well as the "Advanced Photon Source" with an energy of 6 GeV at the Argonne National Laboratory (USA) and a synchrotron with an energy of 6 GeV at the European Synchrotron Radiation Center in Grenoble (France), which are used as sources of hard X-rays. After the successful construction of these facilities, a number of synchrotron radiation sources were created in other places.

A new step towards greater brightness in the infrared to hard X-ray range is associated with the use of "warm" magnetic dipoles with a magnetic field strength of about 1.5 T and much shorter superconducting magnetic dipoles with a field of several Tesla in the bending magnet system. This approach is being implemented in a new synchrotron radiation source being created at the Scherrer Institute in Switzerland, and in the modernization of the source in Berkeley.

The use of synchrotron radiation in scientific research has gained wide scope and continues to expand. The exceptional brightness of such X-ray beams makes it possible to create a new generation of X-ray microscopes for studying biological systems in their normal aquatic environment. This opens up the possibility of rapid analysis of the structure of viruses and proteins for the development of new pharmaceutical preparations with a narrow focus on disease-causing factors and minimal side effects. Bright beams of X-ray radiation can serve as powerful microprobes for detecting even the smallest amounts of impurities and contaminants. They make it possible to very quickly analyze environmental samples in the study of environmental pollution pathways. They can also be used to assess the purity of large silicon wafers before the costly fabrication process of highly complex integrated circuits, and they open up new perspectives for lithography, allowing, in principle, integrated circuits with elements below 100 nm.

ACCELERATORS IN MEDICINE

Accelerators play an important practical role in medical therapy and diagnostics. Many hospitals around the world today have small electron linear accelerators at their disposal that generate intense X-rays that are used to treat tumors. Cyclotrons or synchrotrons generating proton beams are used to a lesser extent. The advantage of protons in tumor therapy over X-rays is in a more localized energy release. Therefore, proton therapy is especially effective in the treatment of brain and eye tumors, when damage to the surrounding healthy tissue should be as minimal as possible. .

CHARGED PARTICLE ACCELERATORS- installations serving to accelerate the charge. particles to high energies. In the usual word usage, accelerators (U.) are called. installations designed to accelerate particles to energies above \ MeV. An energy of 940 GeV has been reached on the record-breaking proton ultrasound, the Tevatron (Fermi Laboratory, USA). The largest electron accelerator LEP (CERN, Switzerland) accelerates colliding beams of electrons and positrons up to an energy of 45 GeV (after installing additional accelerating devices, the energy can be doubled). U. are widely used both in science (the generation of elementary particles, the study of their properties and internal structure, the production of nuclides that do not occur in nature, the study of nuclear reactions, radiobiology, chemical research, work in the field of solid state physics, etc.) and for applied purposes (sterilization of medical equipment, materials, etc., non-destructive testing, manufacturing of microelectronic elements, production of radiopharmacological preparations for medical diagnostics, radiation therapy, radiation technologies in art technology, polymerization of varnishes, modification of material properties, e.g. rubber, manufacturing of heat-shrinkable pipes, etc.).

In all acting U. an increase in energy is charged. particles occurs under the action of external longitudinal (directed along the speed of the accelerated particles) electric. fields. Searches are under way for acceleration using fields created by other moving particles or electromagnets. waves, which are excited or modified by the beam of accelerated particles or other beams ( collective acceleration methods).Collective methods theoretically make it possible to dramatically increase the rate of acceleration (the energy accumulated on \ m path) and the intensity of the beams, but so far have not led to serious success.

U. include the following elements: a source of accelerated particles (electrons, protons, antiparticles); electric generators or el - magn. accelerating fields; a vacuum chamber, in which particles move in the process of acceleration (in a dense gaseous medium, acceleration of charged particles is impossible due to their interaction with gas molecules filling the chamber); devices serving for the inlet () and release (ejection) of the beam from the U .; focusing devices that ensure long, movement of particles without hitting the walls of the vacuum chamber; magnets that bend the trajectories of accelerated particles; devices for research and correction of the position and configuration of the accelerated beams. Depending on the peculiarities of U., one or more of the listed elements may be absent in them.

For the purpose of radiation. security U. are surrounded by protective walls and ceilings (biological protection). The thickness and choice of shielding material depend on the energy and intensity of the accelerated beams. Accelerators with energies higher than several. GeV is usually located underground for safety reasons.

According to the principle of the device, U. of direct action is distinguished, or high voltage accelerators(acceleration in post, electric field), induction accelerators(acceleration in vortex electric. fields arising from a change in magnetic induction) and resonant U., in which during acceleration are used V Ch el - magn. fields. All operating at extremely high energies belong to the latter type.

Modern U. are divided into two large classes: linear accelerators and cyclic accelerators... In linear ultrasound, the trajectories of the accelerated particles are close to straight lines. Accelerating stations are located along the entire length of such U. The largest of the operating linear U. (electronic U. at Stanford) has a length of a mile (3.05 km). Linear U. make it possible to obtain powerful fluxes of particles, but at high energies they turn out to be too expensive. In cyclic. W. "leading" magn. the field bends the trajectories of the accelerated particles, rolling them in a circle ( ring accelerators or synchrotrons) or spirals ( cyclotrons, phasotrons, betatrons and microtrons). Such U. contain one or several accelerating devices, to which the particles repeatedly return during the accelerating cycle.

It should be noted the difference between the U. of light particles (electrons and positrons), which are usually called. electronic U., and U. heavy particles (protons and ions).

Electronic accelerators... Features of electr. are associated with two reasons. The speed of electrons and positrons already at low energies (several MeV) differs little from the speed of light and can usually be considered constant, which greatly simplifies and cheapens U. But, on the other hand, electrons and positrons in magn. fields lose a lot of energy on the electromagnet. radiation ( synchrotron radiation)... In cyclic. These losses lead either to enormous dimensions of the radiation (at large radii of curvature, the losses for synchrotron radiation decrease), or to the need to have powerful accelerating stations, which will greatly increase the cost of radiation. Synchrotron radiation plays and plays a role: it leads to a decrease in the dimensions of the accelerated beam which makes it easier to create drives allowing experiments on colliding beams.

Ring electronic U. are used as sources of synchrotron radiation in UV or X-ray. range. Due to the high radiation density and its sharp directivity, cyclic. W. are unique sources of electromagnet. waves of the indicated ranges. Large losses of electrons to radiation are often forced to give preference to linear Y.

Heavy particle accelerators(predominantly protons) are very different from electronic U. The energy losses for synchrotron radiation in them at the energies reached at the present time (~ \ TeV) are practically absent, and it is usually unprofitable to maintain a high acceleration rate (since the power consumed to power the accelerating stations is proportional to the square of the electric field strength and grows rapidly with an increase in the acceleration rate). The absence of noticeable synchrotron radiation leads to the fact that the amplitude of the transverse particles in the process will accelerate, the cycle decays relatively slowly (like the square root of the momentum of the particles), and stability of motion in the absence of special. measures is violated under the influence of even relatively weak perturbations. All high-energy heavy particles are of the cyclic type. ^ iV

In the 90s. storage and colliding rings, in which dense beams are charged, are gaining more and more importance. particles circulate for a long time, without changing their energy. Such rings are used to carry out reactions between particles moving towards each other (colliding beams), to accumulate ions and particles that do not occur directly in nature (positrons and antiprotons), and also to generate synchrotron radiation. In the interaction of particles moving towards each other, all the energy imparted to them during acceleration can be realized, while in the interaction of accelerated particles with stationary ones, most of the energy is associated with the movement of the center of mass of the particles and does not participate in reactions.

Historical reference... Development of U. began in the 1920s. and was aimed at fission of atomic nuclei. Earlier than others were created electrostatic generators[R. R. Van de Graaf] and cascade generators[J. Cockroft (J. Cockroft) and E. Walton (E. Walton)], belonging to the class U. direct action, and then the first cyclical. resonant W.- [E. E. Lawrence, 1921]. In 1940 D. Kerst built the first W. induction. type - betatron.

In the 40s. appeared theoretical. works in which the stability of the motion of accelerated particles was investigated. In the first works of this cycle [V. I. Veksler and Amer. physicist E. McMillan] considered the stability of longitudinal (phase) motion, formulated the principle aetophasing... Then there were works on the creation of the theory of transverse motion of particles-beta-throne oscillations, which led to the discovery of strong (alternating) focusing [N. Christofilos (N. Christophilos), 1950; E. Curant, M. Livingston, H. Snyder, 1952], underlying all modern. large W.

The rapid development of powerful high-power radio technology. devices that occurred during World War II (1939–45) made it possible to start creating linear ultrasound for high energies. In electronic linear devices, electric is used. field of traveling waves of the decimeter range into the diaphragm. waveguides, in proton - developed by L. Alvarez (L. Alvarez) meter range, loaded with transit tubes. In the beginning. parts of such U. are increasingly used by U. with quadruple high-frequency focusing(English designation RFQ), in the creation of to-ryh basic. the role was played by V.V. Vladimirsky, I.M.Kapchinsky and V.A.Teplyakov.

When constructing, cyclic. U. are increasingly used superconducting magnets. systems. Superconducting magnets are used in cyclotrons to create a post. magn. fields and in synchrotrons proton-to generate slowly (over many seconds) changing magnets. fields. This is how the largest operating proton synchrotron, the Tevatron (USA), works.

Until the 80s. main discoveries in particle physics were made at proton synchrotrons. Nowadays, many interesting results are being obtained at electron-positron and proton-antiproton ring accelerators with colliding beams (kollayderakh). The advantages of such U. over the usual: 1) creatures. an increase in the interaction energy (in the center of mass system); in the ultrarelativistic case, which always occurs in colliding beams, this energy increases from in the collision of fast particles with nuclei of a stationary target before at colliders ( T is the mass of colliding atoms and target atoms, is the total energy of the accelerated particles); 2) a sharp decrease in the background from extraneous reactions. Main the lack of colliders is a significant (by several orders of magnitude) decrease in the number of interactions (over the same time). The technique of ring-shaped u.s. with colliding electron-positron beams was mastered in 1961 (an accelerator for an energy of 2 x 250 MeV in Frascatti, Italy), and installations with colliding proton and antiproton beams appeared only after the methods of electrons were proposed. ronnogo (A.M.Budker, 1967) and with t about khastichsky about [S. S. Van der Meer (1972) cooling of heavy particles (see. Cooling beams charged). More and more attention is paid to the development of non-traditional things. methods of acceleration: collective methods, acceleration on beats of laser fields, acceleration in wake fields, etc. The beginning of this work was laid by V. I. Veksler, A. M. Budker and Ya. B. Fainberg. However, U. based on these ideas has not yet been created.

Direct acting accelerators... In such U. charge. particles increase energy in constant or quasi-constant (not changing during the time during which the particles gain full energy) electric. fields. The energy acquired by the particles is, in this case, equal to their charge multiplied by the passed potential difference. The maximum attainable energy of particles in direct action is determined by the largest potential difference (15-18 MB), which can be created without breakdown in physical. installations. In all practically used U. direct action, the last electrode of the accelerating system is at the earth potential, since only in this case the particles removed from the U. do not lose the acquired energy during further motion.

U. direct action includes electrostatic. generators, cascade generators and rechargeable boosters(or tandem U.). Accelerated particles in such U. move inside and along a pipe made of insulators. material (usually porcelain), a vacuum is created inside the swarm, which is necessary for the unhindered movement of accelerated particles, and outside (under high pressure) a thoroughly dried, oxygen-free gas mixture (most often nitrogen with an admixture of sulfur hexafluoride) is injected, preventing the development of electric. breakdowns. An accelerating potential difference is created between the electrodes located at the ends of the tube (Fig. 1). Electric. the field directed along the axis of the tube is leveled by the metal. will divide. rings connected to Omsk. voltage divider.

In electrostatic U. high voltage is created using a fast moving tape made of insulating material, for example. rubber. In the low-voltage part of the installation, electric is applied to the tape. charge. This charge flows down to the tape from the metal. needles charged from specials. generator up to several. tens of kV. The moving belt transfers the charge to the high-voltage part of the U. located inside the hollow metal. cap. There, the charge is removed from the tape using the same needles and flows from them to the outer surface of the cap. The potential of the bell (and of all the equipment enclosed inside it, including the ion source and the high-voltage electrode of the tube) increases continuously as the charges arrive and is limited only by the breakdown.

Rice. 1. Diagram of the accelerating tube device.

In cascade generators, voltage multiplication circuits are used to create large potential differences.

In the first step, the negative values ​​are accelerated. ions (atoms containing an extra electron), and then, after the removal of two (or several) electrons, will put them in the stripping. ions. Both the source and the output devices of such U. are at ground potential, and the high-voltage electrode equipped with a stripping device is located at cf. parts of U. Rechargeable U. make it possible to obtain double (and with deeper stripping, even higher) values ​​of energy without breakdown.

Induction accelerators... To induction. W. owns betatrons and linear inductors. W.

Rice. 2. Schematic section of the betatron: 1 - magnet poles; 2 -section of the annular vacuum chamber; 3 -core; 4 - electromagnet windings; 5 - magnet yoke.

The diagram of the betatron device is shown in Fig. 2. Accelerated particles (electrons) move in an annular vacuum chamber 2 located in the gap of the electromagnet ( 1 - the poles of the magnet). They are accelerated by a vortex electric. field, a cut is excited when changing the magn. flow penetrating the orbit of accelerated particles. Main part of this flow passes through the core 3 located in the center. parts of the betatron. Windings 4 feed on AC current. Magnet configuration field in the betatron must obey two conditions: 1) magn. induction on the center. the orbit must correspond to the changing energy of the electrons; 2) configuration magn. field in the vacuum chamber should ensure the stability of the transverse motion of electrons or, as they say, the stability of their betatron oscillations(see below). The ring-shaped beveled magnets located above and below the chamber. the poles create the field necessary for such stability, decreasing towards the periphery (Fig. 8, b).

The idea of ​​the betatron acceleration method was expressed in 1922 by J. Slepian, the foundations of the theory were developed in 1948 by R. Wideroe. The first betatron was built in 1940. The simplicity and reliability of betatrons ensured their wide application in technology and medicine (in the energy range 20-50 MeV).

In linear induction accelerators, the power lines are electric. fields (with intensity E) are directed along the axis of the accelerator. Electric. the field is induced by a time-varying magn. by the flux passing through the arranged one behind the other ring ferrite inductors 1 (fig. 3). Magn. the flux is excited in them by short (tens or hundreds of ns) current pulses passed through single-turn windings 2 covering inductors. Focusing is carried out by a longitudinal magnet. field, a cut is created by coils 3 located inside the inductors. Linear induction U. make it possible to obtain record (kiloampere) currents in a pulse; naib. the most powerful of the working U. - ATA (USA) - accelerates electrons to an energy of 43 MeV at a current of 10 kA. The duration of the current pulses is 50 ns.

Rice. 3. Diagram of a linear induction device accelerator: 1 - the core of the inductor; 2 -exciting winding; 3 -Focusing coil.

Resonant accelerators... In resonant U. to increase the energy charge. particles are used HF longitudinal electric. fields. Acceleration in such fields is possible when one of two conditions is met: either the accelerated particles must move together with the electromagnet. wave, maintaining their position relative to it (accelerating and traveling wave), or they must interact with it only at such moments in time when electric. the field has the desired (accelerating) direction and the required magnitude (the resonant Ys proper). Areas on which the particles interact with the accelerating field are called. with a shortcut and a shortcut. On the rest of the path, the particles do not experience the action of the HF field, either because it simply is not there, or because the particles are protected from it by screens.

U. with a traveling wave is used in the main. for the acceleration of light particles (electrons and positrons), the speed of which already at low energies differs little from. Phase velocity of electromagnet. waves in vacuum waveguides always exceed the speed of light; loading the waveguides with a perforated system. diaphragms, you can slow down the speed of the wave, but not very much. Therefore, acceleration of slow particles is not used with a traveling wave.

.

Rice. 4. Diagram of the device of the Wideroe accelerator: 1 - transient f tubes; 2-generator of HF oscillations; 3 - accelerating gaps;

Linear resonance accelerators... The simplest resonant U. is the accelerator Videroe (Fig. 4). Arranged along the course of the beam metallic. The flight tubes are connected (through one) to the poles of the HF generator. In the accelerating gaps (the intervals between oppositely charged flight tubes), a longitudinal electric is created. HF field with a voltage of the order of hundreds of kV. Particles approaching the accelerating gap at the right time are accelerated electrically. field, and then "hide" in the next flight tube. Its length and speed of the particle are coordinated with each other so that the particles approach the next gap at the moment when electric. the field has the correct direction and magnitude, i.e., the same phase as in the previous accelerating gap. For this, it is necessary that the condition

where / is the length of the tube and the accelerating gap; - particle speed, expressed in fractions of the speed of light s; -wavelength el - magn. vibrations (in emptiness); NS-any integer. The accelerated beam consists, i.e., of a chain of bunches of particles (bunches) that have passed through the accelerating gaps with the proper electric. fields. When developing the structure of a linear U. it is important to correctly choose the lengths not only of the span tubes, but also of the accelerating gaps. These lengths must, on the one hand, be large enough to withstand noticeable voltages (hundreds of kV, and sometimes megavolts), and, on the other hand, small enough so that the phase of the HF oscillations does not change too much during the passage of the particle.

As the particle speed increases, the Wideröe accelerators become ineffective and give way to the Aliaretz accelerators. In them, the flight tubes are not connected to the generator, but are located one after another inside a long cylindrical tube. resonator, in Krom excited e - magn. fluctuations. The HF field, a cut far from the transit tubes, is distributed in the same way as in a conventional resonator, at its axis it is concentrated in the accelerating gaps. The layout of the elements "accelerating gap - flight tube - accelerating gap", etc., remains the same as in Wideröe accelerators, but condition (1) takes the form

Linear resonant ultrasound efficiently work if sufficiently fast particles are injected into them, which were previously accelerated with the aid of direct action or with the aid of an alternating high-frequency focusing device. - v

Cyclotrons-simple and historically first U. cyclic. type (fig. 5). In modern Understanding cyclotrons are called resonance cycles. W., working with a leading magnet that does not change in time. field and at a post, the frequency of the accelerating HF field. In conventional cyclotrons, magn. the field is azimuthal and almost independent of the radius; the trajectories of the accelerated particles have the form of unwinding spirals. Conventional cyclotrons are used to accelerate heavy nonrelativistic particles - protons and ions. The vacuum chamber of the cyclotrons is limited to ext. cylindrical wall forms and two flat horizontally located lids. The electromagnet poles of conventional cyclotrons create an almost uniform (slightly falling towards the periphery) magnets in the chamber. field. The accelerating gap is formed by cuts of two electrodes located in the chamber and facing each other, which have the shape of hollow half-cylinders - duant. The dees are connected to the poles of the high voltage generator through quarter-wave lines.

Rice. 5. Diagram of the cyclotron device.

A particle moving in a circle is acted upon by centripetal motion. Lorentz force equal to the centrifugal force where r is the radius of curvature of the trajectory, Ze is the charge of a particle. That., Moving on to more convenient units, we get

where pc-product of particle momentum R at the speed of light with- expressed in MeV, induction magn. fields V measured in tesla, and r in m.

Ultimate energy achievable in conventional cyclotrons; is for protons approx. 20 MeV, and the frequency of the accelerating field (at B = 2 T) - approx. 30 MHz. At high energies, the accelerated particles go out of synchronicity with the accelerating voltage due to the decrease in V from the center to the periphery and due to relativistic effects.

Conventional cyclotrons are widely used for the production of isotopes and in all other cases when protons (or ions) with energies up to 20 MeV (or ~ 20 MeV / nucleon) are needed. If protons with higher energies (up to several hundreds of MeV) are needed, then cyclotrons with azimuthal magnitudes are used. fields. The stability of the transverse motion in such cyclotrons is ensured due to the rejection of the azimuthal symmetry of the magnets. field and the choice of such a configuration, the edge allows you to maintain stability of motion and with increasing (on average) to the periphery of the values ​​of magn. induction.

The acceleration process in cyclotrons occurs continuously: at the same time, some particles just leave the ion source, others are in the middle of the path, and still others finish the acceleration process. Typical current int. beam in cyclotrons is approx. 1 mA, the extracted beam current depends on the ejection efficiency and on the thermal stability of the output foils; usually it is several. tens of μA.

Phazotrons... In phasotrons magn. the field is constant in time and its cylindrical is preserved. symmetry. Magn. the field decreases towards the periphery, the frequency of rotation of particles decreases with increasing energy, and, accordingly, the frequency of the accelerating field decreases. In this case, the restrictions on the energy of accelerated particles disappear, but the intensity of the accelerated beam decreases sharply (by several orders of magnitude). Changing the frequency of the accelerating field leads to the fact that the acceleration process is divided into cycles: a new batch of particles can be introduced into the phasotron only after the acceleration of the previous batch is completed and the frequency is returned to its original value. The usual working area of ​​phasotrons from several. hundreds to thousands of MeV. As the energy increases further, the size of the magnets becomes too large and their weight and cost increase excessively. Recently (90s) new phasotrons have not been built. For energies up to several. hundreds of MeV use cyclotrons with azimuthal variation of magn. fields, and synchrotrons are used to accelerate to high energies.

Synchrotrons used to accelerate particles of all types: synchrotrons proper for electrons and synchrotrons for protons and other ions (the old name is synchrophasotrons, see. Synchrotron proton)... The energy up to which the particles are accelerated in synchrotrons is limited for electrons by the power of synchrotron radiation, and for protons and ions only by the size and cost of Y.

In synchrotrons, the orbit remains constant during acceleration, along which the particles circulate. Leading magn. the field is created only along a narrow path, enclosing an annular vacuum chamber, into which the particles move. As is clear from (3), with the post. radius of magn. induction should increase in proportion. momentum of accelerated particles. The frequency of revolution with (at a constant length of the orbit) is associated with the momentum f-lo

where is the frequency with which a particle moving at the speed of light would turn in the synchrotron. The frequency of the accelerating field can coincide with the frequency of revolution of the particles or be an integer number of times (it is called the frequency) exceed it. Thus, in electronic synchrotrons (for which always p >> mc) the frequency of the accelerating field is constant, while the induction of magn. the margin increases. In proton synchrotrons, during the accelerating cycle, as the induction of magn. field, and the frequency of the accelerating voltage.

Microtrons-cyclic. U. with post. magn. field and with an energy increment per revolution equal to the electron rest energy (0.511 MeV). If the entire increase in energy occurs in one short section, then in fasting. magn. the field of a particle is transferred from one circular orbit to another. All these orbits touch each other at the point where the accelerating device is located. The energy of electrons in such U. reaches several. tens of MeV.

The dimensions of the accelerators. Accelerator complexes... The length of the linear U. is determined by the energy of the accelerated particles and the rate of acceleration, and the radius of curvature of the orbit of ring accelerators is determined by the energy of the particles and the max. induction of the leading magnet. fields.

In modern linear electronic acceleration rate is 10-20 MeV / m, in proton - 2.5-5 MeV / m. The increase in the rate of acceleration runs into two mains. difficulties: the increase in resistive losses in the walls of the resonators and the danger of electric. breakdowns. To reduce resistive losses, superconducting resonators can be used (the first such resonators have already begun to work); to combat breakdowns, the distribution of electricity is carefully leveled. fields in resonators, avoiding local inhomogeneities. It is possible that the rate of acceleration in proton linear ultrasounds can be increased with time by an order of magnitude.

The dimensions of the cyclic U. are associated with the induction of the leading magnet. f-loy fields (3). With the acceleration of singly charged particles and the ring-average value T (which corresponds to this f-la gives (m). In accordance with this, a 1 TeV U. should have a perimeter of ~ 20 km. Such structures are built underground in order to protect them from radiation. The enormous dimensions of the high energy consumption lead to capital expenditures in the billions of dollars.

The above estimates are valid for U., magn. blocks to-rykh contain an iron yoke. Increase B max above 1.8 T turns out to be impossible due to saturation of iron, but this can be done by switching to superconducting magnets. systems. The first such U. - the Tevatron - is already working in the Laboratory. Fermi in the USA. Magn. the field in blocks, wound with a cable with NbTi conductors in a copper matrix, at a temperature of 4 K can be raised to 5-5.5 T, and when the temperature is lowered to 1.8 K or when switching to NbSn, up to 8 -10 T. (The NbSn alloy is not used in the manufacture of accelerators because of its fragility.) fields, but economically unprofitable; the dimensions of the cryogenic plant are decreasing, but the number of expensive and energy-intensive cryogenic equipment is increasing.

Less rigidly defined minimum acceptable values V... In U. with an iron yoke B mines should not be less (6-10). 10 ~ 3 T, since at lower fields the contribution to the total magnitude is too large. induction begins to contribute residual magn. fields, the spatial distribution of which is usually unfavorable. Attitude B max / B min, and, consequently, the ratio of the momenta of the ejected and injected particles in an atmosphere with ordinary magnets cannot therefore exceed 200-300. In superconducting magn. systems, this range turns out to be even smaller, since for small fields in spaces. distribution of magn. induction is strongly affected by eddy currents in superconducting conductors. These restrictions are one of the reasons leading to the fact that all large will accelerate. complexes contain several. serially operating U.: linear U. - injector, one or several. intermediate U.- boosters, finally, the main U., bringing the charge. particles up to ultimate energy, and possibly a storage ring. The circuit will speed up. complex CERN is shown in Fig. 6.

The construction and operation of this complex is carried out and financed by the commonwealth of European countries. Naib. U., which is part of the complex, is an LEP storage-collisional electron-positron ring that accelerates electron and positron beams to an energy of 45 GeV. U. is located in a deep underground tunnel and has a perimeter of 27 km. In this tunnel in the 90s. It is planned to build a large superconducting hadron collider LHC (Large Hadron Collider), designed to accelerate protons and antiprotons to an energy of 7 TeV, and subsequently to accelerate ions.

Rice. 6. Scheme of the CERN accelerator complex (Switzerland).

For injection into the LHC, the SPS (Super Proton Synchrotron) accelerator will be used, at the output of which the protons have an energy of ~ 450 GeV. The perimeter of this accelerator is 6.9 km, it is located underground at a depth of 40 m. The SPS receives heavy particles from the PS proton synchrotron, which, in turn, protons and ions come from the Isolde booster, and electrons and positrons - from the EPA booster.

In Russia, naib. proton (and ionic) urea (70 GeV) operates in Protvino (near Serpukhov, Moscow region). Under him, the construction of an acceleration and storage center (UC) with a perimeter of 21 km began. It is designed to accelerate protons and antiprotons up to an energy of 3 TeV. The International Joint Institute for Nuclear Research (JINR, Dubna, Moscow region) operates a proton synchrotron that accelerates protons to 9 GeV, a phasotron and superconducting ultrasonic ions - a Nuclotron that accelerates ions to an energy of 6 GeV / nucleon.

In Ying-those theoretical. and experiment. Physics (ITEP, Moscow), the proton synchrotron accelerates protons to an energy of 9 GeV.

Phase fluctuations... As already noted, in resonant ultrasonic waves, the beam of accelerated particles spontaneously splits into bunches. Centre. the particles of the bunches once again approach the accelerating gap (in cyclic Y) or to the next accelerating gap (in linear Y) at those times when the phase of the accelerating HF voltage has the required value. Such particles are called. equal. Dr. the particles of the bunch in the process of acceleration oscillate about the equilibrium, sometimes ahead of it, sometimes lagging behind it. These fluctuations are called. phase. They are accompanied by fluctuations in the energy and momentum of the accelerated particles relative to the energy and momentum of an equilibrium particle.

Let us consider the phase motion in linear Y. For simplicity, we will assume that the accelerating gaps are so short that the particles pass through them almost instantly. Let a certain particle come to the gap later than the equilibrium one. In order for it to start catching up with it, it must receive more energy when passing through the gap. On the contrary, a particle that has come to the gap earlier than the equilibrium one should receive less energy.

In fig. 7 sinusoidal curve depicts the time-varying intensity E accelerating HF field. The dashed line marks the tension, the edges must exist at the moment the equilibrium particle passes, so that it comes to the next gap in time. At each period of change E there are two such points: WITH and D... It is easy, however, to see that the motion is stable only at point C. Only at this point at later moments of time the field strength increases, and at earlier moments it decreases.

Rice. 7. To the discussion of the principle of autophasing.

A detailed analysis of the longitudinal motion of particles shows that with a sufficient amplitude of HF oscillations there is always a region of stable phase motion - in this case, the region located around the point C. This statement is called. principle autophasing.

In cyclic acceleration, energy depends not only on the speed of particles, but also on the length of the path they travel from the previous accelerating gap to the next one (if there are several of them), as well as the perimeter of the trajectory. Let's introduce the coefficient. lengthening orbit.

where L- orbital perimeter, R- particle momentum. The change in the time spent by a particle to circulate in the U. depends on its momentum and is described by the f-layer

where the g-Lorentz factor of the particle, In linear Y. a = 0, and the point is stable WITH... In cyclic Y. at point C is stable, and at point D... The energy at which these points change places corresponds to the ratio

and called. Critical energy (in English literature - transition energy). At this point, the phase of the accelerating voltage must be transferred from one "synchronous point" to another. When approaching critical. the energies of the frequency of phase oscillations (in cyclic U. they are often called radial-phase) decrease and the phase sizes of bunches sharply decrease, and the dispersion of particles in momentum (and in energy) increases. At the moment of transition through the critical. energy increases the influence of decomp. kind of instabilities. Depending on the design features of U. - on the value of a - critical. energy can lie within or outside the operating energy range.

Roll stability problem. Betatron oscillations... In large ring-shaped ultrasounds, particles travel a path measured in hundreds of thousands or even millions of kilometers during their acceleration. In will accumulate. systems this way for several more. orders of magnitude more, and in small U. - by several. orders of magnitude less, but it is always very large in comparison with the diameter of the vacuum chamber, the transverse dimensions of a cut usually do not exceed two tens of cm. Collision of particles with the walls of the chamber leads to their loss. Therefore, acceleration is only possible with a carefully calculated and executed focusing system.

For any value of the energy of the accelerated particle (in the region of stability of phase oscillations), there is a closed (stable) orbit in annular ultrasound. Being in the vacuum chamber of U., the particles move near this orbit, making about it betatron oscillations The frequencies of these oscillations significantly exceed the frequencies of the phase oscillations, so that when studying betatron oscillations, the energy of the accelerated particles and the position of the closed orbit can be considered constant.

With theoretical. Considering betatron oscillations, the areas are usually investigated, which are occupied by the accelerated particles in the "phase planes" ( r, p r) and (z, p z), where r and z- horizontal and vertical coordinates of the particle ( r = R - R 0, where R- particle radius, R 0 is the radius of the equilibrium trajectory), a p r and p z- the corresponding components of its impulse. In unperturbed motion, these areas have the shape of an ellipse. According to Liouville's theorem, the values ​​of the areas do not change when moving. In the process of acceleration, particles cross numerous. inhomogeneity of magn. and electric. fields. In this case, the region occupied by the beam in the phase space can acquire a complex shape, so that eff. the size of the area - the area of ​​the described ellipse - increases. In a carefully tuned W., such an increase does not occur. In the presence of a connection between horizontal and vertical movements, not each of the indicated areas is preserved, but the volume occupied by the beam in four-dimensional space ( r, z, p r, p z).

Practical Of interest is usually the region occupied by the beam not in the phase planes, but in the planes ( r, q r), (z, q z) where q r and q z are the angles made up by the velocities of particles with a tangent to the equilibrium orbit. These areas are called. horizontal (or radial) and vertical (or axial m) emittans beam e r and e z... The transition from impulses to angles is given by f-lami

where R- the longitudinal component of the impulse, which practically coincides with the full impulse; R 0 = mc... It follows from Liouville's theorem that the integrals of motion are the quantities p e r and p e z or, respectively, bge r and bge z, to-rye are called. normal

It is clear from what has been said that during acceleration, the normalized emittances remain unchanged, while the usual emittances e r and e z decrease as 1 / bg. The transverse dimensions of the beam are correspondingly reduced.

The most important characteristic of any U. is its akseptan s - naib. emittance, to-ry U. passes without loss. The high intensity of the accelerated beam can be achieved only in ultrasound with a sufficiently large acceptance.

With the given dimensions of the vacuum chamber, the acceptance of W. is proportional to the max. angle, which can be trajectories of particles with an equilibrium orbit, and, therefore, is inversely proportional to the wavelength of betatron oscillations. Vertical and horizontal acceptance of U. are proportional, i.e., to the number of betatron oscillations per revolution Q r and Q z to-rye therefore it is desirable to increase. In all existing U. Q r and Q z are close to each other. If both are less than 1, the focus is called. with l and b about i (m i gk o d), and if more than 1-s or l n about th (f e s t about th).

All integer and half-integer values Q r and Q z are prohibited. With whole Q particles return to magn. elements in the same phase of betatron oscillations, the influence of the field errors adds up and a resonant oscillation build-up occurs (external resonance). Around integer values ​​there are forbidden frequency regions, within which the increase in oscillations, although limited in magnitude, turns out to be unacceptably large, for example. exceeds the dimensions of the vacuum chamber.

Half-integer values Q r and Q z are forbidden due to the occurrence of a parameter and a resonance oscillation, arising due to the irregularities of the gradient magn. fields. In some U., especially in storage devices, higher orders are also reflected.

In cyclic. U. to focus the particles use transverse magnets. fields. In a uniform guide field, there is only horizontal focusing and no vertical focusing ( Q z = 0) This result is easy to understand, noting that when particles move in a uniform (vertical) magn. field ( B r = 0, B z = const) the Lorentz forces do not have a z component and the particles retain the initial value. axial speed. The forces necessary for axial focusing arise only in the presence of the radial component of magn. fields.

Magnet configuration field depends on the shape of the pole pieces. In fig. eight ( a) and 8 ( b) depicts pole pieces in the form of a figure of rotation (about the axis z). In fig. eight ( a) shows flat poles that create a uniform vertical field, such fields do not create axial focusing. In fig. eight ( b) shows the picture of the field arising between the poles creating a gap expanding to the periphery. In this case, the Lorentz force acquires a focusing (returning to the central plane) axial component. However, the appearance of axial focusing is accompanied by a weakening of the radial one: the particles deflected to the periphery return to the equilibrium trajectory more slowly, since they fall into a weaker field.

Rice. eight. a- magnetic forces in a homogeneous field; b- magnetic forces in the field decreasing towards the periphery.

In linear U. the problem of focusing is also important, although it is not as critical as in ring U.: the path length of the particles in linear U. is small and the accelerated particles do not return to the already passed perturbations of the field.

In cyclic U., magn. system to-rykh has azimuthal symmetry, valid f-la

Simultaneous stability of radial and axial betatron oscillations in this case is possible only with, i.e., with weak focusing (see. Focusing particles in an accelerator With strong focusing, the areas focusing in z and defocusing in r, are replaced by areas focusing along the horizontal and defocusing along the vertical coordinates. When followed. the location of such areas and the correct choice of gradients of magn. field and geometry of magnets, the system as a whole turns out to be focusing, and both resulting values ​​of betatron frequencies can significantly exceed unity.

In ultrasound with strong focusing, quadrupole magnets are used. or electric. (at low energies of accelerated particles) of the field. In fig. nine ( a) depicts a quadrupole magn. lens creating vertical focusing (z-axis) and radial defocusing r magn. field. The vacuum chamber is located along the axis of the lens between its poles (not shown in the figure). Positively charged particles "fly" towards the reader. Four such particles and the Lorentz forces acting on them are depicted by dots and arrows. In focusing along the radius (and defocusing along z) lenses magn. poles N and S swap places. In the ring U. magnets that create a leading magnet. field, located between the lenses. They create a uniform z-directional magnet. field. In some U. magnets with combined functions are used. Their magn. the field contains both a dipole (guiding field) and a quadrupole component (Fig. 9, b).

Rx. nine. a- quadrupole magnetic lens; b- magnetic block with combined functions.

For transverse focusing in linear U. one could try to use an electromagnet. wave, edges accelerates particles. However, in ordinary waves E-type points corresponding to stable phase motion are unstable for transverse vibrations and vice versa. To get around this difficulty, one can use alternating phase focusing (points WITH and D in fig. 7 successively replace each other) or abandon the azimuthal symmetry of the electric. fields in the cavity (quadrupole HF focusing). Most often, however, for transverse focusing, quadrupole fields are used, created by special. magn. lenses. Since the 80s. for the manufacture of such lenses began to use the post. magnets (SmCo alloy).

Intensity-related effects... In addition to resonances arising from the interaction of the beam with the external. fields, at high intensities of beams begin to play the role of decomp. a kind of instability associated with the interactions of the beam particles with each other, with the elements of the vacuum chamber and the accelerating system, and in the environment with colliding beams, and with the action of the beams on each other. Naib. the simplest among these effects is the Coulomb shift of the betatron vibration frequency. Electric. the beam field repels external particles to the periphery and does not act on the central particle of the bunch. As a result, the frequencies of the betatron oscillations of the particles in the beam begin to differ from the oscillation frequency of the center of gravity of the beam. If this difference exceeds the distance between the nearest forbidden values Q, then at any setting of Y, part of the beam is inevitably lost. Electrostatic. the repulsion of particles also affects the phase oscillations of the beam (in particular, it leads to the effect of "negative mass").

A beam of accelerated particles interacts with its electrostatic. the image in the vacuum chamber and with the objects located in it (resonators of accelerating stations, sensors of measuring devices, parts and inputs of the vacuum system, etc.). In this case, the force acting on each particle is proportional. the shift of the beam in the chamber relative to the equilibrium trajectory and its linear density. As a result of this interaction, an electromagnet is generated. the fields acting on the later passing particles (the effect of "heads - x in c t") and on the particles themselves that caused the appearance of the fields when these particles return to the excited area. This interaction causes a number of effects leading to the loss of beam stability. In addition to the already mentioned "head-to-tail" effect, there may be a resistive, unstable (interaction with an electric image of the beam running along the camera, a cut lags in phase due to the finite conductivity of the chamber walls), microwave instability (interaction with objects that can be excited at high frequencies), etc.

Colliding beam accelerators (colliders)... When new particles are generated in the act of collision, an energy should be released equal to or greater than the rest energy of the particles being born, i.e. hundreds of MeV, and sometimes many tens of GeV. With such large energy releases, it is not only chemical that loses its importance. the bonding of the particles that make up the target, but also the bonding of nucleons in the nucleus, so that collisions occur with single nucleons or even with single ones that make up a nucleon. T. n. cumulative processes, to-rye can be considered as simultaneously. collision of an accelerated particle with two or several. nucleons, are of scientific interest, but at high energies are observed extremely rarely.

As noted above, in the collision of particles in colliders, all the energy accumulated during acceleration can be realized, while in the collision of a fast proton with a nucleon of a stationary target, only a part of this energy is used. So, to generate J/ y-meson, the proton energy should be 3.7 times higher than the rest energy J/ y-meson, and the generation of the Z 0 -boson requires 50 times the energy. The generation of heavy particles on stationary targets is therefore catastrophically disadvantageous, and it is necessary to go over to colliders. In colliders, particles can move towards each other either in one ring (particles and antiparticles), or in two intersecting rings.

Technique of working with accumulate. rings, in which the colliding beams move, is very complex. The number of nuclear reactions occurring per unit time turns out to be thousands of times less than with stationary targets, due to the extreme rarefaction of the beams. The efficiency of colliders is usually characterized by luminosity,T. That is, the number by which you need to multiply eff. cross section of the studied reaction to obtain the number of such reactions per unit time. Luminosity proportional the product of the intensities of the colliding beams and inversely proportions. the cross-sectional areas of the beams (if they are equal). The colliding beams must, that is, contain many particles and occupy small volumes in the phase space. Cooling of the phase volume of electron and positron beams due to synchrotron radiation was discussed above. At the same time, the phase volume of proton beams decreases with acceleration as only 1 /R, that is, completely insufficient. And the volume occupied by antiproton beams turns out to be very large already during their generation and decreases little later, since antiprotons are formed at high energies (several GeV). Therefore, before collisions, antiproton beams should accumulate and cool down, i.e., they should be compressed in phase space.

There are two ways to cool beams of heavy particles (protons, antiprotons, ions) - electronic and stochastic. Electron cooling occurs when cooled beams interact with a beam of "cold" electrons flying in a certain common section together with cooled particles and having the same avg. speed. (The tempo of a beam is called the average energy of its particles, measured in a coordinate system moving with the beam.)

Static cooling is based on the fact that the number of simultaneously cooled particles is not very large. If there is only one particle inside the device that measures the beam coordinates, then its deviation can be measured by a sensor and then corrected by a corrector. If it measures inside. there will be several devices. particles, the sensor reacts to the position of their electr. center of gravity and there is no correction, but vibration damping (at N particles in the device are corrected by one, not N parameters). Stochastic. the cooling is gradual and requires a large number of revolutions.

Note that electron cooling turns out to be more efficient at low beam energies, while stochastic cooling is more efficient at not too large number of particles.

Prospects for the development of accelerators... Among the projects of large accelerators, to-rye are under development, construction, or have already entered service, the following can be listed.

In Russia (Troitsk, Moscow region), the construction of a "meson factory" with an energy of 600 MeV is nearing completion. current 70 μA. In 1993, she already gave out a beam with an energy of 430 MeV. For the production of isotopes, a proton beam with an energy of 160 MeV and with avg is used. current 100 μA. In Pro-tvino, the construction of an accelerator-storage complex (UNK) is underway, designed to accelerate protons up to 3 TeV. UNK is located in an underground tunnel with a perimeter of 21 km. Pulse intensity is expected 5. 10 12.

In the Federal Republic of Germany (Hamburg), a colliding beams (HERA) unit was commissioned to study the interaction of protons (820 GeV) with electrons and positrons (30 GeV). Design luminosity ~ 2. 10 31 cm -2. with -1. The proton synchrotron contains superconducting magnets, while the electronic synchrotron contains ordinary magnets (so as not to increase losses for synchrotron radiation). In equipping this U. and in the work on it, 37 institutes from different countries take part.

In Germany, a DESY linear collider project with particle energies of 250x250 GeV (1st version) or 500 x 500 GeV (2nd version) is also being developed. At CERN (Switzerland), the construction of the Large Hadron Collider (LHC) for heavy particles begins in the tunnel of the ring electron-positron U. (LEP). It will be possible to study collisions of protons (2x7 TeV), protons and electrons, protons and ions (including lead, 1148 TeV).

Heavy ions can be accelerated at the Nucleotron (Dubna, Russia). Since 1977 at the proton synchrotron in Dubna, dec. ions up to carbon (4.2 GeV / nucleon, and from 1992 - up to 6 GeV / nucleon).

On the W. "Saturn" in Saclay (France), ions are accelerated up to argon (up to 1.15 GeV / nucleon). The SPS accelerator (CERN) can accelerate oxygen and sulfur ions up to 200 GeV / nucleon.

In the USA, the Naib project has been developed. a large superconducting supercollider (SSC) with an energy of 2 x 20 TeV. The construction of this accelerator has been postponed.

In Int. the accelerator committee is considering even larger projects, the implementation of which will require joint efforts of developed countries. The specific project of such a management has not yet been determined. All implemented and developed projects are based on well-known, well-proven principles. The new acceleration methods mentioned above can, if successful, completely change these plans.

Application of accelerators... In addition to scientific U. have practical. application. So, linear U. are used to create neutron generators for radiation testing of materials, electronuclear methods of producing nuclear fuel and accelerating heavy low-charged ions for controlled inertial thermonuclear fusion are being actively discussed. In Loma Linda (USA), the construction of a specialized complex with a proton synchrotron for radiation therapy. A similar project is being considered in Russia.

Lit .: Kolomensky AA, Lebedev AN, Theory of cyclic accelerators, M., 1962; Waldner OA, Vlasov AD, Shalnov AV, Linear accelerators, M., 1969; Brook G., Cyclic accelerators of charged particles, trans. with French., M., 1970; Komar EG, Fundamentals of accelerating technology, M., 1975; Linear ion accelerators, ed. B.P. Murin, t. 1-2, M., 1978; Bakhrushin Yu. P., Anatsky AI, Linear induction accelerators, M., 1978; Lebedev A. N., Shalnov A. V., Fundamentals of physics and technology of accelerators, vol. 3, M., 1981; Moskalev V.A., Betatrons, M., 1981; Kapchinsky I.M., Theory of linear resonance accelerators, Moscow, 1982. L. L. Gol'din.

By discipline

"Concepts of modern natural science"

on the topic " Particle accelerators "

1. Introduction ………………………………………………………………………… .3

2. Modern accelerators of charged particles ……………………………… ... 4

3. Scientific centers for the study of elementary particles …………………… 7

4. Cyclic accelerator ……………………………………………………… 15

5. Beating laser accelerator ………………………………………… ..16

6. Conclusion …………………………………………………………………… ..20

7. List of used literature …………………………………………… 21

Introduction

Currently, charged particle accelerators are widely used in science and technology - installations for producing beams of charged particles (protons, electrons, antiparticles, nuclei of other atoms) of high energies - from tens of keV (103 eV) to several TeV (10 12 eV) ... In technology, such accelerators are used to obtain isotopes, to harden the surfaces of materials and to produce new materials, to create sources of electromagnetic radiation (from microwave to X-ray radiation), are widely used in medicine, etc. However, as before, nuclear physics and high-energy physics are among the main areas of application of accelerators. Modern charged particle accelerators are the main sources of information for physicists studying matter, energy, space and time. The overwhelming majority of elementary particles known today do not occur naturally on Earth and are obtained at accelerators. It is precisely the needs of the physics of elementary particles that are the main stimulus for the development of accelerator technology, and, first of all, for increasing the energy to which charged particles can be accelerated.

Modern charged particle accelerators.

In modern high-energy physics, accelerator installations of two types are used. The traditional scheme of the accelerator experiment is as follows: a beam of charged particles is accelerated to the maximum possible energy and then directed to a stationary target, when colliding with the particles of which a lot of elementary particles are generated. Measurements of the parameters of the particles being born provide the richest experimental information necessary for testing (or creating) the modern theory of elementary particles. The efficiency of the reaction is determined by the energy of the particle colliding with the target in the center of mass system. According to the theory of relativity, with a stationary target and the same rest masses of colliding particles, the reaction energy

Where E is the energy of the particle incident on the target, m 0 is its mass, c is the speed of light. So, in a collision with a stationary target of a proton accelerated to an energy of 1000 GeV, only 42 GeV energy is spent on the creation of new particles, and most of the energy is spent on the kinetic energy of particles born as a result of the reaction.

Proposed at the end of the 60s of the XX century, accelerators on colliding beams (colliders), in which the reaction is carried out in the collision of colliding accelerated beams of charged particles (electrons and positrons, protons and antiprotons, etc.), give a significant gain in the reaction energy. In colliders, the reaction energy is equal to the sum of the energies of colliding particles

E 1 + E 2, that is, at equal energies of particles, the gain is 2E / m 0 c 2. Of course, the efficiency of a collider turns out to be lower than that of an accelerator with a stationary target, since particles of two rarefied beams collide with each other much less frequently than particles of a beam and a dense target. Nonetheless, the main trend in high energy physics is to move into ever higher energies, and most of the largest accelerators today are colliders that sacrifice the number of collisions to achieve record energies.

Modern charged particle accelerators are the largest experimental facilities in the world, and the particle energy in an accelerator is linearly related to its size. For example, the 50 GeV SLC linear electron accelerator at Stanford University (USA) has a length of 3 km, the perimeter of the 900 GeV Tevatron proton synchrotron at the V.I. Fermi (Batavia, USA) is 6.3 km, and the length of the ring under construction in Serpukhov, the accelerator-storage complex UNK, designed for energy 3 TeV, is being built in the 27-kilometer accelerator tunnel of the European Organization for Nuclear Research (CERN) in Geneva.

The ever-increasing size of accelerators has already reached the boundary of a reasonable balance of physical characteristics and financial costs, making the construction of accelerators a national problem. We can say that purely engineering solutions are also close to their limit. Obviously, further progress in accelerator technology should be associated with the search for new approaches and physical solutions that make accelerators more compact and cheaper to build and operate. The latter is also important, since the power consumption of modern accelerators is close to the power consumption of a small city. Applied accelerator science poses an interesting and extremely important problem for modern physics. It is necessary to turn to new advances in radiophysics, plasma physics, quantum electronics and solid state physics in order to find worthy solutions.

Most promising is the search for ways to increase the rate of particle acceleration. In modern accelerators, the rate of particle acceleration is limited by the maximum intensity of the accelerating electric field that can be created in vacuum systems. This value does not exceed 50 MV / m today. In stronger fields, phenomena of electrical breakdown occur on the walls of the resonator and the formation of a plasma that absorbs the energy of the field and prevents the acceleration of particles. In fact, the magnitude of the maximum permissible high frequency field depends on its wavelength. Modern accelerators use electric fields with a wavelength of more than 10 cm. For example, a transition to a wavelength of 1 cm will increase the maximum permissible electric fields several times and thereby reduce the size of the accelerator. Of course, to realize this advantage, it is necessary to develop ultra-powerful radiation sources in this range, capable of generating pulses of electromagnetic waves with a power of hundreds of MW and a pulse duration shorter than 100 ns. This is a major scientific and technical problem, which is being addressed by many research centers around the world.

Another possible way is to abandon traditional vacuum microwave resonance systems and use laser radiation to accelerate charged particles. With the help of modern lasers, it is possible to create electric fields with an intensity much higher than the limiting fields in the microwave range. However, the direct use of laser radiation in a vacuum does not allow achieving the effect of noticeable acceleration of charged particles due to the impossibility of resonant Cherenkov interaction of a wave with a particle, since the speed of light in a vacuum is always greater than the speed of a particle. In recent years, methods of accelerating charged particles by laser radiation in gases and plasmas have been actively studied; moreover, since the ionization of matter and the formation of plasma occurs in strong electric fields, ultimately, we are talking about the acceleration of charged particles by intense laser radiation in plasma.

Scientific centers for the study of elementary particles

Institute for High Energy Physics (IHEP)

The basis for the creation of the institute was the construction in Protvino, located near the city of Serpukhov near Moscow, the world's largest (up to 1972) circular proton synchrotron. The unique experimental technique collected in this scientific center enables scientists to penetrate into the depths of the structure of matter, to understand and reveal the laws of the infinitely diverse and mysterious world of elementary particles unknown to man.

The accelerator was launched in October 1967. In this accelerator, initially protons are formed as a result of a gas discharge, then they are accelerated by the electric field of a high-voltage pulse of the transformer to an energy of 760 keV and enter the linear accelerator - injector, where they are preliminarily accelerated to an energy of 100 MeV, and then enter the main ring. accelerator. In it, protons are already accelerated to an energy of 76 GeV. The number of protons in one accelerator pulse is 3 · 10 12. Repetition of impulses occurs every 7 seconds. The accelerator has a diameter of 472 m. The weight of the electromagnets is 20 thousand tons. The power consumed by the accelerator is 100 MW. The accelerator runs for 3,000 - 4,000 hours annually for physical research.

The scientific center has a mound, under which there is an accelerating ring, and an experimental hall. Experiments at IHEP are carried out both on the internal target of the accelerator and on extracted particle beams.

It only deflects the particle without changing its energy, and sets the orbit along which the particles move.

Accelerators can be basically divided into two large groups. it linear accelerators, where the particle beam passes through the accelerating gaps once, and cyclic accelerators, in which the beams move along closed curves such as circles, passing the accelerating gaps many times. You can also classify accelerators by purpose: colliders, neutron sources, boosters, synchrotron radiation sources, cancer therapy facilities, industrial accelerators.

Accelerator designs

High voltage accelerator (direct action accelerator)

Main article: High voltage accelerator

Accelerator of charged particles (electrons) in which the acceleration of charged particles occurs by an electric field, constant or weakly changing during the entire time of particle acceleration. An important advantage of V.U. in comparison with other types of accelerators - the possibility of obtaining a small spread in energy of particles accelerated in a constant in time and uniform electric field. This type of accelerators is characterized by high efficiency (up to 95%) and the ability to create high-power installations (500 kW and more), which is very important when using accelerators for industrial purposes.

Electrostatic accelerator

Ideologically the simplest linear accelerator. The particles are accelerated by a constant electric field and move rectilinearly along the vacuum chamber, along which the accelerating electrodes are located.

Varieties:

• Van de Graaff accelerator. a van de Graaff generator based on the mechanical transfer of charges by a dielectric tape. The maximum electrical voltages of ~ 20MV determine the maximum particle energy of ~ 20MeV.
• Cascade accelerator. The accelerating voltage is created by a cascade generator, which creates a constant accelerating high voltage of ~ 5 MV, converting the low alternating voltage according to the diode multiplier circuit.

Linear accelerators of low-energy electrons are often used as part of a wide variety of electric vacuum devices (cathode ray tube, kinescope, X-ray tube, etc.).

Cyclotron

Cyclotron device. 1 - place of arrival of particles, 2 - trajectory of their movement, 3 - electrodes, 4 - source of alternating voltage. The magnetic field is directed perpendicular to the plane of the drawing.

The idea behind the cyclotron is simple. Between two semicircular hollow electrodes, the so-called. dees, an alternating electric voltage is applied. Dees are placed between the poles of an electromagnet that creates a constant magnetic field. A particle, rotating around a circle in a magnetic field, is accelerated at each revolution by an electric field in the gap between the dees. For this, it is necessary that the frequency of the change in the polarity of the voltage across the dees be equal to the frequency of revolution of the particle. In other words, the cyclotron is resonant accelerator... It is clear that with increasing energy, at each revolution, the radius of the particle trajectory will increase until it goes beyond the dees.

The cyclotron is the first of the cyclic accelerators. It was first designed and built in the year by Lawrence, for which he was awarded the Nobel Prize of the year. Until now, cyclotrons are used to accelerate heavy particles to relatively low energies, up to 50 MeV / nucleon.

Betatron

Another name: induction accelerator. A cyclic accelerator, in which the acceleration of particles is carried out by a vortex electric field induced by a change in the magnetic flux swept by the orbit of the beam. Since to create a vortex electric field it is necessary to change the magnetic field of the core, and magnetic fields in non-superconducting machines are usually limited by the effects of saturation of iron at a level of ~ 20 kG, an upper limit on the maximum energy of the betatron arises. Betatrons are mainly used to accelerate electrons to energies of 10-100 MeV (the maximum energy reached in the betatron is 300 MeV).

For the first time, the betatron was developed and created by Wideröe in the year, which, however, he failed to launch. The first reliably working betatron was created by D.V. Kerst only in the years. in the USA.

Microtron

Main article: Microtron

It is also a variable speed accelerator. A resonant cyclic accelerator with a constant guiding magnetic field and frequency of the accelerating voltage, like that of a cyclotron. The idea of ​​the microtron is to make the increment in the time of a particle's revolution, resulting from the acceleration at each revolution, a multiple of the oscillation period of the accelerating voltage.

Phazotron (synchrocyclotron)

The fundamental difference from the cyclotron is the frequency of the electric field that changes during acceleration. This allows, due to autophasing, to raise the maximum energy of the accelerated ions in comparison with the limiting value for the cyclotron. The energy in phasotrons reaches 600-700 MeV.

Synchrophasotron

Cyclic accelerator with constant equilibrium orbit length. In order for the particles to remain in the same orbit during acceleration, both the guiding magnetic field and the frequency of the accelerating electric field change. Most modern cyclic accelerators are strong focusing synchrophasotrons. For ultrarelativistic electrons, the revolution frequency practically does not change during acceleration, and synchrotrons are used.

Synchrotron

A cyclic accelerator with a constant orbital length and a constant frequency of the accelerating electric field, but with a changing guiding magnetic field.

Free Electron Laser (FEL)

Main article: Free Electron Laser

A specialized source of coherent X-ray radiation.

Linear accelerator

Also often called linac (short for LINear ACcelerator). An accelerator in which particles fly once. Linear accelerators are most often used for the primary acceleration of particles received from an electron gun or ion source. However, the idea of ​​a full energy linear collider is also not new. The main advantage of linacs is the possibility of obtaining ultra-small emittances and the absence of energy losses for radiation, which grow in proportion to the fourth power (!) Of the particle energy.

Collider

It is also a colliding beam accelerator. Purely experimental installations, the purpose of which is to study the processes of collision of high-energy particles.

Application

• Sterilization (for sterilizing food, medical instruments).
• Medicine (treatment of oncological diseases, radio diagnostics).
• Semiconductor device manufacturing (impurity injection).
• Radiation flaw detection.
• Radiation crosslinking of polymers.
• Radiation treatment of flue gases and waste water.

see also

• Particle detector

Links

• Kolomensky DD, Lebedev AN Theory of cyclic accelerators. Moscow: Fizmatgiz, 1962.
• A. Chao, M. Tigner, Handbook of Accelerator Physics and Engineering, 1999.
• B.S. Ishkhanov, I.M. Kapitonov, E.I. Kabin, Experiment (Web Publishing)
• History, classification, principle of operation, main types of modern accelerators

Wikimedia Foundation. 2010.

• Hölder's condition
• Particle accelerator

See what "Charged particle accelerators" are in other dictionaries:

CHARGED PARTICLE ACCELERATORS- installations serving to accelerate the charge. particles to high energies. In the usual word usage, accelerators (U.) are called. installations designed to accelerate particles to energies above MeV. At the record-setting protons, the Tevatron reached an energy of 940 ... ... Physical encyclopedia

Charged particle accelerators- devices for obtaining charged particles (electrons, protons, atomic nuclei, ions) of high energies. Acceleration is carried out using an electric field capable of changing the energy of particles with an electric charge. Magnetic ... ... Great Soviet Encyclopedia

CHARGED PARTICLE ACCELERATORS- installations for receiving directions. beams of electrons, protons, alpha particles or ions with energies from hundreds of keV to hundreds of GeV. In U. z. h. accelerated charge. particles increase their energy, moving in electric. field (static, inductive or ... ... Big Encyclopedic Polytechnic Dictionary

GOST 22491-87: Charged particle accelerators. Terms and Definitions- Terminology GOST 22491 87: Charged particle accelerators. Terms and definitions original document: 14. Betatron with bias 15. Resonant accelerator Betatron with constant component of magnetic field induction Accelerator ... ...

GOST 4.477-87: System of product quality indicators. Industrial charged particle accelerators. Nomenclature of indicators- Terminology GOST 4.477 87: System of product quality indicators. Industrial charged particle accelerators. Nomenclature of indicators original document: 3. Basic sample An accelerator selected from a group of accelerators, the most ... ... Dictionary-reference book of terms of normative and technical documentation

Particle accelerator- View of the Fermilab accelerator center, USA. Tevatron (ring in the background) and ring injector Charged particle accelerator class of devices for producing charged particles (elementary ... Wikipedia

accelerator (charged particles)- An electrophysical device designed to increase the kinetic energy of charged particles. Note It is assumed that in accelerators the particle energy increases by more than 0.1 MeV. [GOST R 52103 2003] Topics charged accelerators ... ...

charged particle buncher- A device that carries out the phase grouping of charged particles. [GOST R 52103 2003] Topics charged particle accelerators EN charged particle buncher ... Technical translator's guide