A charged particle accelerator is a device in which a beam of electrically charged atomic or subatomic particles moving at near-light speeds is created. The basis of his work is an increase in their energy by an electric field and a change in their trajectory by a magnetic one.
Why are charged particle accelerators needed?
These devices are widely used in various fields of science and industry. Today in the whole world there are more than 30 thousand. For a physicist, charged particle accelerators serve as a tool for fundamental research on the structure of atoms, the nature of nuclear forces, and the properties of nuclei that are not found in nature. The latter include transuranic and other unstable elements.
Using the discharge tube, it became possible to determine the specific charge. Charged particle accelerators are also used for the production of radioisotopes, in industrial radiography, radiation therapy, for sterilization of biological materials, as well as in radiocarbon analysis. The largest installations are used in studies of fundamental interactions.
The lifetime of charged particles at rest relative to the accelerator is shorter than that of particles dispersed to speeds close to the speed of light. This confirms the relativity of the time intervals SRT. For example, in CERN, an increase in the lifetime of muons at a speed of 0.9994c by 29 times was achieved.
This article discusses how the charged particle accelerator is structured and works, its development, various types and distinctive features.
Acceleration Principles
Regardless of which particle accelerators you know, they all share common elements. Firstly, all of them must have a source of electrons in the case of a television picture tube or electrons, protons and their antiparticles in the case of larger installations. In addition, all of them must have electric fields to accelerate particles and magnetic fields to control their trajectory. In addition, the vacuum in the charged particle accelerator (10 -11 mm Hg), i.e., the minimum amount of residual air, is necessary to ensure a long life time of the beams. And, finally, all installations must have means of recording, counting and measuring accelerated particles.
Generation
Electrons and protons, which are most often used in accelerators, are found in all materials, but first they need to be distinguished from them. Electrons, as a rule, are generated in the same way as in a kinescope - in a device called a “gun”. It is a cathode (negative electrode) in a vacuum, which is heated to a state where electrons begin to detach from atoms. Negatively charged particles are attracted to the anode (positive electrode) and pass through the outlet. The gun itself is also the simplest accelerator, since electrons move under the influence of an electric field. The voltage between the cathode and the anode, as a rule, is in the range of 50-150 kV.
In addition to electrons, all materials contain protons, but only nuclei of hydrogen atoms consist of single protons. Therefore, the source of particles for proton accelerators is hydrogen gas. In this case, the gas is ionized and the protons exit through the hole. In large accelerators, protons are often formed in the form of negative hydrogen ions. They are atoms with an additional electron, which are the product of the ionization of a diatomic gas. It is easier to work with negatively charged hydrogen ions in the initial stages. Then they are passed through a thin foil, which robs them of electrons before the final stage of acceleration.
Overclocking
How do charged particle accelerators work? A key feature of any of these is the electric field. The simplest example is a uniform static field between positive and negative electric potentials, similar to that which exists between the terminals of an electric battery. In such a field, an electron carrying a negative charge is subject to the action of a force that directs it to a positive potential. It accelerates it, and if there is nothing to prevent this, its speed and energy increase. Electrons moving towards a positive potential through a wire or even in air collide with atoms and lose energy, but if they are in a vacuum, they are accelerated as they approach the anode.
The voltage between the initial and final position of the electron determines the energy acquired by it. When moving through a potential difference of 1 V, it is equal to 1 electron-volt (eV). This is equivalent to 1.6 × 10 -19 joules. The energy of a flying mosquito is a trillion times greater. In a picture tube, electrons are accelerated by voltages above 10 kV. Many accelerators achieve much higher energies, measured by mega-, giga- and teraelectron-volts.
Varieties
Some of the earliest types of charged particle accelerators, such as the voltage multiplier and the Van de Graaff generator, used constant electric fields created by potentials up to a million volts. It is not easy to work with such high voltages. A more practical alternative is the repeated action of weak electric fields created by low potentials. This principle is used in two types of modern accelerators - linear and cyclic (mainly in cyclotrons and synchrotrons). Briefly speaking, linear accelerators of charged particles pass them once through a sequence of accelerating fields, while in a cyclic one they repeatedly move along a circular path through relatively small electric fields. In both cases, the final energy of the particles depends on the total effect of the fields, so that many small “shocks” are added together to give the combined effect of one large one.
The repeating structure of a linear accelerator to create electric fields naturally involves the use of alternating rather than constant voltage. Positive charged particles accelerate to a negative potential and receive a new impulse if they pass by a positive one. In practice, the voltage should change very quickly. For example, at an energy of 1 MeV, a proton moves at very high speeds of 0.46 the speed of light, passing 1.4 m in 0.01 ms. This means that in a repeating structure several meters long, the electric fields must change direction with a frequency of at least 100 MHz. Linear and cyclic accelerators of charged particles, as a rule, accelerate them using alternating electric fields with a frequency of 100 to 3000 MHz, i.e., in the range from radio waves to microwaves.
An electromagnetic wave is a combination of alternating electric and magnetic fields oscillating perpendicular to each other. The key point of the accelerator is to adjust the wave so that when the particle arrives, the electric field is directed in accordance with the acceleration vector. This can be done using a standing wave - a combination of waves moving in opposite directions in a confined space, like sound waves in an organ pipe. An alternative for very fast moving electrons, whose speed approaches the speed of light, is a traveling wave.
Auto phasing
An important effect during acceleration in an alternating electric field is “autophasing”. In one oscillation cycle, the alternating field passes from zero through the maximum value again to zero, drops to a minimum and rises to zero. Thus, it twice passes through the value necessary for acceleration. If a particle whose speed increases arrives too early, then a field of sufficient force will not act on it, and the impulse will be weak. When she reaches the next site, she will be late and will experience a stronger effect. As a result, autophasing will occur, the particles will be in phase with the field in each accelerating region. Another effect will be their grouping in time with the formation of clots, rather than a continuous flow.
Beam direction
An important role in how the accelerator of charged particles is arranged and works is also played by magnetic fields, since they can change the direction of their motion. This means that they can be used to “bend” the beams along a circular path so that they pass several times through the same accelerating section. In the simplest case, a charged particle moving at right angles to the direction of a uniform magnetic field is affected by a force perpendicular to both its displacement vector and the field. This causes the beam to move along a circular path perpendicular to the field until it leaves the area of its action or another force begins to act on it. This effect is used in cyclic accelerators such as cyclotron and synchrotron. In a cyclotron, a constant field is created by a large magnet. Particles, as their energy grows, move in a spiral outward, accelerating with each revolution. In the synchrotron, the bunches move around the ring with a constant radius, and the field created by the electromagnets around the ring increases as the particles accelerate. The “bend” magnets are dipoles with north and south poles bent in the shape of a horseshoe so that the beam can pass between them.
The second important function of electromagnets is to concentrate the beams so that they are as narrow and intense as possible. The simplest form of a focusing magnet is with four poles (two north and two south) located opposite each other. They push particles toward the center in one direction, but allow them to propagate perpendicularly. Quadrupole magnets focus the beam horizontally, allowing it to exit the focus vertically. To do this, they must be used in pairs. For more accurate focusing, more complex magnets with a large number of poles (6 and 8) are also used.
As the energy of the particles increases, the strength of the magnetic field directing them increases. This keeps the beam on one path. The clot is introduced into the ring and accelerated to the required energy before it is removed and used in experiments. Retraction is achieved by electromagnets, which are turned on to push particles out of the synchrotron ring.
Clash
Charged particle accelerators used in medicine and industry mainly produce a beam for a specific purpose, for example, for radiation therapy or ion implantation. This means that particles are used once. For many years, the same was true for accelerators used in basic research. But in the 1970s, rings were developed in which two beams circulate in opposite directions and collide along the entire contour. The main advantage of such installations is that in a head-on collision, the energy of the particles passes directly into the energy of interaction between them. This contrasts with what happens when the beam collides with a material at rest: in this case, most of the energy is spent driving the target material in accordance with the principle of conservation of momentum.
Some cars with colliding beams are built with two rings intersecting in two or more places in which particles of the same type circulated in opposite directions. Colliders with particles and antiparticles are more common. An antiparticle has the opposite charge of a particle bound to it. For example, the positron is positively charged, and the electron is negatively charged. This means that the field that accelerates the electron slows down the positron moving in the same direction. But if the latter moves in the opposite direction, it will accelerate. Similarly, an electron moving through a magnetic field will bend to the left, and a positron will bend to the right. But if the positron moves towards it, then its path will still deviate to the right, but along the same curve as the electron. Together, this means that these particles can move along the synchrotron ring due to the same magnets and can be accelerated by the same electric fields in opposite directions. By this principle, many powerful colliders on colliding beams have been created, since only one accelerator ring is required.
The beam in the synchrotron does not move continuously, but is united into “clots”. They can be several centimeters in length and a tenth of a millimeter in diameter, and contain about 10 12 particles. This is a low density, since a substance of similar sizes contains about 10 23 atoms. Therefore, when the beams intersect with the colliding beams, there is only a small probability that the particles will interact with each other. In practice, the clots continue to move along the ring and meet again. A deep vacuum in an accelerator of charged particles (10 -11 mm Hg) is necessary so that the particles can circulate for many hours without collision with air molecules. Therefore, the rings are also called cumulative, since the beams are actually stored in them for several hours.
registration
Mostly, charged particle accelerators can detect what happens when particles hit a target or another beam moving in the opposite direction. In a television picture tube, electrons from a gun strike a phosphor on the inner surface of the screen and emit light, which thus recreates the transmitted image. In accelerators, such specialized detectors react to scattered particles, but they are usually designed to create electrical signals that can be converted into computer data and analyzed using computer programs. Only charged elements create electrical signals passing through the material, for example, by excitation or ionization of atoms, and can be detected directly. Neutral particles, such as neutrons or photons, can be detected indirectly through the behavior of charged particles, which are set in motion by them.
There are many specialized detectors. Some of them, such as a Geiger counter, simply count particles, while others are used, for example, to record tracks, measure speed or amount of energy. In terms of size and technology, modern detectors range from small charge-coupled devices to large gas-filled chambers with wires that record ionized traces created by charged particles.
History
Accelerators of charged particles were mainly developed to study the properties of atomic nuclei and elementary particles. Since the discovery by the British physicist Ernest Rutherford in 1919 of the reaction of the nitrogen nucleus and alpha particles, all research in nuclear physics until 1932 was carried out with helium nuclei released as a result of the decay of natural radioactive elements. Natural alpha particles have a kinetic energy of 8 MeV, but Rutherford believed that to observe the decay of heavy nuclei, they must be artificially accelerated to even greater values. At that time, it seemed complicated. However, a calculation made in 1928 by Georgy Gamow (at the University of Göttingen, Germany) showed that ions with significantly lower energies could be used, and this stimulated attempts to build a facility that provided a beam sufficient for nuclear research.
Other events of this period have demonstrated the principles by which charged particle accelerators are built to this day. The first successful experiments with artificially accelerated ions were carried out by Cockcroft and Walton in 1932 at the University of Cambridge. Using a voltage multiplier, they accelerated protons to 710 keV and showed that the latter react with the lithium nucleus to form two alpha particles. By 1931, at the Princeton University of New Jersey, Robert Van de Graaf built the first high potential electrostatic belt generator. - -- - .
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After World War II, the science of accelerating particles to high energies made rapid progress. It was started by Edwin Macmillan in Berkeley and Vladimir Wexler in Moscow. In 1945, they both independently described the principle of phase stability. This concept offers a means of maintaining stable particle orbits in a cyclic accelerator, which removed the restriction on the energy of protons and allowed the creation of magnetic resonance accelerators (synchrotrons) for electrons. Autophasing, the implementation of the principle of phase stability, was confirmed after the construction of a small synchrocyclotron at the University of California and a synchrotron in England. Soon after, the first proton linear resonance accelerator was created. This principle is used in all large proton synchrotrons built since then.
In 1947, William Hansen, at Stanford University in California, built the first linear traveling-wave electron accelerator using microwave technology that was developed for radars during World War II.
Progress in research was made possible by increasing proton energy, which led to the construction of ever larger accelerators. This trend was stopped by the high cost of manufacturing huge ring magnets. The largest weighs about 40,000 tons. Ways to increase energy without increasing the size of machines were demonstrated in 1952 by Livingston, Courant and Snyder using the alternating focusing technique (sometimes called strong focusing). Synchrotrons operating on this principle use magnets 100 times smaller than before. Such focusing is used in all modern synchrotrons.
In 1956, Kerst realized that if two sets of particles were held in intersecting orbits, then their collisions could be observed. The application of this idea required the accumulation of accelerated beams in cycles called accumulative. This technology has allowed to achieve maximum particle interaction energy.