A particle accelerator is a device that creates a beam of electrically charged atomic or subatomic particles moving at near-light speeds. Its work is based on an increase in their energy by an electric field and a change in the trajectory - by a magnetic one.
What are particle accelerators for?
These devices are widely used in various fields of science and industry. Today, there are more than 30 thousand of them all over the world. For a physicist, particle accelerators serve as a tool for fundamental research into the structure of atoms, the nature of nuclear forces, and the properties of nuclei that do not occur in nature. The latter include transuranium and other unstable elements.
With the help of a discharge tube, it became possible to determine the specific charge. Particle accelerators are also used in the production of radioisotopes, in industrial radiography, in radiation therapy, in the sterilization of biological materials, and in radiocarbonanalysis. The largest installations are used in the study of fundamental interactions.
The lifetime of charged particles at rest relative to the accelerator is less than that of particles accelerated to speeds close to the speed of light. This confirms the relativity of SRT time intervals. For example, at CERN, a 29-fold increase in the lifetime of muons at a speed of 0.9994c was achieved.
This article discusses how a particle accelerator works, its development, different types and distinctive features.
Principles of acceleration
Regardless of which particle accelerators you know, they all have common elements. First, they must all have a source of electrons in the case of a television kinescope, or electrons, protons, and their antiparticles in the case of larger installations. In addition, they must all have electric fields to accelerate the particles and magnetic fields to control their trajectory. In addition, the vacuum in the particle accelerator (10-11 mm Hg), i.e. the minimum amount of residual air, is necessary to ensure a long lifetime of the beams. And, finally, all installations must have the means to register, count and measure accelerated particles.
Generation
Electrons and protons, which are most commonly used in accelerators, are found in all materials, but first they need to be isolated from them. Electrons are usually generatedjust like in a kinescope - in a device called a "gun". It is a cathode (negative electrode) in a vacuum, which is heated to the point where electrons begin to break away 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 the electrons move under the action of an electric field. The voltage between the cathode and the anode is usually between 50-150 kV.
In addition to electrons, all materials contain protons, but only the nuclei of hydrogen atoms consist of single protons. Therefore, the source of particles for proton accelerators is gaseous hydrogen. In this case, the gas is ionized and the protons escape through the hole. In large accelerators, protons are often formed as negative hydrogen ions. They are atoms with an extra electron, which are the product of 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 that deprives them of electrons before the final stage of acceleration.
Acceleration
How do particle accelerators work? The key feature of any of them is the electric field. The simplest example is a uniform static field between positive and negative electrical potentials, similar to that which exists between the terminals of an electric battery. In suchfield, an electron carrying a negative charge is subject to a force that directs it toward a positive potential. She accelerates him, and if there is nothing to prevent this, his speed and energy increase. Electrons moving towards a positive potential in a wire or even in air collide with atoms and lose energy, but if they are in a vacuum, they accelerate as they approach the anode.
The voltage between the initial and final position of an 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 kinescope, electrons are accelerated by a voltage of over 10 kV. Many accelerators achieve much higher energies, measured in mega-, giga-, and teraelectronvolts.
Varieties
Some of the earliest types of particle accelerators, such as the voltage multiplier and the Van de Graaff generator, used constant electric fields generated by potentials up to a million volts. It is not easy to work with such high voltages. A more practical alternative is the repetitive action of weak electric fields generated by low potentials. This principle is used in two types of modern accelerators - linear and cyclic (mainly in cyclotrons and synchrotrons). Linear particle accelerators, in short, pass them once through a sequenceaccelerating fields, while in the 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 combined effect of the fields, so that many small "shocks" add up to give the combined effect of one big one.
The repeating structure of a linear accelerator to create electric fields naturally involves the use of AC rather than DC voltage. Positively charged particles are accelerated towards the negative potential and get a new impetus if they pass by the positive one. In practice, the voltage should change very quickly. For example, at an energy of 1 MeV, a proton travels at very high speeds of 0.46 the speed of light, traveling 1.4 m in 0.01 ms. This means that in a repeating pattern several meters long, the electric fields must change direction at 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., ranging from radio waves to microwaves.
An electromagnetic wave is a combination of alternating electric and magnetic fields that oscillate 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 with a standing wave - a combination of waves traveling in opposite directions in a closed loop.space, like sound waves in an organ pipe. An alternative for very fast moving electrons approaching the speed of light is a traveling wave.
Autophasing
An important effect when accelerating in an alternating electric field is "autophasing". In one cycle of oscillation, the alternating field goes from zero through a maximum value again to zero, falls to a minimum and rises to zero. So it goes through the value needed to speed up twice. If the accelerating particle arrives too early, then it will not be affected by a field of sufficient strength, and the push will be weak. When she reaches the next section, she will be late and will experience a stronger impact. As a result, autophasing will occur, the particles will be in phase with the field in each accelerating region. Another effect would be to cluster them over time in clumps rather than a continuous stream.
Beam direction
Magnetic fields also play an important role in how a charged particle accelerator works, as they can change the direction of their movement. This means that they can be used to "bent" the beams along a circular path so that they pass through the same accelerating section several times. In the simplest case, a charged particle moving at right angles to the direction of a uniform magnetic field is subjected to a forceperpendicular both to the vector of its displacement and to the field. This causes the beam to move along a circular trajectory 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 the cyclotron and synchrotron. In a cyclotron, a constant field is generated by a large magnet. The particles, as their energy grows, spiral outward, accelerating with each revolution. In a synchrotron, the bunches move around a ring with a constant radius, and the field created by the electromagnets around the ring increases as the particles accelerate. The "bending" magnets are dipoles with the north and south poles bent in a horseshoe shape 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) opposite each other. They push the particles toward the center in one direction but allow them to propagate in the perpendicular direction. Quadrupole magnets focus the beam horizontally, allowing it to go out of focus vertically. To do this, they must be used in pairs. More complex magnets with more poles (6 and 8) are also used for more precise focusing.
As the energy of the particles increases, the strength of the magnetic field guiding them increases. This keeps the beam on the same path. The clot is introduced into the ring and accelerated torequired energy before it can be withdrawn and used in experiments. Retraction is achieved by electromagnets that turn on to push particles out of the synchrotron ring.
Collision
Particle accelerators used in medicine and industry mainly produce a beam for a specific purpose, such as radiation therapy or ion implantation. This means that the 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 the two beams circulate in opposite directions and collide along the entire circuit. The main advantage of such installations is that in a head-on collision, the energy of the particles goes directly into the energy of interaction between them. This contrasts with what happens when the beam collides with material at rest: in this case, most of the energy is spent on setting the target material in motion, in accordance with the principle of conservation of momentum.
Some colliding beam machines are built with two rings intersecting at two or more places, in which particles of the same type circulate in opposite directions. Colliders with particles and antiparticles are more common. An antiparticle has the opposite charge of its associated particle. For example, a positron is positively charged, while an 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 be accelerated by the same electric fields in opposite directions. Many of the most powerful colliders on colliding beams have been created according to this principle, since only one accelerator ring is required.
The beam in the synchrotron does not move continuously, but is combined into "clumps". They can be several centimeters long and a tenth of a millimeter in diameter, and contain about 1012 particles. This is a small density, since a substance of this size contains about 1023 atoms. Therefore, when beams intersect with oncoming beams, there is only a small chance that the particles will interact with each other. In practice, the bunches continue to move along the ring and meet again. The deep vacuum in the particle accelerator (10-11 mmHg) is necessary so that the particles can circulate for many hours without colliding with air molecules. Therefore, the rings are also called accumulative, since the bundles are actually stored in them for several hours.
Register
Particle accelerators for the most part can register what happens whenwhen particles hit a target or another beam moving in the opposite direction. In a television kinescope, 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 respond to scattered particles, but they are usually designed to generate electrical signals that can be converted into computer data and analyzed using computer programs. Only charged elements create electrical signals by passing through a material, for example by exciting or ionizing atoms, and can be detected directly. Neutral particles such as neutrons or photons can be detected indirectly through the behavior of the charged particles they set in motion.
There are many specialized detectors. Some of them, like the Geiger counter, simply count particles, while others are used, for example, to record tracks, measure speed, or measure the amount of energy. Modern detectors range in size and technology from small charge-coupled devices to large wire-filled gas-filled chambers that detect the ionized trails created by charged particles.
History
Particle accelerators were mainly developed to study the properties of atomic nuclei and elementary particles. From the discovery of the reaction between the nitrogen nucleus and the alpha particle by the British physicist Ernest Rutherford in 1919, all research in nuclear physics up to1932 was spent with helium nuclei released from the decay of natural radioactive elements. Natural alpha particles have a kinetic energy of 8 MeV, but Rutherford believed that in order to observe the decay of heavy nuclei, they must be artificially accelerated to even greater values. At the time it seemed difficult. However, a calculation made in 1928 by Georgy Gamow (at the University of Göttingen, Germany) showed that ions with much 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 demonstrated the principles by which particle accelerators are built to this day. The first successful experiments with artificially accelerated ions were carried out by Cockcroft and W alton 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 Princeton University in New Jersey, Robert van de Graaf had built the first high potential belt electrostatic generator. Cockcroft-W alton voltage multipliers and Van de Graaff generators are still used as power sources for accelerators.
The principle of a linear resonant accelerator was demonstrated by Rolf Wideröe in 1928. At the Rhine-Westphalian University of Technology in Aachen, Germany, he used a high alternating voltage to accelerate sodium and potassium ions to energies twiceexceeding those reported by them. In 1931 in the United States, Ernest Lawrence and his assistant David Sloan of the University of California, Berkeley used high frequency fields to accelerate mercury ions to energies in excess of 1.2 MeV. This work complemented the Wideröe heavy particle accelerator, but ion beams were not useful in nuclear research.
The magnetic resonance accelerator, or cyclotron, was conceived by Lawrence as a modification of the Wideröe installation. Lawrence Livingston's student demonstrated the principle of the cyclotron in 1931 by producing 80 keV ions. In 1932 Lawrence and Livingston announced the acceleration of protons to over 1 MeV. Later in the 1930s, the energy of cyclotrons reached about 25 MeV, and that of Van de Graaff generators reached about 4 MeV. In 1940, Donald Kerst, applying the results of careful orbital calculations to the design of magnets, built the first betatron, a magnetic induction electron accelerator, at the University of Illinois.
Modern physics: particle accelerators
After World War II, the science of accelerating particles to high energies made rapid progress. It was started by Edwin Macmillan at Berkeley and Vladimir Veksler in Moscow. In 1945, both of them independently described the principle of phase stability. This concept offers a means of maintaining stable particle orbits in a cyclic accelerator, which removed the limitation on the energy of protons and made it possible to create magnetic resonance accelerators (syncrotrons) for electrons. Autophasing, the implementation of the principle of phase stability, has been confirmed after constructiona small synchrocyclotron at the University of California and a synchrotron in England. Shortly thereafter, the first proton linear resonant accelerator was created. This principle has been 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 radar during World War II.
Progress in research was made possible by increasing the energy of protons, which led to the construction of ever larger accelerators. This trend has been h alted by the high cost of making 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 in the technique of alternating focusing (sometimes called strong focusing). Synchrotrons based 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 kept in intersecting orbits, they could be observed colliding. The application of this idea required the accumulation of accelerated beams in cycles called storage. This technology made it possible to achieve the maximum interaction energy of particles.
