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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Charged particle accelerator. History of invention and production
Directory / The history of technology, technology, objects around us Modern physics has a tried and tested way to penetrate the secrets of the atomic nucleus - bombard it with particles or irradiate it and see what happens to it. For the very first studies of the atom and its nucleus, the energy of radiations arising from the natural decay of radioactive elements was enough. But soon this energy was not enough, and in order to "look" even deeper into the nucleus, physicists had to think about how to artificially create a stream of high-energy particles. It is known that, having fallen between electrodes with different charges, a charged particle, for example, an electron or a proton, accelerates movement under the action of electric forces. This phenomenon gave rise in the 1930s to the idea of creating the so-called linear accelerator. By design, a linear accelerator is a long straight tube-chamber, inside which a vacuum is maintained. A large number of metal tubes-electrodes are placed along the entire length of the chamber. From a special high-frequency generator, an alternating electrical voltage is applied to the electrodes - so that when the first electrode is charged, let's say positively, the second electrode will be negatively charged. Then again the positive electrode, followed by the negative one.
A beam of electrons is fired from the electron "gun" into the chamber and, under the action of the potential of the first, positive electrode, begins to accelerate, slipping further through it. At the same moment, the phase of the supply voltage changes, and the electrode, just charged positively, becomes negative. Now he repels electrons from himself, as if urging them from behind. And the second electrode, having become positive during this time, attracts electrons to itself, accelerating them even more. Then, when the electrons fly through it, it will turn negative again and push them towards the third electrode. So, as the electrons move forward, they gradually accelerate, reaching near-light speed by the end of the chamber and acquiring an energy of hundreds of millions of electron volts. Through a window installed at the end of the tube, impenetrable to air, a portion of accelerated electrons falls on the studied objects of the microworld - atoms and their nuclei. It is easy to understand that the more energy that we want to impart to the particles, the longer the linear accelerator tube should be - tens or even hundreds of meters. But this is not always possible. Now, if you roll the pipe into a compact spiral. Then such an accelerator could freely be placed in the laboratory. Another physical phenomenon helped bring this idea to life. A charged particle, once in a magnetic field, begins to move not in a straight line, but "curls" around the magnetic field lines. Thus, another type of accelerator appeared - the cyclotron. The first cyclotron was built in 1930 by E. Lawrence in the USA.
The main part of the cyclotron is a powerful electromagnet, between the poles of which a flat cylindrical chamber is placed. It consists of two semicircular metal boxes separated by a small gap. These boxes - dees - serve as electrodes and are connected to the poles of an alternating voltage generator. In the center of the chamber is a source of charged particles - something like an electronic "gun".
Having flown out of the source, the particle (let's assume that it is now a positively charged proton) is immediately attracted to the electrode, which is currently negatively charged. There is no electric field inside the electrode, so the particle flies in it by inertia. Under the influence of a magnetic field, the lines of force of which are perpendicular to the plane of the trajectory, the particle describes a semicircle and flies up to the gap between the electrodes. During this time, the first electrode becomes positive and now pushes the particle out while the other draws it in. Thus, passing from one dee to another, the particle picks up speed and describes an unwinding spiral. Particles are removed from the chamber with the help of special magnets on the target of the experimenters. The closer the speed of the particles in the cyclotron approaches the speed of light, the heavier they become and begin to gradually lag behind the changing sign of the electric voltage on the dees. They no longer fall in time with electric forces and stop accelerating. The limiting energy that can be communicated to particles in a cyclotron is 25-30 MeV. To overcome this barrier, the frequency of the electrical voltage applied alternately to the dees is gradually reduced, adjusting it to the beat of the "heavier" particles. An accelerator of this type is called a synchrocyclotron. One of the largest synchrocyclotrons at the Joint Institute for Nuclear Research in Dubna (near Moscow) produces protons with an energy of 680 MeV and deuterons (heavy hydrogen nuclei - deuterium) with an energy of 380 MeV. To do this, it was necessary to build a vacuum chamber with a diameter of 3 meters and an electromagnet weighing 7000 tons! As physicists penetrated deeper into the structure of the nucleus, higher and higher energy particles were required. It became necessary to build even more powerful accelerators - synchrotrons and synchrophasotrons, in which particles move not in a spiral, but in a closed circle in an annular chamber. In 1944, independently of each other, the Soviet physicist V.I. Veksler and American physicist E.M. Macmillan discovered the principle of autophasing. The essence of the method is as follows: if the fields are chosen in a certain way, the particles will automatically fall into the phase with the accelerating voltage all the time. In 1952, American scientists E. Courant, M. Livingston and H. Snyder proposed the so-called hard focusing, which presses particles to the axis of motion. With the help of these discoveries, it was possible to create synchrophasotrons for arbitrarily high energies. There is another classification system for accelerators - according to the type of accelerating electric field. High-voltage accelerators work due to the high potential difference between the electrodes of the accelerating space, which operates all the time while the particles fly between the electrodes. In induction accelerators, a vortex electric field "works", which is induced (excited) in the place where the particles are currently located. And, finally, resonant accelerators use an electric accelerating field that varies in time and magnitude, synchronously with which, "into resonance", the entire "set" of particles is accelerated. When people talk about modern high-energy particle accelerators, they mean mainly ring resonant accelerators. In yet another type of accelerator - proton - for very high energies, by the end of the acceleration period, the speed of particles approaches the speed of light. They circulate in a circular orbit at a constant frequency. Accelerators for high-energy protons are called proton synchrotrons. The three largest are located in the USA, Switzerland and Russia. The energy of currently operating accelerators reaches tens and hundreds of gigaelectronvolts (1 GeV = 1000 MeV). One of the largest in the world is the U-70 proton synchrophasotron of the Institute for High Energy Physics in the city of Protvino near Moscow, which was put into operation in 1967. The diameter of the accelerating ring is one and a half kilometers, the total mass of 120 magnetic sections reaches 20000 tons. Every two seconds, the accelerator shoots at targets with a volley of 10 to the twelfth power of protons with an energy of 76 GeV (the fourth indicator in the world). To achieve this energy, the particles must complete 400000 revolutions, covering a distance of 60000 kilometers! An underground ring tunnel twenty-one kilometers long for the new accelerator was also built here. Interestingly, launches of accelerators in Dubna or Protvino in Soviet times were carried out only at night, since they were supplied with almost all the electricity not only in Moscow, but also in neighboring regions! In 1973, American physicists put into operation an accelerator in the city of Batavia, in which particles managed to impart an energy of 400 GeV, and then brought it up to 500 GeV. Today, the most powerful accelerator is located in the USA. It is called the "Tevatron" because in its ring more than six kilometers long, with the help of superconducting magnets, protons acquire an energy of about 1 teraelectronvolt (1 TeV is equal to 1000 GeV).
In order to achieve even higher energy of interaction between the beam of accelerated particles and the material of the studied physical object, it is necessary to disperse the "target" towards the "projectile". To do this, organize the collision of particle beams flying towards each other in special accelerators - colliders. Of course, the density of particles in colliding beams is not as high as in the material of a stationary "target", so the so-called accumulators are used to increase it. These are annular vacuum chambers into which particles are thrown "in portions" from the accelerator. The accumulators are equipped with accelerating systems that compensate for the energy loss of particles. It is with colliders that scientists associate the further development of accelerators. So far, only a few of them have been built, and they are located in the most developed countries of the world - in the USA, Japan, Germany, as well as in the European Center for Nuclear Research, based in Switzerland. A modern accelerator is a "factory" for the production of intense particle beams - electrons or protons 2000 times heavier. The beam of particles from the accelerator is directed to a "target" selected on the basis of the tasks of the experiment. When colliding with it, a variety of secondary particles are produced. The birth of new particles is the purpose of the experiments. With the help of special devices - detectors - these particles or their traces are registered, the trajectory of movement is restored, the mass of particles, electric charge, speed and other characteristics are determined. Then, by complex mathematical processing of information received from the detectors, the entire "history" of interaction is restored on computers and, by comparing the measurement results with the theoretical model, conclusions are drawn whether the real processes coincide with the constructed model or not. This is how new knowledge about the properties of intranuclear particles is obtained. The higher the energy acquired by the particle in the accelerator, the stronger it affects the "target" atom or the counter particle in the collider, the smaller the "fragments" will be. With the help of a collider in the United States, for example, experiments are being carried out with the aim of recreating in laboratory conditions the Big Bang, from which our universe is supposed to have begun. Physicists from twenty countries took part in this bold experiment, among which were representatives of Russia. The Russian group in the summer of 2000 directly participated in the experiment, was on duty at the accelerator, and took data. Here is what one of the Russian scientists - participants in this experiment - Candidate of Physical and Mathematical Sciences, Associate Professor of MEPhI Valery Mikhailovich Emelyanov says: "60 miles from New York, on Long Island, the RHIC accelerator - Relativistic Heavy Ion Collider - was built on heavy relativistic ions. "Heavy" - since already this year he began working with beams of nuclei of gold atoms. "Relativistic" - also understandable, we are talking about speeds at which the effects of special relativity manifest themselves in all their glory. And the "collider" (from collide - collide) it is called because in its ring there is a collision of colliding beams of nuclei. By the way, in our country there are no accelerators of this type. The energy that falls on one nucleon is 100 GeV. This is a lot - almost twice as much as before achieved. The first physical collision was recorded on June 25, 2000." The task of scientists was to try to register a new state of nuclear matter - quark-gluon plasma. “The task is very complicated,” continues Emelyanov, “and mathematically it is generally incorrect: the same fixed distribution of secondary particles in terms of momenta and velocities can have completely different causes. And only in a detailed experiment that involves a lot of detectors, calorimeters, multiplicity sensors charged particles, counters registering transition radiation, etc., there is hope to register the subtlest differences inherent in quark-gluon plasma.The mechanism of interaction of nuclei at such high energies is interesting in itself, but more importantly, for the first time in the laboratory explore the origin of our universe." Author: Musskiy S.A.
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