ince Rutherford used a-rays emitted by natural radioactive elements to bombard metal foils in 1919, realizing the first artificial nuclear reaction in the history of human science, physicists realized that in order to understand the nucleus, high-speed particles must be used to bombard the nucleus. Before the advent of particle accelerators, there were two types of particle beams used to study the structure of atomic nuclei: rays from natural radionuclides and cosmic rays from the sky. However, the flow intensity of the former radiation particles is too low and the energy is not high, so the probability of nuclear reaction is very small. The energy of Yushen ray particles can be as high as 102eV, but its intensity is too weak, and the results of visual experiments are unpredictable. Hence, particle accelerators were born. Particle accelerator is a device that uses artificial methods to generate high-speed charged particles. It is an important tool for exploring the properties, internal structure and interaction of atomic nuclei and particles. It is also important in medical and health, industrial and agricultural production, science and technology, etc. And a wide range of practical applications.

One.Development of particle accelerators

　　1. Linear accelerator

　　The first generation of accelerators belonged to linear accelerators. The energy of an electron accelerated by an electric field with a potential difference of 1 volt is 1 electron volt (abbreviated as 1 eV). Protons or electrons are accelerated by a strong electric field. The main goal of such an accelerator is to obtain the highest possible voltage.In 1932, American scientist Kircroft and Irish scientist Walton built the world's first DC accelerator, the Kircroft-Walton DC High Voltage Accelerator..., a proton beam with an energy of 400,000 electron volts. A nuclear reaction experiment of alpha particles and helium obtained by bombarding a lithium target. It was the first time in history that a nuclear reaction had been achieved using artificially accelerated particles, for which the two won the Nobel Prize in Physics in 1951.Although linear accelerators have achieved a series of results, to further increase the energy of accelerated particles, it is necessary to further increase the voltage, which has become a bottleneck in the development of linear accelerators. The particle energy accelerated by the linear accelerator is relatively low, which does not play a big role in exploring the nucleus and discovering new particles. Physicists urgently need higher energy particles to bombard the nucleus. Explore a deeper physical world.

Two.Cyclotron

　　The second generation of accelerators is the cyclotron. In 1930, the American experimental physicist Lawrence proposed the working principle of the cyclotron, and in 1932 the first cyclotron 4 was built. And use it to produce artificial radioisotopes, which won the Nobel Prize in Physics in 1939. This is the first person in the history of accelerator development to receive this honor. The main structure of the cyclotron is that there are two semicircular metal flat boxes (D-shaped boxes) in the vacuum chamber between the magnetic poles, and there is a constant magnetic field in the vertical direction of the upper box. An alternating voltage is applied to the D-shaped box, and an alternating electric field is generated at the gap. The particle source placed in the center produces charged particles, which are accelerated by the electric field, and are not subjected to the electric field in the D-shaped box. The frequency of the alternating voltage applied to the D-shaped box is exactly equal to the frequency of the circular motion of the dry particles in the magnetic field. Since the above-mentioned time for the particle to make a half-circle is independent of the speed of the particle, the particle is accelerated once every half-circle, and the circle radius increases. After many times of acceleration, the particles are drawn out from the edge of the D-shaped bin along a spiral trajectory. The energy of this cyclotron is controlled by the relativistic effect that increases with the particle velocity, the mass of the particle increases, and the particle orbiting period becomes longer, which gradually deviates from the acceleration state of the alternating electric field. Cyclotrons can generally only accelerate protons to about 25MeV. If the strength of the accelerator's magnetic field is designed to increase synchronously with the particle energy along the radial direction, the protons can be accelerated to hundreds of MeV, which is called an isochronous cyclotron. In order to further explore the structure of atomic nuclei and generate new elementary particles, it is necessary to study the principle of building higher-energy particle accelerators. In 1945, former Soviet scientist VIVeksler and American scientist E.MMcMillan independently discovered the principle of automatic phase stabilization. The discovery of the principle of automatic phase stabilization was a major revolution in the history of accelerator development, which led to the creation of a series of new accelerators that can break through the energy limit of cyclotrons: synchrocyclotrons (the frequency of the high-frequency accelerating electric field decreases with the increase of the energy of the multiplying particles, Keep the particle cyclotron frequency synchronized with the accelerating electric field), modern proton linear accelerators, synchrotrons (using ring magnets whose magnetic field strength increases with particle energy to maintain the circular trajectory of particle motion, but maintain the high frequency of the accelerating field unchanged. )Wait.

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