What Is a Linear Accelerator?

Linear accelerators generally refer to accelerators that use high-frequency electromagnetic fields to accelerate, and at the same time, the trajectories of the accelerated particles are linear. High-frequency linear accelerator (abbreviated as linear accelerator) refers to a device that accelerates charged particles with a high-frequency electric field distributed along a linear orbit.

According to the types of accelerated particles, they can be divided into electron linear accelerators, proton linear accelerators, heavy ion linear accelerators, and superconducting linear accelerators. ^[1]

Chinese name: High-frequency linear accelerator

Foreign name: high-frequency linear accelerator
Short name: Linear Accelerator

Linear accelerator history

The prototype of the linear accelerator was first proposed by British scientist G. Ising in 1924. In 1924, he proposed a design of a linear accelerator in an article entitled "Principle of the Method for Generating High-Voltage Polar Tunnel Rays". According to G. Ising's article, a linear accelerator consists of a straight vacuum tube and a series of perforated metal drift tubes. The acceleration of particles is accomplished by a pulsed electric field between adjacent drift tubes, and the synchronization of the electric field and the particles is achieved by the time delay of the transmission line length between the voltage source and the corresponding drift tube. At the same time, he wrote in the article: "It is too early to discuss the details of the realization of this idea and the difficulties that may be encountered, and I hope to do an experiment soon." This proposal was limited by the level of electromagnetic technology at the time It is indeed difficult to achieve. But this concept is quite important and has had a landmark impact on the development of linear accelerators. By 1928, the concept of linear accelerator was officially proposed by German scientist Rolf Widederoe, who completed the world's first linear accelerator. R. Wideroe described the principle of this accelerator in the article "New Principles of High Voltage Generation". Unlike G. Ising's concept, the drift tube of the accelerator is alternately connected to high-frequency power and ground. The length of the pipette becomes longer as the particle speed increases, ensuring that the particles can be accelerated at the correct time each time they reach the gap. In this accelerator, the beam is first formed into clusters and then accelerated with high efficiency. During the acceleration time, the beam is in the acceleration gap to feel the acceleration electric field. When the electric field is reversed, the beam is in the drift tube. At this time, the drift tube shields the deceleration electric field, so that the entire process is an acceleration process. ^[2]

In 1928 E. Widlow proposed the principle of acceleration. Early use of an alternating electric field with a relatively low frequency to accelerate charged particles.

Proton linear accelerator Use radio frequency microwaves to accelerate charged particles. Inputting microwave into a cylindrical metal tube (waveguide) can excite various modes of electromagnetic waves. One mode has a larger electric field along the axis direction and can be used to accelerate charged particles. In order to keep the charged particles running along the axis always in an accelerated state, the phase velocity of electromagnetic waves in the waveguide needs to be reduced to synchronize with the movement of the accelerated particles. This can be achieved by arranging diaphragms or drift tubes with circular holes in the waveguide at certain intervals. achieve. The mass of the electron is very small, only a few megaelectron volts.

When the energy of the acceleration cavity of the 35MeV proton linear accelerator of the Institute of High Energy Physics of the Chinese Academy of Sciences, the speed of electrons is close to the speed of light. A diaphragm device with a circular hole is suitable for accelerating electrons. Use a device with a drift tube. The American Stanford Linear Linear Accelerator tube, built in 1966, is 3,050 meters long, has an electron energy of up to 22 giga-electron volts, a pulsed electron current intensity of about 80 milliamps, and an average current intensity of 48 microamps.

Linear accelerator principle

The accelerator is composed of three high pillars made of insulating material and an accelerator tube between them. Accelerator by vacuum pump

Medical linear accelerator Keep the vacuum. The streamlined appearance is not only for aesthetics, but also to prevent accidental discharge from any corners or protrusions.

There are metal rings in the accelerator tube, and the way they are connected to the high voltage generator can make the negative pressure of a series of metal rings gradually increase from the bottom to the top. A proton-producing ion source is installed at the upper end of the accelerator tube. The positively charged protons are shot down because they are attracted by the negatively charged metal ring-as the negative voltage of the metal ring below increases, the speed of the protons also increases. Under the floor at the ground end of the accelerator tube, there is a small chamber equipped with a receiver, where protons can collide with matter, and in the process, bombardment can cause the nucleus to degenerate.

Linear accelerator main features

The injection and extraction of the beam is very convenient, the beam is strong, the transmission efficiency is high, and the beam quality is good. It can be designed, manufactured and debugged from front to back. Because the accelerator does not have the synchrotron radiation limitation of the deflected beam, it can accelerate the electron beam to very high energy, and is the only candidate for the next generation of ultra-high energy colliders (see Colliders). In order for the accelerator to have a proper length, the acceleration electric field strength on the axis is generally 5-25 megavolts / meter, and a large microwave power source is required, so the unit beam power requires a high cost and operating cost. Superconducting accelerators proposed today can effectively reduce operating costs. ^[1]

Linear accelerator traveling wave and standing wave acceleration

Charged particles are accelerated in the high-frequency linear accelerator using the axial component of the high-frequency (or microwave) electric field. According to the adopted acceleration wave classification, there are two types of traveling wave and standing wave. The former uses a cylindrical waveguide as an acceleration structure, and a disc load is periodically set along its axis in the waveguide, so that the phase velocity propagating in the waveguide is less than or equal to the speed of light to accelerate the particles synchronously. The mode of the acceleration field is similar to TM ₀₁ It provides the largest axial electric field component in the paraxial region. The latter uses a cylindrical resonant cavity, and the electrode (or drift tube) load is periodically set along the axis to improve the effective acceleration electric field strength. The mode of the acceleration field is similar to TM ₀₁₀ , which also provides the largest Axial electric field component. There are two main parameters for measuring the performance of the acceleration structure: one is the parameters related to the acceleration efficiency, especially the effective shunt impedance. It indicates how high an accelerating electric field the structure can establish given a high frequency power loss. The level of the shunt impedance depends on the selected frequency, the geometric size and shape of the structure, and the amount of high-frequency phase changes between adjacent acceleration units (working mode). Generally, the higher the frequency, the smaller the structure size, and the higher the shunt impedance and acceleration efficiency. The second is the stability of the accelerated structure, which characterizes the effect of beam errors due to structural errors and adjacent non-accelerated modes. For the standing wave acceleration structure, the main way to achieve stability is to adopt the so-called double-period structure, that is, in addition to the periodic acceleration unit formed by the load, a periodic coupling unit is also introduced to adjust the position and size of the coupling unit. Improve the interference resistance of the structure. ^[1]

Linear accelerator classification

According to the type of accelerated particles, they can be divided into electron, proton and heavy ion linear accelerators.

Linear accelerator electronic linear accelerator

Traveling or standing waves can be used to accelerate particles. When traveling wave acceleration is used, the structure can be designed to be of constant impedance or constant gradient. The isoimpedance type is a uniform acceleration structure, that is, the dimensions of the structure are unchanged along the axis, which is convenient for design and manufacturing. The disadvantage is that the microwave power is not uniformly lost in the structure. For longer linear accelerators, the Structure temperature control is not easy. The constant-gradient acceleration structure avoids this disadvantage, at the cost of slow changes in the size of the structure along the axis, which makes the design and manufacturing more complicated.

Linear accelerator proton linear accelerator

The static mass of protons is more than 1,800 times that of electrons. In its long acceleration range, the speed is far less than or less than the speed of light. Therefore, a standing wave acceleration structure is used to obtain higher effective shunt impedance and acceleration efficiency. The kinetic energy of protons ranges from 1 megaelectron volts to 1,000 megaelectron volts, and its velocity ranges from 4.6% to 87.5% at the speed of light. In order to make the structure have higher acceleration efficiency in different energy regions, different structures need to be adopted. For example: The kinetic energy of a proton is accelerated from less than 1 megavolt to several megavolts. A radio frequency quadrupole acceleration structure (RFQ) can be used. At the center of a cylindrical cavity, four axial high-frequency electrodes are symmetrically arranged at the azimuth angle. In the paraxial area surrounded by them, a quadrupole focusing electric field is generated to focus the beam in a radial direction; the axis can be adjusted periodically along the axis. The radial size of each electrode is varied to obtain the axial electric field that is clustered and accelerated in the axial direction. It has several functions of focusing, focusing, and accelerating. It is an acceleration structure that emerged in the 1970s. The selected frequency is 200-400 MHz. Proton kinetic energy should be accelerated from several meg volts to about 150 meg volts. A drift tube structure (also known as Alvarez structure) can be adopted. It was first proposed and constructed by L. Alvarez in the late 1940s. of. Within the cylindrical cavity, electrodes whose length gradually increases with energy are provided along the axis. When the high-frequency electric field is in the positive half cycle, the proton cluster is accelerated between the electrodes; when in the negative half cycle, the proton cluster hides in the electrode and drifts forward without being affected by the negative half-cycle deceleration field, so it is also called an electrode. Drift tube. A quadrupole magnet is placed in the drift tube to focus the beam in the radial direction, and the selected frequency is 200-400 MHz. When the proton kinetic energy is to be accelerated from 150 megaelectron volts to higher energy, a coupling cavity acceleration structure is usually used. Radial focusing of the proton beam in this energy region is relatively easy. The quadrupole magnet can be moved outside the acceleration cavity to increase the frequency to 800-1,300 MHz to improve acceleration efficiency. This structure can also be used to accelerate electrons, usually operating at 1,300-3,000 MHz.

Linear accelerator heavy ion linear accelerator

It is closer to a proton linear accelerator, but at the same kinetic energy, the particle moves at a lower speed, so the working frequency is lower, generally around 27-150 MHz. Early accelerators of this type used the Vedro acceleration structure. Modern accelerators of this type can use high-frequency quadrupole or Avales type. Today's developed heavy ion acceleration structures, such as cylindrical and planar spiral structures, separation ring resonator structures, etc., are characterized by smaller radial dimensions, looser tolerance requirements, and can be made into many short cavities to form a complete accelerator. , Not only facilitate the use of superconducting technology, but also conducive to expanding the range of heavy ions and continuously variable energy requirements.

Linear accelerator superconducting linear accelerator

The structure made of superconducting material has almost negligible power consumption, so a higher acceleration electric field can be established with less microwave power. Most of these acceleration chambers are made of pure niobium material coated with an oxidation protection layer on the inner surface. They are placed in a low-temperature container with liquid nitrogen and liquid helium stepwise cooling, which can be cooled to 4.2K or lower. The acceleration electric field can reach several megavolts / meter to more than 20 megavolts / meter. The advantages of using a superconducting cavity for a high-energy linear accelerator are even more significant. For high-energy proton linear accelerators (approximately 150-1,000 mega-electron volts), because the power consumption can be ignored, a structure with a larger beam channel diameter can be selected, which can effectively avoid the serious loss of high-energy high-current beams along the way. Radioactive pollution. In addition, it is also conducive to increasing the acceleration field strength and reducing equipment scale and operating costs. The proposed superconducting positron-positron linear collider (TESLA) uses a much lower frequency (1,300 MHz) and a larger beam aperture than other similar collider solutions (5,700-11,400 MHz). In addition to a high acceleration electric field (about 25 megavolts / meter), the wake field induced by the beam on the cavity wall is relatively small, which makes it easier to ensure the high quality of the beam (small emission, small energy dissipation, etc.).
Linear accelerators are the most widely used type of accelerator of all types (see particle accelerators). ^[1]

Linear Accelerator Medical Application

Linear Accelerator Product Use

Two-photon medical linear accelerator is a large-scale medical device used for cancer radiotherapy. It generates X-rays and electron rays to directly irradiate tumors in patients, thereby achieving the purpose of eliminating or reducing tumors.

Linear Accelerator Product Features

There are many energy bins and a wide energy range.

Medical linear accelerator Designed with perfect multi-level safety interlock to ensure the safety of personnel and equipment.

Fully digital design, the whole machine is controlled by computer, and the operation software adopts graphic interface, which makes the operation easier. Control systems such as automatic frequency control (AFC), automatic beam current control (AIC), dose monitoring, and automatic evenness control (ADC) are all controlled by microprocessors for more stable doses.

Independent dual-channel ionization chamber design ensures the accuracy of dose measurement. The deflection system adopts ski-type anti-dispersion structure to obtain better beam distribution.

The accelerating tube adopts the traveling wave feedback system, which has the characteristics of wide energy range, high energy stability, good beam energy spectrum, and fast transient response. With high-power microwave feedback system, the maximum microwave energy is up to 6MW.

The upper and lower diaphragms of the beam limiting device can be independently moved to meet the needs of different types of treatment. Center accuracy is high. Can be equipped with external X-knife, multi-leaf grating and other conformal treatment systems. With remote fault diagnosis function, it can assist users in maintenance through the Internet, making maintenance easier.