Linear accelerator

Linear accelerators usually refer to accelerators that use high-frequency electromagnetic fields to accelerate, while the trajectory of the accelerated particles is straight. High frequency linear accelerator (high - frequencylinearaccelerator) linear accelerator, is refers to the use of high frequency electric field distribution along a linear path to accelerate charged particles. According to the type of accelerated particles, it can be divided into electron linear accelerator, proton linear accelerator, heavy ion linear accelerator and superconducting linear accelerator.

The prototype concept of the linear accelerator was first proposed by the British scientist G.Ising in 1924, in 1924, he proposed a linear accelerator design pattern in an article called "Principle of the method of generating high pressure tunnel rays". According to G.Ising's article, a linear accelerator consists of a straight vacuum tube and a series of metal drift tubes with holes. The acceleration of the particles is accomplished by the pulsed electric field between the 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. "It is too early to go into the details and possible difficulties of implementing this idea," he wrote in the article. "I hope to do an experiment soon." This proposal was difficult to implement at the time due to the level of electromagnetic technology. But the concept was so important that it had a landmark impact on the development of linear accelerators. In 1928, the concept of linear accelerator was formally proposed by German scientist RolfWideroe, who completed the world's first linear accelerator. R.ideroe described the principle of the accelerator in his article "New Principles for Generating High Voltages", and unlike the idea of GSING, the accelerator's drift tubes were alternately connected to a high-frequency power source and grounded. The length of the pipette grows as the velocity of the particles increases, ensuring that the particles reach the gap at the right time each time to be accelerated. In this accelerator, the beam is first formed into a bundle 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, and the drift tube shields the decelerating electric field, so that the whole process is an acceleration process.
 

Various modes of electromagnetic wave can be excited by the input of microwave in the cylindrical metal hollow tube (waveguide), and the electric field along the axis of one mode has a large component, which can be used to accelerate the charged particles. In order to keep the charged particles running along the axis in an accelerated state, the phase velocity of the electromagnetic wave in the waveguide is required to be reduced to synchronize with the motion of the accelerated particles, which can be achieved by placing a diaphragm or drift tube with circular holes in the waveguide at a certain interval. The mass of the electron is very small, only a few meV. When the energy of the accelerator cavity of 35MeV proton linear accelerator in Institute of High Energy Physics of Chinese Academy of Sciences is used, the electron speed is close to the speed of light. The mass of protons or ions is larger and their velocity is lower, and devices with drift tubes are often used. The Stanford electron linear accelerator tube built in 1966 is 3050 meters long, the electron energy is as high as 22 giga electron volts, the pulse electron current strength is about 80 mA, and the average current strength is 48 microamps.  The accelerator consists of three tall columns made of insulating material and an accelerator tube in between them. The accelerator relies on a vacuum pump。
 

Medical linear accelerators maintain vacuum. The appearance is streamlined, not only for aesthetics, but also to prevent accidental discharges from forming from any edges or protrusions.

In the accelerator tube there are metal rings, which are connected to the high voltage generator in such a way that the negative pressure of a series of metal rings gradually increases from the bottom to the top. The ion source for producing protons is installed at the upper end of the accelerator tube. The positively charged protons are drawn down the tube by the negatively charged metal-increasing the velocity of the protons as the negative voltage of the metal below increases. Under the floor at the bottom of the accelerator tube is a chamber containing a receiver where protons can collide with matter, and in the process, the bombardment causes the disintegration of atomic nuclei.

 

 

The beam injection and extraction is very convenient, the beam current is strong, the transmission efficiency is high, the beam quality is good, and it can be designed, manufactured and adjusted from front to back. Because the accelerator does not have the synchrotron radiation limitation of the deflecting beam, it accelerates the electron beam to very high energies and is the only candidate for the next generation of ultra-high energy colliders (see Collider). In order to make the accelerator have the appropriate length, the axial acceleration electric field strength is generally 5-25 meV/meter, which requires a large microwave power source, so the cost and operation cost per unit beam power are high. The superconducting accelerators proposed today can effectively reduce the operating cost.

Charged particles in high frequency linear accelerators are accelerated by the axial component of a high frequency (or microwave) electric field. According to the acceleration wave classification, there are two types: traveling wave and standing wave. The former uses a cylindrical waveguide as an accelerating structure, and periodically sets a disk load along its axis to make the phase velocity propagating in the waveguide less than or equal to the speed of light, so as to accelerate the particles synchronously. The mode of the acceleration field is class -TM01, which provides the largest axial electric field component in the paraxial region. The latter uses a cylindrical resonator, and also periodically sets the electrode (or drift tube) load along the axis to improve the effective acceleration electric field intensity, and its acceleration field mode is class -TM010, which also provides the largest axial electric field component in the near-axis region. There are two main parameters to measure the performance of the accelerated structure: one is the parameter related to the acceleration efficiency, especially the effective shunt impedance. It represents how high an accelerating electric field the structure can build for a given high frequency power loss. The level of shunt impedance depends on the selected frequency, the geometry and shape of the structure, and the amount of change in high-frequency phase between adjacent acceleration units (operating 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 represents the influence of structural errors and adjacent non-accelerating modes on the beam. For the standing wave acceleration structure, the main way to achieve stability is to adopt the so-called two-period structure, that is, in addition to the periodic acceleration unit formed by the load, the periodic coupling unit is also introduced, and the position and size of the coupling unit can be adjusted to improve the anti-interference of the structure.

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

Folded electron line

Particles can be accelerated by travelling or standing waves. When the traveling wave acceleration is adopted, the structure can be designed as equal impedance or equal gradient. Equal impedance type is a uniform acceleration structure, that is, the dimensions of the structure are unchanged along the axis, easy to design and manufacture, the disadvantage is that the loss of microwave power in the structure is not uniform, for a long linear accelerator, the structural temperature control along the axis is not easy. The equal 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 manufacture more complicated.

Folded proton line

The rest mass of the proton is more than 1,800 times that of the electron, and in its very long acceleration range, the speed is much less than or less than the speed of light, so the standing wave acceleration structure is used to obtain a high effective shunt impedance and acceleration efficiency. The kinetic energy of a proton ranges from 1 MeV to 1,000 MeV, and its speed ranges from 4.6% to 87.5% of the speed of light. In order to make the structure have higher acceleration efficiency in different energy regions, different structures should be used. For example :① the kinetic energy of the proton is accelerated from less than 1 megavolt to several megavolts, and the high-frequency quadrupole (RFQ) can be used. In the central part of a cylindrical cavity, four axial high frequency electrodes are arranged symmetrically in the azimuth Angle. In the paraxial region surrounded by them, a four-pole focusing electric field is generated to focus the beam radially. The radial dimensions of each electrode can be periodically modulated along the axis to obtain the axial electric field that gathers and accelerates the beam axially. It has several functions of bunching, focusing and accelerating, and is an acceleration structure that arose in the 1970s, with a frequency of 200-400 MHZ. ② The kinetic energy of the proton should be accelerated from a few MeV to about 150 meV, and the drift tube structure (also known as the Alvarez structure) can be used, which was developed by L. Alvarez first proposed and built it. In the cylindrical cavity, electrodes whose length increases with energy are periodically arranged along the axis. When the high frequency electric field is in positive half circle, the proton bundle is accelerated between the electrodes. When in the negative half cycle, the proton bundle hiding in the electrode is not affected by the negative half cycle deceleration field and drift forward, so it is also called the electrode drift tube. A quadrupole magnet placed in the drift tube can focus the beam radially, and the selected frequency is 200-400 MHZ. When the kinetic energy of the proton is to be accelerated from 150 MeV to a higher energy, the coupled cavity acceleration structure is usually used. Radial focusing of the proton beam in this energy region is already easier, and the quadrupole magnet can be moved outside the acceleration cavity to increase the frequency to 800-1,300 MHZ to improve the acceleration efficiency. This structure can also be used to accelerate electrons, typically operating at 1,300-3,000 MHZ.

Fold heavy ion lines

It is closer to the proton linear accelerator, but under the same kinetic energy, the particle motion is lower, so the operating frequency is also lower, generally around 27-150 MHZ. Early accelerators of this type used the Vidro acceleration structure. Modern accelerators of this type can be high-frequency quadrupole or Avaleze according to the energy region. The heavy ion acceleration structures developed today, such as cylindrical and planar helical structures, separated ring resonator structures, etc., are characterized by small radial size, loose tolerance requirements, and can be made into many short cavities combined into a whole accelerator, which is convenient for the use of superconducting technology, and is conducive to expanding the range of heavy ions and the continuous variable energy needs.

Folded superconducting line

With structures made of superconducting materials, the power consumption is almost negligible, so a higher accelerating electric field can be established with a smaller microwave power. Most of these accelerators are made of pure niobium material with an inner surface coated with an oxide protective layer and placed in a cryogenic vessel with liquid nitrogen and liquid helium cooled step by step to 4.2K or less. The accelerating electric field can reach several megavs/m to more than 20 megavs/m. Using superconducting cavity in high energy linear accelerator has more obvious advantages. If used in the high-energy segment of the high-current proton linear accelerator (about 150-1,000 MeV), because the power consumption can be ignored, the structure with a larger beam channel aperture can be selected, which can effectively avoid serious radioactive pollution caused by the loss of high-energy high-current beams along the way. In addition, it is also beneficial to improve the acceleration field strength, reduce the size of the equipment and operating costs. The proposed superconducting electron-Positron Linear Collider (TESLA) uses a much lower frequency (1,300 MHZ) and a larger beam aperture than other similar collider schemes (5,700-11,400 MHZ). In addition to still having a high accelerating electric field (about 25 megavolts/m), the tail field induced by the beam on the cavity wall is relatively small. It is easier to ensure the high quality of the beam (small emittance, small energy dispersion, etc.).

Two-photon medical linear accelerator is a large medical equipment used in cancer radiation therapy. It can directly irradiate the tumor in the patient by generating X-ray and electron line, so as to achieve the purpose of eliminating or reducing the tumor.

The medical linear accelerator design has a perfect multilevel safety interlock to ensure the safety of personnel and equipment.

② Full digital design, the whole machine adopts computer control, the operation software adopts graphical interface, the operation is easier. Control systems such as automatic frequency control (AFC), automatic beam control (AIC), dose monitoring and automatic evenness control (ADC) are all microprocessor controlled, and the dose is more stable.

③ Independent dual-channel ionization chamber design to ensure the accuracy of dose measurement. The deflection system adopts ski-type dispersive structure to obtain better beam distribution.

(4) 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 reaction. With high-power microwave feedback system, the maximum microwave energy is up to 6MW.

(5) The upper and lower stops of the beam limiting device can be moved independently to meet the needs of different treatment types. High center accuracy. It can be equipped with external X-knife, multi-leaf grating and other conformal treatment systems. With remote fault diagnosis function, users can be assisted with maintenance through the Internet, and maintenance is easier.