RU2529372C2 - Linear electron accelerator - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to physics and charged particle beam engineering, specifically to linear accelerators. The disclosed linear electron accelerator can be sued in physics, medicine and radiation technologies for sterilising medical articles, X-ray inspection of large cargo and inspecting thick-walled metal objects. The accelerator comprises an electron injector, an accelerating cavity in the form of a biperiodic line of coupled cells, a microwave generator, a vacuum pump, power supply and a control device. In order to use the accelerator with microwave generators of different power rating without replacing the accelerating cavity, the microwave power input cell includes a piston for adjusting the resonance frequency thereof, and the dimensions of the coupling slit are controlled using removable contact plates.

EFFECT: high reliability and service life, longer continuous operation, enabling use of different microwave sources to obtain electron beams of different power.

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Description

FIELD OF THE INVENTION

The invention relates to the field of acceleration of charged particles, specifically to the technique of linear electron accelerators. It can be used to create compact microwave electron accelerators with high beam intensities needed for research in the field of physics and radiation chemistry, for radiation therapy of cancer, as well as for radiation technologies - sterilization of medical devices and medical waste, radiographic inspection of bulky goods, control manufacturing quality of thick-walled metal objects.

State of the art

Linear electron accelerators are often used to produce beams of accelerated electrons with energies of the order of units or tens of megaelectron-volts (MeV). Currently, there is a great need for accelerators of intense electron beams designed for radiation technology.

In particular, the urgent task is to create installations for radiation sterilization of medical devices and materials using accelerated electrons. Typically, in such installations, the energy of accelerated electrons is from 5 to 10 MeV, which provides sufficient depth and uniformity of transmission of boxes with products.

Recently, great interest has been shown in the creation of X-ray inspection complexes, which are designed for the rapid control of bulky goods (sea and automobile containers, trucks and cars) in order to detect explosives and smuggling. Beams of accelerated electrons with energies of 5–10 MeV are also necessary for such complexes.

An urgent task is the creation of linear electron accelerators with local biological protection made of metal blocks. The use of local protection avoids the construction of buildings with thick (up to 3 meters) concrete walls and significantly accelerates and cheapens the creation of radiation-technological installations. In particular, the use of accelerators with local radiation protection will make it possible to create installations for radiation sterilization of medical devices, which can be located in ordinary rooms in clinics and in small enterprises.

At the indicated energies of accelerated electrons, for reliable protection of personnel from radiation, it is necessary to have a thickness of protection made of steel of the order of 0.5 meters and even more. Such protection can weigh several tens of tons. Therefore, the size of the accelerator is of critical importance, which may determine the practical feasibility of creating the installation.

Typically, a linear electron accelerator comprises an electron beam injector and a microwave frequency accelerating system. The accelerator also contains a microwave generator that feeds the accelerating system, as well as devices for pumping the volume of the accelerator to high vacuum, water cooling of the resonator, power and control. As a rule, accelerators for sterilization and inspection operate in a pulsed mode.

As a microwave accelerating system, a standing wave system is currently commonly used. It is a resonator containing a biperiodic structure in the form of a chain of connected cells located on one axis in which a standing π / 2 wave is excited. In the resonator, accelerating cells alternate with communication cells, the phase shift of the microwave field between adjacent cells is π / 2, and the fields in the accelerating cells are antiphase. This biperiodic structure has minimal transverse dimensions compared to other known structures, which is extremely important for installations with local protection.

An electron injector typically uses a thermionic cathode. To reduce the length of the accelerator, the injector is located directly next to the accelerating resonator, so that between the injector and the first cavity cell there is only a dividing wall.

The development of installations with local radiation protection became possible after the creation of a compact linear accelerator, in which there are no special devices for focusing the electron beam. In such an accelerator, the bunching of an electron beam into bunches, the acceleration of bunches, and the focusing of the beam are carried out by an electromagnetic field in the cavity (see, for example, Belugin V.M., Pirozhenko V.M., Rozanov N.E., Simonov K.G. “Source penetrating radiation. ”RF Patent No. 225588 dated 02/14/2003 and Andreev N.V., Belugin V.M., Pirozhenko V.M., Rozanov N.E.“ Linear Accelerator. RF Patent No. 2392782). The accelerator described in patent 2392782, can be considered as a prototype.

The proposed accelerator makes it possible to obtain a large electron beam power and, accordingly, a high performance of a sterilization or inspection installation due to the introduction of an aperture filter into the design of the accelerator and a gradual or stepwise increase in the diameter of the aperture in drift tubes. The design of the resonator of such an accelerator and the parameters of the electron beam are calculated based on the power of the microwave generator. The resonator has a predetermined connection with the waveguide path, which is determined by the power of microwave losses in the resonator and the beam load. The magnitude of this connection is determined by the size of the slit in the input cell, and the input cell itself, like all other cavity cells, must be tuned to the operating frequency with great accuracy from 0.03% to 0.1% relative to the operating frequency of the resonator. The setup of the input cell (as well as the other cells of the resonator), including the selection of the size of the communication gap, is performed mechanically by changing (grooving) of one or another size. After manufacturing the resonator (soldering the cells into a single unit) in a hydrogen furnace in the described design, the input cell cannot be rearranged in frequency. Accordingly, there is no possibility of changing the magnitude of the coupling of the resonator with the waveguide path after the manufacture of the resonator, which limits the possibility of changing the microwave power introduced into the resonator. In the case of, for example, the use of a more powerful microwave generator in order to increase the beam power (and the development and implementation of more powerful vacuum devices is ongoing), it will be necessary to increase the coupling of the waveguide with the resonator (increasing the coupling gap). An increase in the coupling gap will decrease the frequency of the input cell; accordingly, the distribution of electromagnetic fields in the cavity cells will change relative to the optimal one and the acceleration of the electron beam will be disrupted, which will lead to an increase in the loss of the electron beam during acceleration and a decrease in the power of the electron beam. In this case, in order to maintain the optimal process of acceleration of the electron beam, it will be necessary to replace the accelerating resonator.

Disclosure of invention

Currently, there is an urgent need for compact linear electron accelerators with high electron beam power and local radiation protection. The accelerator designed for radiation technology should have high reliability and a long service life, including a long duration of non-stop operation to replace failed elements, and also when changing a microwave source to a more or less powerful source, be able to increase or decrease the power of the accelerated beam electrons.

The problem of creating an accelerator with such characteristics can be solved as follows. The linear electron accelerator comprises an electron beam injector, a microwave accelerating cavity, a microwave generator for supplying an accelerating system, a vacuum pumping device, water cooling, power supply and control. The accelerating resonator is made in the form of a biperiodic chain of connected cells located on the same axis and operating on a standing π / 2 wave.

The accelerating cavity cells have an optimal, Ω-shaped shape, which includes rounded external walls and drift tubes on the axis. An exception may be a microwave power input cell, which, for structural reasons, is advisable to make in the form of a cylinder.

The communication gap in the microwave power input cell is made with a margin, and the required size is selected using removable plates, the microwave contact of which with the cell is provided with screws and contact protrusions on the plates located along the narrow waveguide wall. A change in the resonator frequency with a change in the size of the coupling gap with the waveguide is eliminated with the help of a plunger in the form of a metal cylinder located on the cylindrical wall of the resonator. When the cylinder is inserted inside the cell, it pushes the lines of force to the axis of the cell and its action is equivalent to a decrease in the radius of the cell; an increase in the resonance frequency occurs. In order to be able to both increase and decrease the frequency of the cell, the initial setting of the cell can be carried out with a plunger inserted into the cell at half possible movement.

Brief Description of the Drawings

Figure 1 - Linear electron accelerator. Sectional view.

The implementation of the invention

The linear electron accelerator shown in figure 1, contains the following devices:

- electron injector 1;

- accelerating resonator 2;

- input cell microwave power 3;

- removable plates 4;

- tuning plunger 5;

- microwave generator 6;

- high voltage power supply and control devices 7.

The electron source (item 1) is a thermionic cathode with a heater. The cathode is mounted on an insulator and is at a high negative potential, which is supplied from power and control devices.

The accelerating resonator (pos. 2) is made in the form of a biperiodic structure of connected cells. The resonator contains a grouping section, in which the formation and preliminary acceleration of electron beam clumps following with a frequency of microwave oscillations are performed, and an accelerating section, in which the main acceleration of the electron beam is performed.

The main cells of the grouping and accelerating sections have an optimal shape, in particular, drift tubes are made on the axis to concentrate the electric field, and the outer walls are rounded to reduce the path of microwave currents. This form provides a high microwave accelerating field strength, a high acceleration rate and a minimum microwave power loss. The communication cells are made in the form of flat cylindrical volumes. The cells have an electromagnetic coupling with each other using communication windows in the walls.

The microwave power input cell (pos. 3) has a cylindrical shape and a rectangular slot is made in the cylindrical wall. This gap is partially overlapped by two removable contact plates, with the help of the selection of the sizes of which the required value of the connection with the waveguide path is obtained.

A trimming plunger (pos. 4) is installed on the cylindrical wall of the input cell, by changing the immersion depth of which the cell is tuned to the operating frequency when changing the dimensions of the communication gap with the waveguide using contact removable plates.

The microwave generator (pos. 6) is connected to the microwave power input cell (pos. 3) using a waveguide path.

The linear accelerator operates as follows. A high voltage of negative polarity is applied to the injector’s cathode from the power supply system, which creates an electric field in the injector’s cathode-anode gap, which draws an electron beam from the cathode, accelerates it, focuses it and feeds it to the accelerating resonator. A microwave generator, using a waveguide feeder, excites an electromagnetic field in the cavity cells. The cells are tuned due to the appropriate choice of their geometrical dimensions so that a standing π / 2 wave is established in the resonator, in which the electric fields in neighboring accelerating cells are in antiphase, and the field amplitudes correspond to the calculated law. The longitudinal component of the electric field in the first cell accelerates those electrons of the injected beam that hit the positive half-wave of microwave oscillations. The longitudinal sizes of the cells and the field amplitudes in them are selected in such a way that in subsequent cells of the resonator, the electrons captured in the acceleration mode gradually increase their energy and speed. At the same time, the radial components of the electric field and the azimuthal components of the magnetic field in the grouping cells focus a certain part of the electron beam and defocus the other part. As a result of this, some electrons move near the axis, and some gradually move away from it, i.e. the beam gradually expands. Due to the fact that the aperture of the acceleration channel also expands from the beginning to the end of the accelerator, the losses of the accelerated electron beam are small. Electrons that fall into the negative, decelerating half-wave of microwave oscillations in the first cell stop, scatter in different directions.

All cells of the accelerating resonator must be tuned with high accuracy to a single resonant frequency, which corresponds to the frequency of the microwave supply generator. Typically, the required cell frequency tuning accuracy is from 0.03% to 0.1%.

When replacing the supply microwave generator with a generator of another power due to removable contact plates mounted on the input cell, the connection between the generator and the resonator is adjusted, and the frequency of the input cell is adjusted by the plunger.

The implementation of this invention allows for the replacement of the microwave power source of a linear electron accelerator with a greater or lesser output power and obtaining the optimal acceleration mode without replacing the accelerating resonator.

Claims (1)

A linear electron accelerator containing an electron beam injector with a thermionic emission cathode, an accelerating cavity made in the form of a biperiodic chain of coupled cells, a microwave generator for supplying an accelerating cavity, a vacuum pumping device, power supply and control, characterized in that a tuning plunger is installed in the microwave power input cell , and the communication gap is partially blocked by two removable contact plates.