RU41387U1 - EXPLOSIVE EMISSION CATHODE OF A HIGH-CURRENT ELECTRON ACCELERATOR - Google Patents

Abstract

The explosion-emission cathode of a high-current electron accelerator made of a carbon-containing material and electrically connected to a power source, characterized in that an open-porous glassy carbon material is used as the material for the cathode.

Description

The proposed device relates to high-current pulsed technology and can be used to create electron accelerators, sources of powerful x-ray and microwave pulses.

High-current accelerators are known in which various metals, such as copper, tungsten, and molybdenum, are the electron source: (1. Powerful nanosecond pulses of X-ray radiation / G.A. Mesyats, S.A. Ivanov, N.I. Komyak, E.A. .Pelix // Moscow, Energoatomizdat, 1983, p. 68; 2. Intensive electron beams / E. A. Abramyan, B. A. Alterkop, G. D. Kuleshov // Moscow, Energoatomizdat, 1984, p. 9, 186 )

Powerful microwave radiation generators are also known, in the electron accelerators of which carbon-containing materials are used as materials for performing explosive emission cathodes. (3. Theoretical and experimental studies of microwave devices with a virtual cathode / V.D. Selemir, B.V. Alekhin, V.E. Vatrunin, etc. // Plasma Physics, 1994, Volume 20, No. 7, 8 pp. 689-708; 4. Experimental study of virtual cathode oscillator in uniform magnetic field / KGKostov, NANikolov at. Al. // Appl. Phys. Lett. 60 (21), 1992, p.2548.)

The generation of high-current high-energy electron beams is based on the phenomenon of explosive electron emission. The essence of this phenomenon is that when an electric field accelerator with a strength of more than 200-300 kV / cm is applied to the anode-cathode gap, heating and explosion of microinhomogeneities on the cathode surface occurs. The plasma formed during microexplosions serves as a source of electrons in high-current accelerators. Microcraters formed after microexplosions on the cathode surface subsequently serve as emission

centers. It should be noted that current density levels realized on the basis of explosive emission can be more than 10 ⁹ A / cm ² , which is unattainable with other types of electronic emission: thermionic and field emission. The experience of many years of research and operation of accelerators has shown that metal cathodes (stainless steel, tungsten, molybdenum, etc.) are mainly used in the form of tips, i.e. when the emitting surface is either a point (or a set of them) or blades.

If it is necessary to form electron beams with a large area, cathodes based on carbon-containing materials are used, as a rule. The disadvantages of such traditional cathodes operating in high-current accelerators in a pulse-periodic mode are the instability of the emission from pulse to pulse and the uneven distribution of it over the cathode area. This is due to both the negative effect on the emission ability of the sputtering products of the anode beam and the vacuum chamber, and the fundamental reasons associated with the mechanism of beam generation during explosive emission due to the formation of new emission centers in previous pulses.

A high-current explosion-emission cathode of the electron accelerator, which is part of a powerful microwave generator, was selected as the prototype. (Theoretical and experimental studies of microwave devices with a virtual cathode / V.D.Selemir, B.V. Alekhin, V.E. Vatrunin and others // Plasma Physics, 1994, Volume 20, No. 7, 8 pp. 689- 708).

In the prototype accelerator, an explosive emission cathode in the form of a flat disk with a diameter of 20 ... 40 mm from a carbon-containing material electrically connected to a power source was used as an electron source. The surface structure of the cathode of the prototype is a combination of finely dispersed graphite particles pressed together.

The disadvantage of this design is the inability to provide the required stability of the electron current and the uniformity of their emission over the cathode area. It should be noted that emission centers in this

case are formed during cathode operation. As a result, in devices based on accelerators made according to the prototype scheme, for example, in high-power microwave generators, x-ray machines, lasers with beam excitation of the active medium, the output characteristics from pulse to pulse are unstable.

It is known that explosive emission cathodes in high-power accelerators form high-current electron beams. When they bombard the surfaces of the anode and the vacuum chamber, a high-temperature plasma is formed, the ions of which, deposited on the surface of the cathode, pollute it, thereby significantly impairing the emissivity. As a result, the amplitude of the electron beam current is unstable and the inhomogeneity of electron emission over the cathode area increases. These processes inevitably lead to a decrease in the parameters of devices that use electronic accelerators.

The task is to improve the cathode assembly of a high-current electron accelerator.

The technical result consists in increasing the stability of the amplitude and uniformity of the distribution of the electron beam current over the cathode area.

The technical result is achieved by the fact that, in contrast to the known explosive emission cathode made of a carbon-containing material and electrically connected to a power source, the proposed device uses open-porous glassy carbon material as the material for the cathode.

The structure of the material proposed for use is a collection of open pores, separated by glassy carbon partitions, facing the ends to the surface. Features when using this material as the material of the explosive emission cathode before the material of the prototype are that the surface of the open-porous glassy carbon material is sharply inhomogeneous and

contains many thin carbon partitions, facing the ends to the surface. It is natural to assume that they can play the role of emission centers when the cathode is connected to a power source and there is no need to form them in the process of previous pulses. Moreover, these centers are distributed fairly uniformly over the cathode area. The high porosity of the proposed material allows you to effectively absorb the spray products of the anode and the walls of the vacuum chamber without compromising the emission properties of the cathode during operation.

Thus, the feature of the properties of open-porous glassy carbon material suggests the possibility of using it as a cathode material from the point of view of the cathode forming a stable and uniform distribution of the electron beam current over the cathode area.

Figure 1 presents a photograph of the surface of this material, made using an electron microscope.

Figure 2 shows a diagram of a microwave generator based on a high-current electron accelerator with an explosive emission cathode made of open-porous glassy carbon material

Where:

1 - power source;

2 - an explosive emission cathode of open-porous glassy carbon material;

3 - anode in the form of a grid;

4 - parabolic drift chamber;

5 - output window.

Figure 3 presents the oscillograms of the electron currents of the accelerator (upper beam) and microwave pulses (lower beam) of a microwave generator with an explosion-emission cathode from reactor graphite.

Figure 4 presents the oscillograms of the electron currents of the accelerator (upper beam) and microwave pulses (lower beam) of a microwave generator with

explosive emission cathode of open-porous glassy carbon material.

To test the possibility of using open-porous glassy carbon material as a material for performing an explosive emission cathode, comparative experiments were conducted on a microwave generator (see Fig. 2), which includes a power source 1, electrically connected to a flat cathode 2 and an anode in the form of a grid 3 electron accelerator, a parabolic drift chamber 4 and an exit window 5 for outputting radiation into the atmosphere. As a control sample of the cathode material, finely dispersed reactor graphite widely used in accelerator technology was used. As a material for performing a cathode according to the claimed sample, for example, an open-porous glass-carbon material obtained using the technology proposed in the patent No. 2116279 dated July 27, 1998 was used. This method of producing an open-porous glass-carbon material by selecting the fractional composition of the pore former allows you to adjust the pore size in range from 10 to 100 microns.

The microwave radiation generator operates as follows. A high voltage pulse of negative polarity from the power source 1, made in the form of a GIN, is applied to the cathode 2. The anode in the form of a grid 3, the parabolic drift chamber 4 are electrically connected to each other, grounded and connected to the positive pole of the power source. As a result of explosive emission, an electron beam is formed from the surface of the cathode, which, accelerating, passes through the anode and forms a virtual cathode in the drift chamber. Electrons trapped in a potential well between the real and virtual cathode oscillate and emit an electromagnetic wave, which leaves the system through the exit window 5.

During these experiments, the total electron current from the accelerator cathode and the stability of its amplitude from pulse to pulse were recorded.

Note that the characteristics of the output pulse of microwave radiation serve as a characteristic of the cathode both in terms of the amplitude of the current and the uniformity of its distribution over the area. To control the uniformity of emission over the area, we additionally used the method of registration with the help of a pinhole camera of x-ray radiation arising from the deceleration of an electron beam in an anode foil. The experiments were carried out for both types of cathodes: from reactor graphite and from open-porous glassy carbon material with pore sizes of the order of 20 μm and porosity at the level of 0.6 m ² / g. The performed studies showed that the replacement of reactor graphite with open-porous glassy carbon material made it possible to increase the stability of the amplitude of the electron beam current and the uniformity of its emission over the cathode area.

The experimental results showed that, from the point of view of the output parameters of the microwave generator, the cathode of open-porous glassy carbon material allowed us to increase the energy of the generated microwave radiation by at least 50% and ensure its stability from pulse to pulse within ± 10% (see figure 3 and figure 4).

In addition, the use of a glass-carbon explosive emission cathode made it possible to substantially increase the range of variation of the anode-cathode gap of the accelerator without noticeable deterioration in the energy characteristics of microwave radiation, which also indicates an increase in the cathode's emission ability compared to reactor graphite.

Pictures of the deceleration of the electron beam on the target, obtained using a pinhole camera, also indicate greater stability and uniformity in the area of emission of the electron beam when using the cathode of the claimed type.

Thus, from the point of view of the formation of a cathode stable in amplitude and uniformly distributed over the current area of the electron beam, it seems appropriate to use open-porous glassy carbon material in high-current accelerators.

Claims (1)

The explosion-emission cathode of a high-current electron accelerator made of a carbon-containing material and electrically connected to a power source, characterized in that an open-porous glassy carbon material is used as the material for the cathode.