RU2427940C1 - Plasma emitter of electrones - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: in plasma emitter of electrons consisting of hollow cathode with outlet aperture and igniter electrode on internal surface of cathode cavity there is installed fragment of metal possessing lower threshold current of cathode spot life, than metal of hollow cathode and facilitating sputtering film of fragment metal on whole internal surface of cathode cavity during discharge burning. Additionally, on an end of the hollow cathode with an outlet aperture there is installed a screen with a coaxial aperture either under floating potential by means of ceramic isolators, or it is made of refractory material and possessing the same potential as the hollow cathode. The outlet aperture of the hollow cathode can be made in form of a bush of refractory material.

EFFECT: simple and reliable design of plasma emitter on base of arc discharge with cathode spot operating in range of discharge currents of values from units to tens of amperes at maintaining long term resource of cathode including at operation with chemically active gases.

3 cl, 2 dwg

Description

The invention relates to techniques for producing low-temperature plasma and can be used in sources of electron and ion beams as an emitter.

Plasma emitters based on an arc discharge with a cold (not heated to thermionic temperatures) hollow cathode are widely used in devices for generating low-temperature plasma. One of the main advantages of devices based on a low-pressure arc discharge with a hollow cathode is the dusting of the material of the internal walls of the cavity evaporated in the cathode spot with its relatively small output through the cathode aperture, which significantly reduces the erosion of the cavity walls. In contrast to thermionic cathodes, cold hollow cathodes allow the generation of chemically active gas plasma without a significant reduction in resource and do not require significant energy consumption for the filament circuit. Despite the significantly greater resource of the cold hollow cathode compared with the thermionic cathode, the currently achieved resource of such systems is still insufficient for their wide industrial use. Therefore, one of the main requirements for cold arc hollow cathodes in technological installations is to ensure their high resource.

A device is known in which a plasma electron emitter based on an arc discharge with a cathode spot is used for electron emission, the cathode spot functioning on the inner surface of the cathode cavity [1]. For stable functioning of the arc discharge under conditions of reduced pressure, an external magnetic field formed by a magnetic coil is superimposed on the hollow cathode. The disadvantage of such a plasma emitter is the intense erosion of the inner surface of the side walls of the hollow cathode, caused by the radial movement of the cathode spot at the maximum of the magnetic field (groove formation). The problem is partially solved either by moving the magnetic coil, or using a system of several coils and scanning the magnetic field along the cathode. However, in both cases, the intensive removal of material from the side walls of the cathode and its deposition at the ends of the cathode necessitates the periodic replacement of the entire cathode.

The closest device for the most common features to the proposed invention and taken as a prototype is a cold hollow arc cathode with the retention of the cathode spots of the arc inside it using a self-heating refractory sleeve. Due to the high temperature of the walls of the sleeve, the metal of the working cavity of the cathode, sprayed in the cathode spots of the arc, evaporates from them and again settles on the surface of the working cavity. The dimensions of the sleeve, ensuring the optimum evaporation temperature, are found from the power balance emitted by the discharge on the walls and radiation removed from them, or experimentally [2].

The reason that impedes the achievement of the technical result indicated below when using the specified device is the presence of glowing parts, which reduces the cathode resource, especially when working with reactive gases, as well as the need to select the optimal temperature regime for the self-heating tube when changing the operating parameters of the discharge.

The problem solved by the invention is to provide a simple and reliable design of a plasma emitter based on an arc discharge, operating in the range of discharge currents from units to tens of amperes, while maintaining a long cathode resource, including when working with chemically active gases.

The problem is solved during the implementation of the invention due to the fact that in a plasma electron emitter based on an arc discharge with a cathode spot, consisting of a hollow cathode with an output aperture and an ignition electrode, according to the invention, a metal fragment having a lower threshold current is installed on the inner surface of the cathode cavity the existence of a cathode spot than the metal of which the hollow cathode is made, and which ensures the deposition and restoration of a low-pore metal film during the discharge burning process burned current on the entire inner surface of the hollow cathode.

During the combustion of the discharge, a metal film with a lower threshold current (working material) covers the entire inner surface of the cathode cavity, which ensures the complete absence of metal erosion with a high threshold current (case) at a discharge current in the range between the threshold currents for the existence of a cathode spot for these two metals .

To prevent the cathode spot from exiting the cavity to the outer surface, a screen with a coaxial aperture can be installed at the end of the hollow cathode with the output aperture, either under the floating potential using ceramic insulators or made of refractory material and having the same potential as the hollow cathode.

Also, to prevent the cathode spot from leaving the cavity to the external surface, the output aperture of the hollow cathode can be made in the form of a sleeve of refractory material.

The essence of the invention lies in the fact that the arc discharge operates in the range of discharge currents between the threshold currents of two selected cathode materials. The cathode is kept integrally cold, there is no external magnetic field and no glowing elements. In this case, the cathode spot functions mainly on the surface of the working material, contributing to its uniform distribution over the entire surface of the cathode body. As the local production of the working material to the base with a high threshold current, the cathode spot passes to an adjacent section of the working material with a lower threshold current. The hollow shape of the cathode contributes to dusting the developed area with a new film of material with a low threshold current. With a sufficient amount of working material, the cathode body practically does not undergo erosion and does not require replacement during the entire period of operation. Moreover, the physical principle of operation of such an emitter does not require the determination of the optimum temperature operating parameters of its elements.

In order to select a suitable working material for the invention, experimental studies were conducted of the minimum stable current of a stationary arc contracted discharge of various working materials of a hollow cathode.

The cathode housing can be made of copper or other material with a high threshold current of an arc discharge (for example, tantalum).

The results of the experiment are presented in the table.

The minimum stable current of a stationary arc contracted discharge for various working materials of the cathode Element Sn Ag Al Bi Mg Minimum arc current, A eighteen 12 10 3 2.5

Studies have shown that bismuth or magnesium can be used as a working material for the inner film coating of the cathode, having a minimum current of stable combustion of the arc discharge (2.5-3 A), which will provide more stable functioning of the emitter compared to other materials with the same same arc current.

The design of the plasma emitter is schematically represented in figure 1. The device comprises a water-cooled hollow cathode 1, coated on the inside with a film of working material 2, an igniting electrode 3 made in the form of a tip, an opening for inlet of working gas 4, an output aperture 5, anode 6, insulator 7, and screen 8.

Figure 2 shows a design variant of a plasma emitter in which, to prevent the cathode spot from leaving the cavity on the outer surface of the cathode, a sleeve 9 is used made of a refractory material with a high threshold current for the formation of a cathode spot, for example, tungsten or molybdenum.

The device operates as follows. The vacuum chamber on which the plasma emitter is mounted is pumped out to a residual pressure of no higher than 1 · 10 ^-4 Torr. The working gas is supplied to the cavity of the hollow cathode 1. A constant potential (Udis) is applied to the cathode 1 and anode 6. An arc discharge is initiated by applying a firing high-voltage high-current pulse between the firing electrode 3 and the hollow cathode 1 (Utrig) or by briefly increasing the gas pressure in the cathode cavity. The preliminary plasma formed as a result of the breakdown of the discharge gap in the hollow cathode provides ignition of an arc contracted discharge in a continuous form between cathode 1 and anode 6 with a burning voltage of (20-60) V.

For the initial deposition of the working film on the walls, a fragment of a working material 2 of arbitrary shape having electrical contact with it is placed in the cavity of the cathode 1. Uniform dusting of the walls of the cathode 1 with a film of working material 2 is carried out during the first few hours of operation. This is due to the fact that when the arc discharge current is less than the threshold current for the cathode material 1, the cathode spots function mainly on the surface of the working material 2 with a lower threshold current. After a few hours, the inner walls of the hollow cathode 1 are completely covered with a film of working material 2 and the cathode spots function only on the film surface. Moreover, the housing of the hollow cathode 1 is not subjected to erosion and has an unlimited service life. In the absence of a magnetic field, the cathode spot moves in an arbitrary manner along the entire inner surface of the cavity, which contributes to its uniform generation and dusting. Good adhesion of the film of the working material to the walls of the cooled hollow cathode ensures its effective cooling. Thus, the only transfer mechanism of the working material 2 within the cavity is transfer as a result of the functioning of the cathode spots. The output of the working material 2 from the cavity depends on the ratio of the area of the output aperture 5 to the area of the total inner surface of the hollow cathode 1. The use of refractory materials for the body of the hollow cathode 1 provides an extension of the working range of the arc discharge current to a region of large values.

Thus, this invention, in contrast to the previously mentioned devices, provides a simple and reliable design of a plasma emitter having a long life, including when working with chemically active gases.

Literature

1. Patent RU No. 2227962. Grigoryev S.V. Koval N.N. Shchanin P.M. 06.06.17.

2. Donin V.I., Shipilov A.F., Grigoriev V.A. Powerful cw ion lasers with extended lifetime // Quantum Electronics, 6, 1979, No. 2.

Claims (3)

1. Plasma electron emitter based on an arc discharge with a cathode spot, consisting of a hollow cathode with an output aperture and an ignition electrode, characterized in that the inner surface of the cathode cavity is covered with a metal film having a lower threshold current for the existence of a cathode spot than the metal from which a hollow cathode is made, or a metal fragment is installed on the inner surface of the cathode cavity, which ensures the deposition of this metal film during the discharge burning process. 2. The plasma electron emitter according to claim 1, characterized in that a screen with a coaxial aperture is installed at the end of the hollow cathode with the output aperture, either under the floating potential by means of ceramic insulators or made of refractory material and having the same potential as the hollow cathode. 3. The plasma electron emitter according to claim 1, characterized in that the output aperture of the hollow cathode is made in the form of a sleeve of refractory material.

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