Generation of Large‐Cross‐Section Beams in Plasma‐Cathode Systems

Abstract

Large-cross-section electron beams (LCSEBs) are attractive for applications involving large plane objects or bulk media [1]. They have found use in radiation technologies, in surface modification of structural materials, in pumping the active media of gas lasers, and in other fields. In plasma-cathode sources, the production of beams of this type is accomplished by the extraction of electrons from the surface of volumetric plasma. For the production of LCSEBs, plasma cathodes have an extra advantage over hot cathodes, in that it is much easier to form homogeneous bulk plasma than it is to form a uniformly heated, large, hot-cathode emission surface [2].In electron sources that produce large-cross-section beams, as a rule the emission surface is comparable to the beam in dimensions. Thus voltage division in the region of beam formation and acceleration is impeded, and the acceleration gap is conventionally represented by a diode system with the high-voltage electrodes having a greater surface area. Because of the reduced electric field strength of acceleration gaps with developed electrode surfaces, the upper limit of electron energies is usually not above 250-300 kV. The wide range of possible ways of forming dense, large-volume, homogeneous plasmas makes the creation of plasma-cathode sources of large-cross-section beams rather simple. Beams of this type can be square in cross-section, or radially convergent or divergent. The beam cross-sectional area is dictated by the dimensions of the required region to be exposed to the beam, and it may range from 10 to 10 5 cm 2 . Let us consider some examples of plasma sources of large-cross-section electron beams. Electron Sources with High Pulsed Energy DensityA high-voltage plasma-cathode electron source (see Fig. 4.1) [3] was developed to determine the maximum parameters of devices of this type, and was also used in experiments on surface modification of various materials by an electron beam with high pulsed beam energy. In a vacuum chamber 1, made of stainless steel, on polyethylene bushing insulator 2, is mounted a plasma electron emitter 3. Plasma is generated by a constricted arc [4] or a cascade mode of arc [5,6]. The discharge is 95Plasma Cathode Electron Sources. Efim Oks powered by an LC pulse-forming network line (PFN) whose parameters determine the cathode (discharge) current I d and the pulse duration s d . Usually I d = 10-1000 A and s d = 10 -2 -10 -5 s. Electrons are extracted from the plasma through an emission hole of diameter d e = 60 mm which is covered with a fine stainless-steel grid with mesh size 0.5´0.5 mm. A DC accelerating voltage U acc (up to 200 kV) is applied between the hollow anode 5 and the vacuum chamber. During operation of the electron source, the pressure in the hollow anode, which is determined by the flow rate of the plasma-generating gas (argon), was maintained in the range 0.01-0.1 Pa, whereas in the acceleration gap and in the beam drift space the pressure was lower by about an order of magnitude.