Fore-vacuum plasma electron gun of ribbon beam

Plasma electron gun for ribbon beam generation was designed on the basis of glow discharge with hollow cathode. Electrons were extracted through emission hole in the anode from plasma boundary, stabilized by metal mesh, and accelerated by the voltage applied between the anode and extractor. Electron beam was of 25 cm width, 1 cm thickness. Beam current and energy were of 0.1-1 A and 2-6 keV respectively, at gas pressure of 10 - 60 mTorr. Maximum parameters are defined mostly by the acceleration gap geometry. Current density distribution along the beam width depends on the gas pressure and total beam current. At pressures higher than 30 mTorr local current maximums appear in the electron beam. They look as streams, and their positions are determined by the anode mesh deviation from flatness, but they are always at the edges of the beam. Our experiments show that in the absence of electron emission plasma density distribution in a hollow cathode maintains maximums at edges but their amounts are not more than 5 percents. At the same time, local beam maximums are about two times more. It means there is another reason of nonuniformity. We believe this intensifying is caused by gas ionization in the acceleration gap and back-stream ion flow to discharge plasma. Recharging in plasma, these ions increase plasma density and that, in its turn, leads to stream intensifying and so on. Local plasma density growth is balanced by ion diffusion from this excite zone., Lower pressure, lower ion back flow and lower plasma non-uniformity.