Some Effects During Narrow Focusing Beam Generation by Plasma Electron Gun in Forevacuum Pressure Range

As it was already reported earlier at previous ICOPS, ability to generate electron beam under high operation pressures (up to 10 Pa and more) is one of peculiarities of plasma gun. At the same time there are some obstacles, which stand on the way of such gun and beam wide application. The most important one is secondary plasma producing on the way of beam propagation. This plasma appears, if beam current density becomes more any value (usually 10-100 mA/mm²), depended on gas pressure and electron energy. Most of all conditions for plasma appearance are fulfilled in beam crossover. Further effects are connected with beam-plasma interaction. These are: beam energy spectra broadening, beam diameter growth and beam current density decreasing. Investigations of secondary plasma show that previous effects are accompanied by sharp plasma density and its electron temperature growth. Lines with low exiting energy enrich optical spectra. All these give foundation for concluding about collective character of beam-plasma interaction. The way to suppress them is electron energy and gas pressure taking up. The other problem is special manner of accelerating gap electric breakdown. It is initiated by plasma penetration from main discharge region to accelerating gap through holes in emission electrode and realized as discharge switching from anode to accelerating electrode. Penetrating plasma surface area plays important role in this effect. Lowering of this area by axial magnetic field reduces this switching probability and gives possibility to take up as limiting beam current so working gas pressure.