Generation of beam plasma near a dielectric target irradiated by a pulsed large-radius electron beam in the forevacuum pressure range

Abstract

We have investigated generation of beam plasma near a dielectric (ceramic) target irradiated by a pulsed large-radius electron beam in the forevacuum pressure range 3–15 Pa. The electron beam was accelerated with voltage up to 8 kV. Under a certain threshold accelerating voltage increase in the plasma density was observed near the dielectric target as compared with beam plasma density without the target. A decrease in gas pressure leads to smaller value of this threshold voltage. The observed dependencies are apparently caused by two factors: secondary electrons being emitted from the ceramic target and charging of surface of the ceramic target by the e-beam. The secondary electrons, emitted from the ceramic target, provide “additional” ionization of the gas. The charge on the dielectric target surface creates a field that accelerates the slow secondary electrons, which provides an increase in plasma density near the ceramic target. The negative charge on the target slows down the beam electrons, and thus increases total ionization cross section of gas by electron impact. The value of negative charge on the dielectric surface increases with decreasing gas pressure.