The discharge threshold is investigated both experimentally and theoretically with a model that a positive DC potential is applied to a metal plate with an electron beam injected into a weakly ionized plasma. The experiments are performed systematically by controlling the beam injection energy and the plasma density. In the absence of the plasma, beam injection with an energy larger than the ionization voltage reduces the discharge threshold strongly, whereas the injection has remarkably little influence due to the plasma supply which reduces the threshold. Space charge distributions are also calculated by solving Poisson's equation at a pre-discharge stage. The threshold as functions of the beam energy, the plasma density and the gas pressure is numerically obtained. It is assumed in the calculation that an additional ionization in the plasma begins to occur just at the moment when the electric field becomes zero in the region close to the plate. This implies that the threshold is given when a V-shaped potential profile formed at a pre-discharge stage collapses into a flat profile.
Plasma production by the trapping of energetic atoms