THE HOLLOW CATHODE PHASE OF PSEUDOSPARK DISCHARGES

Abstract

We report here a series of experiments and model calculation to investigate the hollow cathode processes occurring in the pseudospark. Measurements of the time dependence of the total current were made in pseudospark discharges in helium at 0.4 torr and for applied voltages from 5 to 10kV. In addition to their potential as high power switches, hollow cathode processes occurring in psedudospark discharges are also sources of intense pulses of electron beams. The electron beam current was measured as a function of time with a Faraday cup which was separated form the back of the hollow anode by a drift region. A variable strength magnetic field was applied in the drift region and information about the beam energy as a function of time was also obtained. The model we have developed of the initiation or hollow cathode phase of pseudospark discharges shows that the initial, rapid current rise is associated with the formation of a plasma and its expansion in the hollow cathode. The space charge distortion of the applied field just as the plasma beings to fill the hollow cathode is such that the electron multiplication is a maximum at this point in time, and the consequent rapid increase in the charged particle densities leads to a rapid increase in the total discharge current. The electron beam observed experimentally during the current rise is predicted by our model. Electrons created in the hollow cathode backspace are largely confined by the high field sheaths in the hollow cathode until they lose most of their total energy in collisions. These low energy electrons are trapped in the low field region on the axis behind the cathode hole through which they diffuse into the anode-cathode gap. These electrons are then accelerated in any potential remaining between the anode and the cathode form face, and they comprise the observed electron beam. The experiments results and the model prediction of the time dependence are in good qualitative agreement with experiment. Guidelines for optimization of the pseudospark properties can be deduced from these results. 1) J.P. Boeuf and L.C. Pitchford, IEEE Trans. Plasma Sci. 19, 286 (1991).