Theoretical and experimental study of pseudospark electron beam generation

Abstract

Pseudospark hollow cathode discharges (HCD) are sources of intense electron beams. Reported in this paper are theoretical and experimental studies of the HCD processes and the related electron beam production. The purpose of the work is to develop a predictive model to guide the development of this high brightness electron beam. According to the model, the initial, rapid current rise is associated with the formation of a plasma and its expansion in the hollow cathode region (HCR). The space charge distortion of the applied field just as the plasma begins to fill the HCR is such that electron multiplication is a maximum at this point in time, and there is a consequent rapid increase in the charged particle densities. The electron beam observed experimentally during the current rise is predicted by the model. Electrons created in the HCR are largely confined by the high field sheaths until they lose most of their total energy in collisions. These low energy electrons are trapped in the low field region on axis behind the cathode hole through which they diffuse into the cathode-anode gap, and then are accelerated in the remaining potential within the gap. These electrons comprise the observed electron beam. The model indicates that the beam is a direct consequence of the HCD and is therefore produced by a plasma cathode. The difficulty in modeling an actual electron emitting metal surface can therefore be overcome. Experimental results of a hydrogen HCD electron beam are also presented. The pulse-length is usually 10's of nsecs, a peak beam current of 170 A and an efficiency of 21% was measured at -20 kV applied voltage. The experimental results and model predictions are in good qualitative agreement, and demonstrate the potential for developing a first principles predictive model for electron beam current, emittance and brightness. >