High current electron beam generation in a vacuum transient hollow cathode discharge

Experimental observations are presented of prebreakdown electron beam generation in a Transient Hollow Cathode Discharge (THCD) in a vacuum. The discharge driver consists of a 400 kV maximum voltage, 25 nF Marx operated at 450 J stored energy coupled to a 120 ns, 1.5 Ω coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd-YAG laser is used to produce a plasma at the back of the cathode surface, inside the Hollow Cathode Region (HCR). It is found that a plasma density of a few 1018 cm-3 in a volume of a few mm3 is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10--200 mtorr range