Hollow cathode effects in charge development processes in transient hollow cathode discharges

A detailed experimental study of space charge formation and ionization growth in transient hollow cathode discharges (THCD) is presented. The experiment was performed with an applied step voltage up to 30 kV, with rise time less than 50 ns. The discharge was operated in different gases, at pressures in the range 50-750 mTorr, with cathode apertures ranging from 1 to 5 mm diameter and 5 to 20 mm long, with 10 cm electrode separation. Spatial charge formation, both in the hollow cathode region (HCR) and inter electrode space, has been studied with a capacitive probe array. Properties of high energy electron beams have been measured with a beam-target scintillator-photomultiplier arrangement. Detailed correlations of the electron beam evolution with the charge probe signals inside and outside the HCR clearly demonstrate the role of the electron beam in the initial formation and late evolution of a virtual anode and, in turn, the field enhanced ionization when the anode potential is brought close to the HCR. These results clearly identify the different regimes in which the Hollow Cathode plays a significant role in ionization growth in the inter electrode space and in the processes which eventually lead to electric breakdown