Cathode processes of the early phases of the pseudospark discharge

The early phases of the pseudospark discharge (hollow cathode discharge-discharge within the cathode aperture-ignition of the first cathode spots) were investigated in a circuit with low rate of current rise (dI/dt<10/sup 9/ A/s) where a prolongation of the phases is achieved. A system with 3 mm cathode aperture and 3 mm gap distance was used. After ignition, a hollow cathode discharge develops with a typical forward voltage drop (FWD) of 500 to 1000 V. This discharge is stable up to currents of 600 to 800 A. With rising current a ring shaped metal vapour cloud develops at the inner surface of the cathode aperture with a thickness of 250 /spl mu/m. When the current density within the cathode aperture exceeds 10/sup 3/ A/cm/sup 2/ the FWD decreases to 200-400 V, the intensity of metal vapour ring increases, and the light emission of the hollow cathode ceases. Fast shutter camera photographs with a telescope optics (resolution of 3 pm per pixel) of the ring shaped emission showed a homogeneous and diffuse discharge. Self sputtering of cathode material seems to be the relevant cathode mechanism. This phase is terminated at currents between 1 and 2 kA (depending on anode voltage, gas pressure, and the dimension of the cathode aperture) by the ignition of a first cathode spot of type A at the edge of the cathode aperture or by quenching of the discharge. It is only at higher current that cathode spots are observed at the cathode surface facing the main gap.