Space and time resolved observations of hot spot dynamics in a vacuum spark discharge

Experimental observations of the plasma formations in a Vacuum Spark discharge are presented. A low power Nd:YAG laser pulse incident onto a Titanium cathode initiates the discharge. The evolution of the Titanium plasma electron density and temperature is followed both in the visible and the soft X-ray part of the spectrum. The former uses a novel micro holographic interferometric technique permitting a spatial resolution better than 20 µm. The probing beam is a 6 ns frequency doubled Nd:YAG laser. The latter emission is resolved using a number of different methods. The spatial information is derived from a 1 ns multi framing camera X-ray camera which projects the plasma image using four different slit wire pinhole images and six pinhole images, each aperture being filtered differently. The temporal evolution of the emission of each discharge is followed using several Silicon PIN diodes. The x-ray spectrum is deconvolved from the filter and detector response and interpreted using a collisional radiative package. The hot spots are seen to form in a submilimetre pinch stemming from the incident laser focus which has a life time of about 20 ns. The hot spots are much shorter events, reaching substantially higher densities, but involve only part of the line density of the pinch column.