Experimental investigations of a low impedence line driven vacuum spark

Observations of vacuum spark discharge are presented. The discharge is initiated by a 35 ns FWHM Nd:YAG laser pulse. The anode is a truncated cone with a 2.5 mm perforation which allows the laser light to be focussed onto the cathode. Anode-cathode separation is 8 m. The driver is a 1.5 Ω, 120 ns coaxial line which was operated at a maximum current of of 80 kA. The line gap is shorted-out to allow a slow rising Voltage to be applied across the electrodes before Initiation of the discharge. Under these conditions a dI/dt much faster than conventional capacitive drivers used in previous vacuum spark experiments is obtained and on the time scale of the current disruptions the generator behaves as a resistive driver. The diagnostics include multiple filtered PIN diodes, XRD, pin-hole x-ray camera and holographic interferometry. The x-ray pin-hole photographs show reproducible hot spot formation if the energy in the laser pulse is above 350 mJ. At energies below this only strong anode emission is observed. If peak current is below 50 kA even though the laser energy is greater than 400 mJ no hot spots are observed.