Laser Induced Breakdown and High Voltage Induced Vacuum Breakdown on Metal Surfaces

Abstract

Breakdown and plasma formation on surfaces are fundamental processes in laser target interaction experiments as well as in other areas of pulsed power technology. The initial plasma formation on the surface of a laser irradiated metal target is very non-uniform. Micron-sized plasma spots form within nanoseconds. Quite similar, the initial plasma formation on the surface of a cathode of a vacuum arc, vacuum diode, and many other discharges is highly non-uniform. Micron-sized cathode spots form within nanoseconds. The concept of explosive electron emission from a cathode spot is well established in the literature. However, the details of the breakdown process were not well understood. Unipolar arcing represents a discharge form which easily leads to explosive plasma formation. Power dissipation for an unipolar arc is considerably higher than for field emitted or space charge limited current flow. Using a laser produced plasma it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have now been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of E=lMV/2.5 cm. The experimental results show that cathodes spots are formed by unipolar arcing. The localized build-up of plasma above an electron emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of an unipolar arc provides explosive plasma formation.