Onset of breakdown in a vacuum diode

Breakdown and plasma formation on electrodes is a fundamental process in pulsed power technology. The initial “explosive” plasma formation on the surface of a cathode of a vacuum diode and many other discharge 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. The current density j, the electric field E, and sheath effects influence field emission of electrons, joule heating, and plasma formation. Estimates of j for a cathode spot vary by orders of magnitude. This indicates that the details of the explosive electron emission process are not yet well understood. Unipolar arcing represents a discharge form which easily leads to explosive plasma formation. Power dissipation for an unipolar arc is considerable 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, E=O. Similar unipolar arc craters have now been observed on the cathode surface of a pulsed (25 ns) vacuum diode with an externally applied field of E=1MV /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 electric field distributions and pressure gradients which drive the unipolar arc. The unipolar arc current has not previously been included in estimates and measurements of cathode-anode discharge currents. This may explain the large discrepancy in data published on current densities in cathode spots.