Explosive Plasma Formation On Electrodes

Breakdown and "explosive" plasma formation on electrodes are basic processes in pulsed power discharges. Despite their fundamental importance, the details of cathode spot formation have not been well understood. Breakdown in a vacuum diode is initiated by field emitted electrons. Besides joule heating of the emitting spot, the electrons also ionize desorbed gas layers. These ions fall back to the cathode, heating, the surface of the electron emitting spot. Ion surface heating is initially orders of magnitude larger than joule heating. Surface heating and the build-up of positive space charge rapidly lead to further enhanced field emission and, finally, thermionic electron emission. The localized build-up of plasma above the electron emitting spot naturally leads to pressure and electric field distributions which ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions provide the "explosive" plasma formation of a cathode spot. Experimental results will be presented concerning the plasma formation on the cathode of a vacuum diode.