The physics of ´vacuum´ breakdown

Summary form only given. A self-consistent physical model which describes the onset of vacuum breakdown and the formation of a cathode spot plasma is presented. The development of a current with j > j/sub FE/ (vacuum) requires that ions exist in front of the electron emitting spot. The initial ionization must occur in the cathode-anode gap near the electron emitting spot. Ionization of desorbed neutrals provides the mechanism. As the diode voltage increases, the 100 V equipotential surface which moves towards the cathode is met by the desorbed neutrals moving away from the cathode. These two regions proceed from no overlap to a significant amount of overlap on a nanosecond time scale. This results in a delayed, sharp risetime for the onset of ionization. Ions produced in the ionization region, a few /spl mu/m from the electron emitting spot, are accelerated back. Surface heating by these /spl sim/ 100 eV ions is initially orders of magnitude more efficient than Joule heating. As more ions are produced, a positive space charge layer forms which now increases the effective E and thus strongly enhances j/sub FE/.