The physics of "vacuum" breakdown

Breakdown in a vacuum diode is characterized by the "explosive" like formation of cathode spots. A source of high energy density is required to ionize cathode material within nanoseconds. Enhanced field emission of electrons begins from a growing number of small spots on the cathode surface when an increasing voltage is applied. The prebreakdown field emission current is selflimiting. Its negative space charge in the cathode-anode gap reduces the effective electric field on the cathode surface. The development of a current larger than the field emission current from a cold cathode 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 by field emitted electrons provides the mechanism. This ionization process requires considerably less current than the ionization of solid material by Joule heating. Ions produced a few pm from the electron emitting spot are accelerated back. Surface heating by these ions is orders of magnitude more efficient than Joule heating. As more ions are produced, a positive space charge layer forms which now increases the effective electric field and thus strongly enhances the field emission current. Experimental results agree with this self-consistent physical model which describes the onset of vacuum breakdown and the formation of a cathode spot plasma.