ONSET OF BREAKDOWN AND FORMATION OF CATHODE SPOTS

When an increasing diode voltage is applied, enhanced field,,emission of electrons begins from a growing number of small spots or whiskers o n the cathode surface. This stimulates desorption of weakly bound adso rbates from the surface of a whisker. The cross section for ionization of the neutrals has a maximum for almost-equal-to 100 eV electrons. A s the diode voltage increases, the 100 V equiptotential surface which moves towards the cathode is met by the desorbed neutrals moving away from the cathode. This results in the sharp risetime for the onset of ionization of desorbed neutrals by field emitted electrons. Positive i ons produced in the ionization region, a few mum from the electron emi tting spot are accelerated back to the electron emitting spot. This bo mbardment with almost-equal-to 100 eV ions leads to surface heating of the spot. Since the ion energy is deposited only within a few atomic layers at a time instead of an entire whisker volume, and since the ne utral contaminants are only loosely bound to the surface, the onset of breakdown by this mechanism requires much less current than the Joule heating mechanism. The localized buildup of plasma above the electron emitting spot then naturally leads to pressure and electric field dis tributions which ignite unipolar arcs. The high current density of the unipolar arc and the associated surface heating by ions provide the ' 'explosive'' formation of a cathode spot plasma.