Prebreakdown currents in ultrahigh-vacuum gaps between aluminium electrodes

Abstract

Measurements of prebreakdown currents have been made for highly polished plane parallel aluminium electrodes in an ultrahigh vacuum (10−9 torr), as a function of electrode separation, in the range 0.076–2.8 mm. The prebreakdown currents are found to be consistent with the Fowler-Nordheim field-emission theory. The field-in tensification factor is calculated at each gap separation. For gap separations larger than 0.89 mm, a large increase in the prebreakdown current, at a particular applied voltage (ignition), is observed. The ratio of the current increase, depending on electrode separation, is observed to be in the range 10–160. The ignition voltage is determined as a function of gap separation and found to increase with increasing gap separation. It is suggested that the ignition can only occur at a given gap separation, when the prerequisite values of a prebreakdown current, combined with an applied voltage, are simultaneously present. At short gap separations (≪0.76 mm), the ignition phenomenon is absent. It is shown that this is because the condition for the required current value cannot be satisfied at the required ignition voltage.