Prebreakdown currents in ultrahigh-vacuum gaps between aluminium electrodes

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.