Modulation of the Desorbed Ion Current in Bayard–Alpert Gauges

Abstract

The modulation characteristics of a Bayard–Alpert gauge have been investigated in the presence of a large residual current of desorbed ions, following exposure of the gauge to oxygen. Having observed an increase in the collector current on modulation, when the gas phase ion current was relatively small, modulation factors were measured for the desorbed ion current and were found to vary in magnitude up to 0.2. A sharp peak, which increased the amplitude of the mean residual current by approximately 30%, was also found in the modulation curve between Vmg=−10 and −35 V. Computed potential distributions and ion trajectories have shown that this peak can be attributed to the focussing action of the electrostatic field on energetic ions desorbed from the modulator. Experimental verification of this hypothesis was obtained by selective cleaning of the grid and modulator using electron bombardment. The presence of such a peak would produce errors in the proposed alternative modes of modulation in which a modulator-to-grid potential of −20 V is used.