Static characteristics of the three-grid modulated Bayard–Alpert gauge in intermittent-grid-modulation mode

Abstract

A new modulation mode, intermittent-grid-modulation mode, was applied to a specially prepared three-grid Bayard–Alpert gauge which has two additional concentric grids inside the electron collector grid (G1). With this mode, the modulation factors for the noise currents are very small, so that only the signal current corresponding to pressure is singled out, and the modulation can be repeated automatically without significant difficulties. Potentials of the filament and G1 are kept constant at 400 and 500 V, respectively. The innermost grid (G3) was also kept at a constant potential of 300 V to keep a stationary emission of the x-ray photoelectrons from the ion collector. To modulate the true ion current, the potential of the intermittent grid (G2) was changed manually (on:350 V/off:50 V). In the ’’on’’ and ’’off’’ states, all the thermal electrons bombard only G1, and the pressure change induced by the change of the electron stimulated gas desorption rate was not detected. The collector currents in both states for various pressures were measured, and it was confirmed that the difference, Ion−Ioff, was proportional to the pressure down to the lower pressure limit [8×10−9Pa (6×10−11Torr)] of the vacuum system employed in this study. However, the lowest measurable pressure of this gauge is believed to be much lower than this value. The modulation factor k for the true ion current was about 0.81, and the sensitivity S was about 1.3×10−2/Pa.