Low pressure measurement using a Bayard-Alpert gauge with magnetic field modulation

Abstract

The measurement of low pressures with a conventional BA gauge1 is limited by the X-ray effect and the electron desorption effect2,3,4. Redhead described the use of a modulator electrode to extend the low-pressure limit of the BA gauge2,5. A residual collector current, which, as he supposed, is constant and unaffected by the modulation, can be distinguished from the true ion current. From measurements made by Hobson6 and Appelt7 we know that the residual current will also be modulated. They have found methods to estimate the modulation factor of the residual current. Lange and Singleton8 modified the modulation technique of Redhead. They have shown that a slight change in the potentials applied to the modulator electrode offers advantages over the original method. This reduces pressure changes due to electron desorption and subsequent adsorption on the modulator, since the modulator is continuously bombarded by electrons. For gauges with grid end caps, operation in the Lange and Singleton mode cannot be applied. For this mode of operation the modulation factor of the ion current equals approximately unity. The magnetic field modulation which we suggest enables determination of the true ion current and the residual current of a conventional BA gauge. It is shown that for a modulated BA gauge with grid end caps the pressure measurement with magnetic field modulation is more accurate than that using Redhead's method.