New design of modulated Bayard–Alpert gauge

Abstract

The fundamental equations of the modulated Bayard–Alpert gauge (MBAG) introduced by Redhead are briefly discussed. X-ray effects from the modulator may be weakened appreciably by using a ’’pin-to-pin’’ (PTP) collector–modulator structure; the figure-of-merit (Q), which measures the degree of extension of the lowest measurable pressure with respect to a Bayard–Alpert gauge (BAG), can be directly determined from the experimental results. A new operating mode is proposed such that the modulator potential does not switch from the grid potential to the collector potential, but varies always positively with respect to the cathode with an alternating voltage of very low frequency (typically 1 Hz) superimposed on a chosen dc potential. The collector current is then detected by a two-phase lock-in amplifier. The modulator–collector capacitance of the PTP gauge is only 1.7×10−5 F. The figure-of-merit Q calibrated is about 100 so that the lowest measurable pressure of this gauge is in the order of 10−10 Pa (10−12 Torr).