Total Pressure Measurement of Ultrahigh Vacuum with Modulated Bayard-Alpert Gauge

Abstract

Two Bayard-Alpert type ionization gauges with modulator have been used to measure a total pressure of ultrahigh vacuum system, which is made of stainless steel and is evacuated by a sputter ion pump, a titanium sublimation pump and a cryosorption pump (with liquid helium). By using a cold cathode Magnetron gauge (NRC #552) as a reference gauge, the ultimate pressure of the system is estimated to be about 1.2×10-12 Torr. With the application of the modulation methodes, an original method proposed by Redhead and a modified method by Tuzi, the two errors, namely the soft×-ray photocurrent error and the electronic desorbed ion current error, which appear independently, have been investigated. In the method by Redhead, the modulated photocurrent corresponds to a pressure of 7.2× 10-12 Torr, while in the method by Tuzi et al., the error of the modulated photocurrent can not be clearly measured, rather it is found that the range of lowest measurable pressure is extended to 6.2×10-12 Torr. The other error due to electronic desorbed ion current is investigated with an operation of the B-A gauge under CO gas pressure from 1.8×10 -11 Torr to 1.1×10-9 Torr, and it is concluded that this error can be reduced to the order of several per cent when the gauge is operated at a higher electron emission current (≥ 5 mA).