MODULATION OF RESIDUAL CURRENT IN A MODULATED BAYARD-ALPERT GAUGE

The modulation of the residual current in a Bayard-Alpert ion gauge (B AG) was studied in conditions where a positive ion current resulting f rom the electron bombardement of a gas layer adsorbed on the grid (the ESD-current) was a major component of the residual current. Measureme nts were carried out with BAG with an internal modulator and a closed grid in a UHV system. The collector current vs emission current curves were measured with the modulator at both the grid and collector poten tials separately at a raised pressure of electronically desorbable gas es. An analysis, according to Lavson's theory was used to obtain the E SD-current as a function of the emission current. The modulation facto r of the ESD-current computed from these results was found to be as hi gh as 80% of the modulation factor of the gas-phase ion current. Since the modulation factor of the ESD-current is so close to that of the g as-phase ion current the modulated BAG cannot measure pressure reliabl y when the electronegative gases such as O-2 or CO are present in a va cuum system.