Hot-cathode-ionization-gauge system with a self-compensating circuit for errors caused by an external-electron source (2004)

A hot-cathode-ionization-gauge system, consisting of a gauge head with a correcting electrode, an automated-pressure-compensating circuit, and a shield tube, succeeded in overcoming two kinds of erroneous pressure indications with hot-cathode-ionization gauges. Several tens of hot-cathode-ionization gauges in the SPring-8 storage ring have indicated abnormally low pressures (of the order from 10−8 to 10−9 Pa) at stored-electron-beam conditions due to an influx of photoelectrons. Some of these gauges, located near photon absorbers, have indicated negative pressures (from −2×10−9 to −2×10−7 Pa). To investigate these pressure-measurement errors, simulated experiments to reproduce the phenomena were carried out using an external-electron source which was located near a hot-cathode-ionization-gauge head. The kinetic energy of incident electrons to the gauge head from the external-electron source was varied from 10 to 90 eV. The maximum total-electron-beam current from the external-electron source at the position of the gauge head was about 0.12 mA (90 eV), which was 3% of the normal emission current from the gauge filament. In the pressure range of 10−7 Pa, the pressure-measurement errors which occur in the ring were reproduced. During the experiment, the variation of the indicated emission current was less than 1% of the normal emission current. With no emission from the gauge filament, negative currents were detected at the grid and the collector of the gauge from the external-electron source at the same pressures as in the simulated experiment. It was found that detected negative current at the grid of the gauge was 50%–60% of the total-electron-beam current from the external-electron source and it was confirmed that the detected negative current at the collector depends on the kinetic energy of the incident electrons. From these results and calculations, it was also found that the variation of the emission of the gauge filament did not cause the negative-pressure indications at all. Furthermore, it was also confirmed that the hot-cathode-ionization gauge indicated abnormally low pressures or negative pressures when the net current detected at the collector in operation of the gauge was extremely small or negative, respectively, due to the influx of many electrons from the external environment. These experiments were done to simulate operation conditions at the SPring-8 storage ring. In the simulated experiments for abnormally low-pressure indications, pressure measurements using a hot-cathode-ionization-gauge head with a correcting electrode and an automated-pressure-compensating circuit were carried out in the pressure range from 10−6 to 10−8 Pa. It was found that the compensated pressure indicated the actual pressure within an error range of ±15% for incident electrons with 20 eV, although the indicated pressure of the ionization-gauge controller was in a different order than that of the actual pressure and the error current was less than the order of 10−10 A. In the simulated experiment for negative-pressure indications, it was found that the shield tube could reduce incident electrons from the external-electron source by a factor of about 1/10.