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

Abstract

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.

Thermionic Emitters

“…14.7, measurements of the current due to externally-produced electrons are made at the correcting electrode in an auxilliary gauge which does not contain a thermionic emitter filament, and these measurements, applied to the current measurement from the normal gauge, can be used to make corrections to the measured ion current and calculated pressure. The details of this are dealt with in a recent paper [28], and background information will be found in earlier papers [29][30][31][32][33][34][35]. Figure 14.7 shows the side view of the gauge design and its logic.…”

Precise method of compensating radiation-induced errors in a hot-cathode-ionization gauge with correcting electrode

“…It was previously reported that nude-type hot-cathode-ionization gauges (Bayard-Alpert gauges [1]) located in the SPring-8 storage ring, had indicated abnormally low pressures or negative pressures, due to the influx of photoelectrons into the vacuum-gauge heads, [2,3] and due to radiation-induced current in the vacuum-gauge cables. [4] This phenomenon had been observed from the commissioning of the SPring-8 storage ring in 1997 to 2000.…”

“…[7,8] In simulation experiments with excess electrons and with actual synchrotron radiation, the method could displayed the resulting pressure with a pressure measurement error of less than 22 %. [3,4,9] However the method based on an approximate formula was restricted to use in a specific pressure range, related to the influx of photoelectrons, and displayed large errors for pressures below 8 10 -8 Pa. [3,9] Furthermore to solve these problems, a precise method [5] based on an exact formula, was developed without use of the shield tube and the existing vacuum-gauge controller together with the automated-pressure-compensating circuit. In an experiment with excess electrons simulating a synchrotron radiation environment in the SPring-8 storage ring, this precise method without the vacuum-gauge cable could obtain the actual pressure with a pressure-measurement error less than several percent.…”

“…[5] The phenomenon has been well known and observed in many electron storage rings since the 1970s. [6] To overcome this problem, a hot-cathode-ionization-gauge head with correcting electrode [2] was developed and was operated by an existing vacuum-gauge controller together with an automated-pressure-compensating circuit, [3] a shield tube, and a vacuum-gauge cable with long-term resistance to radiation. [7,8] In simulation experiments with excess electrons and with actual synchrotron radiation, the method could displayed the resulting pressure with a pressure measurement error of less than 22 %.…”

Pressure compensation for a radiation-induced current caused in a vacuum gauge cable in the SPring-8 storage ring

“…2,3 To reduce the pressure-measurement error due to radiation, one can cover the collector cable with a lead sheet. This pressuremeasurement error is caused by an influx of photoelectrons into the vacuum gauge head, and by radiation-induced currents in the collector cable due to radiation scattered by the photon absorbers.…”