Modulated Bayard-Alpert Gauge Research Articles

Comparison of ionization vacuum gauges close to their low pressure limit

Parts of CERN’s accelerator complex and experiments, especially in the antimatter field, require a vacuum in the 10−12mbar range or better. Thus gauges are needed to reliably measure XHV during experimental operation and in order to study the vacuum science needed for those experiments. We therefore built a setup to reach 1⋅10−13mbar in order to simultaneously compare different hot cathode ionization gauges with the ability to measure high UHV and XHV close to their lower pressure limit: Barion extended, Extractor IE514, a modulated Bayard–Alpert gauge and two Improved Helmer gauges. All gauges but the Extractor behave similarly with respect to small pressure variations around the limit pressure, while the Extractor seems to overestimate high UHV hydrogen pressure. We show how gauge operation determines our ultimate achievable pressure due to outgassing, which was comparable for all gauges and in the order of Q∼10−10mbarls−1. Further we show the disturbances caused in the static system due to gauge pumping (visible only as electronic pumping), and report some of the possible difficulties and origins of noise when measuring pressures in the XHV range, including the thermoelectric effect.

3D-simulation of ionisation gauges and comparison with measurements

3D-simulations using the Vector Fields OPERA 3D software have been carried out on the CERN-type modulated Bayard-Alpert gauge. The program allows to simulate the ion creation inside the gauge and takes into account space charge effects. Parameters such as sensitivity, ion and electron path lengths inside and outside the ionisation grid, location of ion creation, collection efficiency, and potential distribution were studied as a function of emission current. This investigation resulted in a deeper understanding about the behaviour of the gauge, in particular about the effect of space charge. The achieved results were compared with experimental measurements; the results are satisfactory and encourage further studies.

Modulation of residual current in a modulated Bayard-Alpert gauge

The modulation of the residual current in a Bayard-Alpert ion gauge (BAG) was studied in conditions where a positive ion current resulting from the electron bombardement of a gas layer adsorbed on the grid (the ESD-current) was a major component of the residual current. Measurements 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 potentials separately at a raised pressure of electronically desorbable gases. An analysis, according to Lavson's theory, was used to obtain the ESD-current as a function of the emission current. The modulation factor of the ESD-current computed from these results was found to be as high 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 gas-phase ion current the modulated BAG cannot measure pressure reliably when the electronegative gases such as O 2 or CO are present in a vacuum system.

Ultrahigh vacuum measurements in China

Research and development on UHV gauges were started in China in 1953. Since then incessant efforts have been devoted to improvements of Bayard–Alpert gauges, suppressor gauges, extractor gauges, deflector gauges, and modulated Bayard–Alpert gauges. Performances of various gauges, from a special gauge with all electrodes degassed by direct resistance heating, to a shielding gauge with no photocurrent effect are briefly reviewed in the present paper. Finally, a method for UHV gauge calibration is suggested and a new quadrupole structure for partial pressure measurement is described.

Residual currents in several commercial ultrahigh vacuum Bayard–Alpert gauges

Residual currents have been determined for several commercial UHV Bayard–Alpert gauges, including two modulated gauges, by comparison against an extractor gauge. Residual currents in the modulated gauges were also determined by the modulation method (Redhead mode I) and these results are compared with those of the first method. Residual current modulation factors were estimated by combining the results of these two methods. A significant residual current modulation factor (1.33) was found for one of the modulated Bayard–Alpert gauges. One notable aspect of this work is the finding that residual current values can differ by more than a factor of 10 among nominally identical gauges. Another interesting and somewhat surprising feature of these measurements, which were performed in H2 at pressures in the range 4×10−9 to 4×10−5 Pa, is that no hysteresis was detected in the gauges’ response to large, rapid decreases in the H2 pressure.

The residual current of the modulated Bayard-Alpert gauge

Studies have been made of a Bayard-Alpert gauge with a modulator. The modulation of the current due to cathode evaporation has been measured. The modulation coefficient of the total residual current has been calculated by using results of this measurement and its value has been found in very good agreement with previously published data. Modulation of this current due to cathode evaporation has been proposed as the process contributing to the residual current modulation of this gauge.

Static characteristics of the three-grid modulated Bayard–Alpert gauge in intermittent-grid-modulation mode

A new modulation mode, intermittent-grid-modulation mode, was applied to a specially prepared three-grid Bayard–Alpert gauge which has two additional concentric grids inside the electron collector grid (G1). With this mode, the modulation factors for the noise currents are very small, so that only the signal current corresponding to pressure is singled out, and the modulation can be repeated automatically without significant difficulties. Potentials of the filament and G1 are kept constant at 400 and 500 V, respectively. The innermost grid (G3) was also kept at a constant potential of 300 V to keep a stationary emission of the x-ray photoelectrons from the ion collector. To modulate the true ion current, the potential of the intermittent grid (G2) was changed manually (on:350 V/off:50 V). In the ’’on’’ and ’’off’’ states, all the thermal electrons bombard only G1, and the pressure change induced by the change of the electron stimulated gas desorption rate was not detected. The collector currents in both states for various pressures were measured, and it was confirmed that the difference, Ion−Ioff, was proportional to the pressure down to the lower pressure limit [8×10−9Pa (6×10−11Torr)] of the vacuum system employed in this study. However, the lowest measurable pressure of this gauge is believed to be much lower than this value. The modulation factor k for the true ion current was about 0.81, and the sensitivity S was about 1.3×10−2/Pa.

New design of modulated Bayard–Alpert gauge

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).

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

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).

The tungsten evaporation limit of hot-cathode ionization gauges

Comparative measurements of pressures in the low 10−12-Torr range with hot-filament total and partial pressure gauges have shown discrepancies which can be partly explained by the influence of tungsten vapor from the hot filament on the total pressure gauge readings (tungsten evaporation limit). The method proposed by Alpert and Buritz, with a slight modification, was used for estimates of the tungsten evaporation limit; i.e., a second filament served as an auxiliary source of evaporated tungsten atoms. Three gauges were tested, one gauge with a buried collector and two modulated Bayard-Alpert gauges. The experiments showed that the tungsten evaporation limits of all three gauges fell in the range (1–2)×10−12 Torr (nitrogen equivalent pressure) with an electron emission of 10 mA. Provided the variation of the gauge sensitivity with the emission is known, it is recommended to operate at reduced emission (e.g., 4 mA), at which level the tungsten limit is substantially lower. Low-work-function emitters, as a means to obtain a low-temperature electron source, were not covered by the present study, which aimed at determining some performance limitations at very low pressures of a large number of existing gauges equipped with tungsten filaments.

Spatial Initial Distribution of Ions in an MBAG and the Modulation Factor

The experimental sequence of the values of the modulation factor in a modulated Bayard-Alpert gauge for He, Ne, At, and Kr is explained on ground of the differences in shape of the corresponding ionization cross-section curves. A short discussion concerning the dependence of the modulation factor on electron current and filament-grid spacing is also given.

The modulation characteristics of a modulated Bayard-Alpert ionization gauge

When a modulated Bayard-Alpert gauge is used in the Redhead 1 mode three phenomena occur which may introduce errors into a pressure measurement: (i) the modulation factor may vary with gas and emission ; (ii) pressure changes may occur in the system ; (iii) the residual current may be modulated. Experimental results are presented on the magnitude of effects (i) and (ii). A new method of using a modulated Bayard-Alpert gauge is suggested which reduces the magnitude of the previously listed effects while maintaining the advantage of a high modulation factor.

Measurement of Extreme High Vacuum by Negative Potential Modulation Method with a Modulated Bayard-Alpert Gauge

Measurement of Extreme High Vacuum by Negative Potential Modulation Method with a Modulated Bayard-Alpert Gauge

A comparative study of modulated Bayard-Alpert gauges and an extractor gauge

A comparative study of modulated Bayard-Alpert gauges and an extractor gauge

Behavior of hydrogen in ion sources with hot filaments at very low pressures

Experiments designed to establish the approximate magnitudes of the physical processes occurring in ion sources with hot filaments at pressures of hydrogen near 10−11 Torr are described. The objective has been to provide the user of such ion sources with a dynamic description of their behavior as measuring instruments for very low pressures of hydrogen. The measurements were carried out in ultrahigh vacuum apparatus of conventional design near background conditions. It was found that behavior was dominated by a surface phase of hydrogen on the walls and on the electron collector. This surface phase is in quasi-equilibrium with the gas phase and contains orders of magnitude more hydrogen than the gas phase. This large reservoir of hydrogen is very sensitive to small thermal changes. Electron bombardment of the electron collector of the ion source generates a current identified as protons, which is modulated in the modulated Bayard–Alpert gauge as if it were a true ion current from the gas phase.

674. A comparative study of modulated Bayard-Alpert gauges and an extractor gauge: J S Cleaver and W H Zakrewski, Vacuum, 18 (2), Feb 1968, 73–78

674. A comparative study of modulated Bayard-Alpert gauges and an extractor gauge: J S Cleaver and W H Zakrewski, Vacuum, 18 (2), Feb 1968, 73–78

The measurement of hydrogen pressure with a modulated Bayard-Alpert Gauge at very low pressures: J P Hobson and J W Earnshaw, Radio & Electrical Engineering Division, National Research Council, Ottawa, Canada

Abstract : Experiments have been performed with a modulated Bayard-Alpert gauge at a pressure of hydrogen less than 10 to the minus 10 torr. A component of ion current was found which behaved, under modulation, as if it originated in the gas phase, but which was localized to the gauge under study. This component was identified as protons, on the basis of parallel experiments with a mass spectrometer in the same ultra-high vacuum system. This proton current introduces an error into pressure measurement made with the modulated Bayard-Alpert gauge. The error can be reduced by operating the gauge with a relatively high electron current (8 ma). (Author)

Total pressure measurements below 10 -10 torr with non-magnetic ionization gauges

The modulation method has been applied to (i) the Bayard-Alpert gauge and (ii) the suppressor gauge to reduce their lower pressure limits and measurements made with these two gauges to pressures as low as 10-14 torr. It is shown that the modulated Bayard-Alpert gauge can measure pressure to 3 × 10-11 torr (10% error) without correction for the residual-current modulation effect, and measurements can be extended to 3 × 10-13 torr if corrections for the residual-current modulation are made. The residual limit of the suppressor gauge has been measured (approximately 2 × 10-13 torr). With modulation applied to the suppressor gauge the lowest measureable pressure, set by amplifier noise, is about 10-13 torr for a 10: 1 signal-to-noise ratio. Electronic desorption of chemisorbed material from the grid of a gauge and/or a molecular flux in the gauge are shown to cause a limitation to the lowest measurable pressure; these limits may be higher than the x-ray limit under certain conditions.

Measurements with a modulated Bayard-Alpert gauge in alumino-silicate glass at pressures below 10−12 torr: (USA)J P Hobson, J Vac Sci & Tech, 1 (1), Sept/Oct 1964, 1–6

Measurements with a modulated Bayard-Alpert gauge in alumino-silicate glass at pressures below 10−12 torr: (USA)J P Hobson, J Vac Sci & Tech, 1 (1), Sept/Oct 1964, 1–6

Measurements with a Modulated Bayard–Alpert Gauge in Aluminosilicate Glass at Pressures below 10−12 Torr

The background conditions achieved in a simple ultrahigh-vacuum system with an envelope of aluminosilicate glass (Corning 1720) have been studied. Pressures were measured by a dynamic method and also with a modulated Bayard–Alpert gauge. Good agreement between the two methods was obtained using a modified analysis for the modulated gauge. A total pressure of 8 × 10−13 Torr (equivalent nitrogen) was measured with the whole system at room temperature, and one of 7 × 10−15 Torr (equivalent nitrogen) was measured with a portion of the system immersed in liquid helium. A discussion is given of variations in the residual modulation factor of the gauge in the presence of extremely low gas pressures.