Ionization Gauge for Transient Gas Pressure MeasurementsE. A. Valsamakis Citation: Review of Scientific Instruments 37, 1318 (1966); doi: 10.1063/1.1719969 View online: http://dx.doi.org/10.1063/1.1719969 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/37/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Miniature Penning ionization gauge for pulsed gas measurements Rev. Sci. Instrum. 55, 1663 (1984); 10.1063/1.1137637 A modified ionization gauge for measurement of relatively high pressures Rev. Sci. Instrum. 45, 19 (1974); 10.1063/1.1686439 Measuring Hydrocarbon Gas Pressure with an Ionization Gauge J. Vac. Sci. Technol. 10, 212 (1973); 10.1116/1.1317943 Surface Tension Gauges for the Measurement of Low Transient Pressures Rev. Sci. Instrum. 33, 1473 (1962); 10.1063/1.1717816 Ionization Gauge for Transient Gas Pressures Rev. Sci. Instrum. 32, 717 (1961); 10.1063/1.1717478

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 37, NUMBER 10 OCTOBER 1966

Ionization Gauge for Transient Gas Pressure Measurements

E. A. V ALSAMAKIS

Research Department, Grumman Aircraft Engineering Corporation, Bethpage, New York 1l7H

(Received 17 June 1966)

A gauge has been developed that is particularly suited for measuring transient gas pressures (e.g., gas bursts expanding in vacuum). A commercial subminiature pentode has been used in a suitable circuit for this purpose. Calibration curves for helium, nitrogen, neon, argon, krypton, and xenon at room temperature are given in the 1 to 400 IS Hg range. The features of this gauge are its small size, short rise time, and simple construction and installation. The serious disadvantage is its short lifetime, about 3 weeks of continuous use, mainly due to filament failure.

INTRODUCTION

A N ionization gauge system capable of measuring tran-sient gas pressures in the range of 1 to 400 It Hg is

described. The gauge is to be used with gas bursts expand­ing in vacuum. Such an instrument is quite useful for studying the gas profile in pulsed plasma accelerators utilizing gas propellants. l

The operation of the ionization gauge depends on the ionizing collisions of electrons with neutral gas molecules. In general, the ion current ic in an ionization gauge is related to the electron emission current ia by2

iclia=Kpn,

where p is the gas pressure, 11 a constant, and K the gauge sensitivity. In general, the gauge sensitivity is a function of the gauge geometry, the electrode potentials, and gas pressure. The calibration of the gauge is simplified if the gauge response is linear. This, however, requires that the sensitivity and emission current be independent of pressure.

Some high pressure ionization gauge design consider­ations have been given by Schulz and Phelps.s Their de­ductions led to the development of a miniature triode gauge, the Westinghouse WX-414S. Commercial triodes3

and pentodes or tetrodes4 have also been used as ioni­zation gauges. These tubes, however, are larger than the commerical subminiature tube used in this investigation. The small size of the subminiature tubes results in two ad­vantages for gas pressure measurements. First, they are suitable for gas profile measurements because the pressure is averaged over a smaller volume. Second, with proper circuitry, to be described, their linear response extends to higher pressures.2

I R. Small, A. Lind, G. Connell, and E. A. Valsamakis "Third Hypervelocity Techniques Symp. Proc.," Denver, Coloddo (Co­sponsored by Denver Research Institute, University of Denver and Arnold Engineering Developu:ent Center ARO, Inc.) (March 1%4); and T. W. Karras, B. GoroWltz, and P. Gloersen AIAA Preprints 65-341 (1965) and 66-241 (1966). '

2 E. A. Valsamakis, Grumman Research Department Memorandum RM-268 (February 1965).

: G. J. Schul~ and A. V. Phelps, Rev. Sci. Instr.28, 1051 (1957). J. Marshall m .Proc. Fourth Lockheed Symp. M agnetohydrod,)namics,

Plasma Acceieratwn, S. W. Kash, Ed. (Stanford University Press Stanford, California, 1960), p. 60. '

1318

PREPARATION AND HANDLING OF PENTODE TUBE FOR USE AS GAUGE

The tube used as an ionization gauge is the CK-S702, a beam power pentode having a high second grid power rating. Before the CK-S70Z is inserted in a vacuum system and used as an ionization gauge, its glass envelope is re­moved. The tube cathode has an oxide coating that can suffer hydration when exposed to the atmosphere, unless the filament temperature is kept at 120°C." In our work the hydration of the oxide was not entirely eliminated because the filament temperature was raised to 80°C in atmosphere, with a 10 V filament voltage. (Higher volt­age destroys the filaments in atmosphere.) The filaments were kept at 10 V in the early stages of pumpdown, with the voltage gradually reduced to Z V under vacuum.

Before setting the gauge for the appropriate filament emission current, the vacuum system should be flushed several times with the gas to be used in the experiments. The gauge can be degassed before each measurement by increasing its emission current. The different electrodes are degassed by ion or electron bombardment as well as bv radiation from the cathode. •

The accelerating grid potential must be removed when the gauge is not being used to measure pressures, but the filament is kept at working temperatures. This step is essential to avoid gas cleanup effects.6 In our experience, the lifetime of a tube has been, on the average, about 3 weeks. The oxide coating deteriorates as the tube gets older, and the filament is kept at a considerably higher voltage than rated, sometimes as high as 18 V, in order to keep the rated emission current.

IONIZATION GAUGE CIRCUIT PARAMETER CHOICE

The circuit used with the ionization gauge is shown in Fig. 1. The electrons emitted by the cathode are acceler­ated to the second grid, and perform collisions with the neutral gas molecules in the vicinity of the second grid.

• J. W. Kelly, J. Sci. Instr. 39, 473 (1962). 6 S. Dushman in Scientific Foundations oj Vacuum Technique, J. M.

Lafferty, Ed. (John Wiley & Sons, Inc., New York 1962) 2nd ed pp. 301-353, 649..{i68. ' , .,

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IONIZATION GAUGE 1319

The plate and suppressor grid are both at a slightly nega­tive potential in order to collect the ions, whereas the second grid collects the secondary electrons. The first grid acts as a control element for the emission current.

When the tube is used as a triode the emission current was found to vary with pressure.2 If the sensitivity of the gauge is independent of pressure, then the gauge can be made to behave linearly by removing the pressure depend­ence of the emission current. This is accomplished by the circuit of Fig. 1. Both R02 and Rk act as negative current feedback elements; the resistance values selected provide enough feedback to keep the emission current fairly con­stant and eliminate a steady state drift.

The value of Rp in the plate circuit must be low to pre­vent the plate voltage from changing and affecting the ion collection, which depends on the second grid to sup­pressor grid potential distribution. Rp has been chosen so that at the low pressure limit of the gauge, about 1 JJ. Hg, an adequate signal can be developed across Rp for measure­ment with an oscilloscope. For most gases, the ionization efficiency by electron impact shows a maximum for elec­trons of about 100 eV.7 Therefore, E02 is set above 100 V; in our case 165 V proved effective.

The plate power supply voltage Ep was chosen from the plate characteristics, a plot of the plate current I p versus Ep with pressure as parameter (Fig. 2). At about -5 V I p reaches a maximum, which becomes quite pronounced at high pressures. Some plausible explanations for this maximum have been offered.2 The plate characteristics suggest that the gauge operate with the plate at -30 V. This eliminates the abrupt change of I p with pressure, thereby extending the linear range of the gauge.

The emission current I g2 was chosen from the current

Plate

Suppressor

Second Grid --- - ----_______ --,

First Grid

Cathode

~ = 100 n

Rg2 = 27 k Eg2 =165V' R =3.9k

Ig2 =3ma P

FIG. 1. Ionization gauge circuit.

Rfl = lOOn

Rf2 = 200n

Cf=0.02~F

Tube: CK-5702

7 A. Von Engel, "Ionization in Gases by Electrons in Electric Fields" in Handbuch der Physik, Vd. XXI, Gas Discharges I, S. Flugge, Ed. (Springer-Verlag, Berlin, 1956), p. 508.

400

200

100

---o~---o __ -<> __ --o 21301' Bb

_<>--0----<>---0.-__ <>-_-<>-___ 10'oHO

),......--<>--0---0------<>---_>__ __ >__ __ 52.6,101 Hg

t::====t::::====t:::::=='L:==::::;c::=:::::s~=~ 16. 62.&1 Hg U U • a •

FIG. 2. Plate characteristics for nitrogen; 102=3 rnA; E02=165 V.

characteristic (Fig. 3) which is the plot of the plate cur­rent I p versus 102 with pressure as a parameter. At an optimum emission current of about 2.5 rnA, this character­istic shows a plate current peak, being quite pronounced at high pressures. Some plausible explanations for the existence of the peak in the current characteristics have also been given.2 Operation of the gauge at the optimum emission current results in a departure of the gauge response from linearity. Because the circuit drifts at low emission currents, with a resultant unsteady operation, it is advisable to operate at an emission current greater than the optimum current; we chose an emission current of 3 rnA.

GAUGE CALmRATION

The ionization gauge was developed for measuring the pressures of expanding gases, and therefore, it is neces­sary for it to have a fast time response. The circuit be­havior was tested under dynamic conditions using an elec­tromagnetically driven valve to control the inlet of gas

200

150

213 ~Hg 100

~104~Hg 50

O~--~-----L ____ ~ __ ~ o 1 2 3

FIG. 3. Current characteristics for nitrogen; Ep= -30 V; E g2 = 165 V.

4

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1320 E. A. VALSAMAKIS

F~G. 4. G.auge r~sponse to nitrogen gas puffs; gauge 125 cm from gas mlet or;fice. Time scale-2 msec/div; E p=-20 V; 102=3 rnA; Eo, = 170 v; upper traces--collector current 1 p measured across R p =3.9 kll, 0.2 V/div; and lower traces-emission or cathode current measured across R k =100 Il, 0.05 V/div. (a) Pressure-8 J1. Hg; (b) pressure--lc8 J1. Hg.

through an oririce. Some typical pictures of the gauge re­sponse are f,riven in Fig. -1. The ion current is measured by monitoring the voltage across the plate resistor Rp. A simultaneous display of the emission and plate currents is essential, hecause the cathode temperature is affected ,vhen the gauge is exposed to micron pressure gases, and the electron emission characteristics change. The emission current is measured by monitoring the voltage across the cathode resistor Rio. The results indicate that the emission current does not change appreciably at lower pressures. At high pressures, about 400 j.1 Hg, the emission current may change as much as 20%.

100'~------------~----------~~~----------~

10~------------~------------~------------~

1000 p ~Hg

FIG. 5. Comparison of static and dynamic characteristics of guage ior nitrogen; Ep= -20 V; 102=3 mAo

100 ~----------+-----------~--~---~~~~~~r-~

1.0

0.1 0.1 1.0 10 100

P ~Hg

FIG. 6. Gauge calibration curves; E02 = 165 V; Ep= -30 V; 102=3 rnA.

1000

The risetime of the ionization gauge is about 20 j.1sec. This value has been obtained by noting the time it takes for the gauge to rise from 10 to 90% of the first peak of the voltage across Rp. The gauge might actually have a better risetime, because the gas puff might not necessarily have a steep front.

The dynamic characteristic of the gauge (shown in Fig. 5) is obtained by plotting the value of the first maximum of the plate current, acquired from traces similar to those of Fig. 4, versus the static pressure recorded on an Alpha­tron gauge. A comparison of the dynamic plot with that obtained under static conditions is also included in Fig. 5. There is no significant difference between the static and dynamic characteristics in the pressure range of interest. The closeness of the static and dynamic characteristics of the gauge might be due to the gas being at equilibrium by the time it reaches the gauge, and thus having no appreci­able pressure fronts. As the distance to the orifice is re­duced to less than 50 em, the pressure fronts become more pronounced.

The gauge calibration was performed statically for vari­ous gases (Fig. 6). The close agreement between the static and dynamic characteristics (Fig. 5) leads us to believe that the static calibration curves of Fig. 6 can be used in the dynamic mode without appreciable error.

ACKNOWLEDGMENTS

The author would like to thank Drs. W. B. Ericson and A. L. Loeffler for their useful criticism. Dr. R. L. Small obtained the rise time measurement of the gauge. J. H. Baer calibrated the gauge for neon, xenon, and krypton.

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