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An orbitron type ionisation gauge with an external electron source
An orbitron type ionisation gauge with an external electron source received 12 February 1969; accepted 7 March 1969 R K Fitch, T J Norris and W J Thatcher, Physics Department, University of Aston in Birmingham
An orbitron-type ionization gauge, in which electrons are injected into the radial electrostatic field from an external electron gun, is described. The gauge has a sensitivity of up to 20,000 torr" and does not exhibit the troublesome ion current instabilities frequently observed with the orbitron.
1. Introduction Although the Bayard-Alpert ionization gauge is now used widely in high vacuum technology, its main disadvantage is that it has a low sensitivity. Thus it is necessary to operate it with comparatively large emission currents and therefore high filament temperatures. This can produce undesirable effects such as chemical pumping, dissociation of the gas molecules and outgassing. Furthermore the low pressure limit, due to both X-rays and electron bombardment of adsorbed gas layers, is at least partly dependent on the magnitude of the emission current. Successful modifications have been made to reduce this low pressure limit in, for example, the modulator gauge (Redhead, 1967) 7 and the extractor gauge (Helmer and Haywood 1966)'. An alternative approach is to use a gauge of very much higher sensitivity and thus operate it at much smaller emission currents. One of the most sensitive thermionic ionization gauges, which does not employ a magnetic field, is the orbitron. However, a number of workers--Meyer and Herb (1965) s, Redhead (1965) ° and G a m m o n (1967)~--have reported severe ion current instabilities with small changes in filament bias potential. Such devices are unsuitable for pressure measurements because small changes in bias potential can cause large changes in gauge sensitivity. However, recently Fitch and Thatcher (1968) Z have described a small volume orbitron of reasonably high sensitivity, up to 5000 torr -x, which exhibited stable ion current characteristics. They suggested that one important factor in this respect is the manner in which the electric field is terminated at the cylinder ends. This was achieved using a glass insulating rod in the upper end of the cylinder so that the field lines were not terminated abruptly as they are, for example, using a reflector plate. More recent work in this laboratory on an orbitron of this basic design, in which the diameter of the anode wire was reduced and the position of the vacuum pumping port and ion collector terminal was changed, have produced sensitivities up to I0,000 torr-L However, with these higher sensitivities small irregularities were observed in the characteristic curve of gauge sensitivity against filament bias voltage, figure (!). These irregularities are not as serious as those referred to earlier in this section, but such a device does demand a reasonable degree of stability of the filament bias supply, in order to maintain constant gauge sensitivity. It is thought that with this increased sensitivity, the undesirable effect of the field distortion around the filament, as suggested by Walker and Pacey (1968) 8, has become more significant. However, the effect of the reflecting cylinder at the Vacuum/volume 19/number 5.
12,000 p =3 xlO-7~,0rr i== IpLA 10,OOC
T
800C
I
>
600C
~
4000
360V
2000
6
12
18
24
30
36
42"
Filoment bios voltoge - Vb
Figure I lower end of the orbitron may also be important at these higher sensitivities. With the orbitron it is an inherent difficulty that this potential disturbance, however small, must be present in order that electrons can be ejected into favourable orbits. A possible solution to this problem is to apply the principle as used by Gabor (1962) 3 for an electrostatic ion pump in which electrons were injected into the radial field from an external electron gun. However, although some workers---e.g. Gammon (1967) 1 have suggested that this method could be applied to an ionization gauge, the authors are not aware of any published work using this approach. The purpose of this paper is to report preliminary but reasonably successful results of an orbitron type ionization gauge using an external electron source.
2. Description of the gauge and experimental procedure A number of designs of electron gun were tested and the most successful, together with complete experimental tube, is shown in Figure 2. The gauge envelope is constructed from a 5 cm diameter pyrex glass tube of length 20 cm, and the positive ion collector consists of an opaque evaporated layer of gold. The 0.3 mm diameter tungsten anode was supported in the centre of the hemi-spherical ends of the envelope by 1 mm diameter tungsten rods sheathed in glass. This design was used in order to produce
Pergamon Press LtdlPrinted in Great Britain
227
R K Fitch, T d Norris and W J Thatcher: A n orbitron type ionisation gauge with an external electron s o u r c e
vo / I collector terminal
~
=---
" To vacuum pump '
Gold ion collector Gun anode
'l V~
E'egu'nOn
-I,1,1,1,11,1
Modu,ator Gouge anode
Gun orbitron circuit
Figure 3
Orbitron type ionisotion 9ouqe with external electron gun
Figure 2 favourable conditions for reflection of the electrons 2 at both ends of the cylinder. The envelope of the electron gun is made from an 18 mm diameter glass tube and is joined to the main envelope at the mid point along the cylinder. It can be seen from the figure that the electron beam was directed to a point about half way between the central anode and the cylindrical gold collector. The electron gun consists of a 0.15 mm diameter "V" shaped tungsten filament, and a nickel cylindrical modulator and gun anode, both of length 1 cm and diameter 1 cm. The modulator and gun anode have 2 mm diameter apertures and are separated by a distance of 3 ram. The tip of the tungsten filament was made to just protrude through the aperture in the modulator electrode. The complete electrode assembly was mounted onto a 4 pin glass base using tungsten leads. The voltages on the gauge anode, V, the gun anode, Vu, and the modulator, V,,, were supplied by dry batteries and a variac and 6.3V isolating transformer were used for the filament supply. The gauge anode current, i , was measured with a reflecting galvanometer, the gun anode current, ig, with a unipivot microammeter and the ion current to the collector, ip, with a dc amplifier. A diagram of the complete circuit is shown in Figure 3. It can be seen that the filament is maintained at a small positive bias voltage, lib, in order to ensure that electrons do not reach the gold ion collector which is at earth potential. Furthermore the bias lead is centre-tapped between two 22 D resistors across the transformer secondary to maintain the tip of the filament at the correct bias voltage. The modulator was always maintained at a small negative potential with respect to the filament. The gauges were evacuated and tested on a stainless steel vacuum system comprising a 5 litre/sec, sputter ion pump and a zeolite sorption pump. Calibration of the gauges was made against a Mullard Ionization Gauge, IOGI2, and the pressure was varied by admitting dry air into the system. 228
3. Results and discussion The variation of gauge sensitivity, torr -1, as a function of the gun accelerating voltage, V, is shown in Figure 4 for four values of V. All these measurements were taken at a pressure of 3 × 10 -7 torr and with io==0.5/,A, V , , : 5 V and Vb=6 V. It can be seen that the sensitivity changes smoothly with increasing V, and does not exhibit any significant ion current instabilities. However, all the characteristics show a maximum value of sensitivity for each value of Va. This optimum condition is to be expected because this implies that the electrons are being injected into the device at the most favourable average energy for the particular geometrical configuration of the electron gun relative to the gauge envelope. At V : 6 0 0 V, the maximum sensitivity is about 17,000 torr -x, but even at Vo=240 V the sensitivity is about 5000 torr-L This latter figure is comparable with those quoted for the orbitron 1."." but it can be seen that the characteristic is very stable over a large range of values of V,. With this experimental gauge the anode current was only about 10% of the emission current from the filament, the remainder being collected at the gun anode. In another design, in which the apertured disc was omitted from the gun anode this 20,000
i
Vo=6OOV 16,000
+
~. 12,000
p =3 xlO-Ttorr io=0.5/zA
/ Vo:480~
/p
I
Vo=3Sov ~ .+-
•~
8000
Vo: 240V 4000
0 Figure 4
ne~
,.~.~.-+-o-o-o. ~ . . o . . . , . ~ . . ~
20
40 60 80 Gun onode voltoge- Vq
~o== ~
I00
120
R K Fitch, T J Norris and W J Thatcher: An orbitron type Ionisation gauge with an external electron source was increased to about 35 per cent. However, this was not found to be any real advantage because it was then necessary to increase the filament temperature to obtain the same value of emission. This is on account of the weaker accelerating field between the gun anode and the filament. The effect o f varying the bias voltage was also investigated. This was performed whilst still maintaining the potential difference between V and Vb constant. However, provided Vb was greater than about 6V, the sensitivity was not critically dependent on the value of Vb. F o r Vb>6V the sensitivity decreased smoothly with increasing V and no instabilities were observed with variation in this parameter. This is not very surprising because in this device the purpose of this small filament bias potential is only to ensure that electrons do not reach the positive ion collector. This is not the case with the orbitron 5 because then the filament bias potential, and its consequent disturbance on the logarithmic electrostatic field, is necessary to cause electrons to be ejected into favourable orbits. The effect of variation of Vm was as expected and it produced no change in gauge sensitivity. It was usually maintained at IV negative with respect to the filament and its importance is in only in relation to the focusing properties of the electron gun. The full extent of the capabilities of the gauge have not yet been investigated and calibration measurements have only been made in the range 10 -s to 10 -a torr. In the range 10 -8 to 10 -4 torr the positive ion current varied linearly with pressure. However at pressures greater than 10 -4 torr the ion current did not increase linearly with pressure due to the onset of space charge limitation. At present it has not yet been possible to investigate the low pressure limit.
4. Conclusions This investigation has shown that, with a gauge of this design, sensitivities of up to 20,000 torr -~ are obtainable which are free of the serious instabilities frequently experienced with the n o r m a l orbitron gauge. This gauge design has essentially two main advantages com-
pared with the orbitron. Firstly the external gun enables the electrons to be injected into the radial field without seriously disturbing the field in the way in which the internal filament does in the orbitron. It is thought that the disturbance of the field in the region of the external gun is relatively unimportant because in this case the disturbance is at a place where the electric field is weakest. However, future designs could minimise this still further by reducing the diameter of the electron gun envelope and electrode assembly. In the second place the symmetry of this device enables the electric field to be terminated smoothly in the axial direction at both ends of the cylinder in the same manner. With the present design it has only been possible to inject about 10 per cent of the filament emission into the main gauge envelope. This is not a serious limitation but it is hoped that further work on the design of the electron gun may improve this considerably. It is perhaps rather surprising that a reasonable a m o u n t of effort appears to have been spent on the orbitron type gauge rather than on the arrangement originally proposed by G a b o r 3 for an electrostatic ion pump. The present work seems to have shown that the use of an external electron source is more appropriate. However, further work is necessary before the full potential of the device can be assessed.
5. Acknowledgements The authors would like to thank Dr C S Bull, and M r T Mulvey for their helpful discussion in this work and Mr B Cutforth and M r R Herrick for their assistance in glassblowing.
References I R B Gammon, Vtwumn, 17 (7), July 1967, 379. 2 R K Fitch and W J Thatcher, J ofPhys Ed, 1, 1968, 317. 3 D Gabor, British Patent No 887, 251, 1962. 4 j C Helmer and W H Haywood, Rev Sci hastrum, 37, 1966, 1652. 5 E A Meyer and R G Herb, J Vac Sci Technol, 2, 1965, 5. 6 p A Redhead, N.R.C. Bulletia, September, 1965, p. 44. 7 p A Redhead, J Vac Sci Technol, 4, 1967, 57. 8 R Walker and D J Pacey, Paper presented at 4th hlternational Vacuum Congress, Manchester, 1968.
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An orbitron-type ionization gauge, in which electrons are injected into the radial electrostatic field from an external electron gun, is described. The gauge has a sensitivity of up to 20,000 torr" and does not exhibit the troublesome ion current instabilities frequently observed with the orbitron.
1. Introduction Although the Bayard-Alpert ionization gauge is now used widely in high vacuum technology, its main disadvantage is that it has a low sensitivity. Thus it is necessary to operate it with comparatively large emission currents and therefore high filament temperatures. This can produce undesirable effects such as chemical pumping, dissociation of the gas molecules and outgassing. Furthermore the low pressure limit, due to both X-rays and electron bombardment of adsorbed gas layers, is at least partly dependent on the magnitude of the emission current. Successful modifications have been made to reduce this low pressure limit in, for example, the modulator gauge (Redhead, 1967) 7 and the extractor gauge (Helmer and Haywood 1966)'. An alternative approach is to use a gauge of very much higher sensitivity and thus operate it at much smaller emission currents. One of the most sensitive thermionic ionization gauges, which does not employ a magnetic field, is the orbitron. However, a number of workers--Meyer and Herb (1965) s, Redhead (1965) ° and G a m m o n (1967)~--have reported severe ion current instabilities with small changes in filament bias potential. Such devices are unsuitable for pressure measurements because small changes in bias potential can cause large changes in gauge sensitivity. However, recently Fitch and Thatcher (1968) Z have described a small volume orbitron of reasonably high sensitivity, up to 5000 torr -x, which exhibited stable ion current characteristics. They suggested that one important factor in this respect is the manner in which the electric field is terminated at the cylinder ends. This was achieved using a glass insulating rod in the upper end of the cylinder so that the field lines were not terminated abruptly as they are, for example, using a reflector plate. More recent work in this laboratory on an orbitron of this basic design, in which the diameter of the anode wire was reduced and the position of the vacuum pumping port and ion collector terminal was changed, have produced sensitivities up to I0,000 torr-L However, with these higher sensitivities small irregularities were observed in the characteristic curve of gauge sensitivity against filament bias voltage, figure (!). These irregularities are not as serious as those referred to earlier in this section, but such a device does demand a reasonable degree of stability of the filament bias supply, in order to maintain constant gauge sensitivity. It is thought that with this increased sensitivity, the undesirable effect of the field distortion around the filament, as suggested by Walker and Pacey (1968) 8, has become more significant. However, the effect of the reflecting cylinder at the Vacuum/volume 19/number 5.
12,000 p =3 xlO-7~,0rr i== IpLA 10,OOC
T
800C
I
>
600C
~
4000
360V
2000
6
12
18
24
30
36
42"
Filoment bios voltoge - Vb
Figure I lower end of the orbitron may also be important at these higher sensitivities. With the orbitron it is an inherent difficulty that this potential disturbance, however small, must be present in order that electrons can be ejected into favourable orbits. A possible solution to this problem is to apply the principle as used by Gabor (1962) 3 for an electrostatic ion pump in which electrons were injected into the radial field from an external electron gun. However, although some workers---e.g. Gammon (1967) 1 have suggested that this method could be applied to an ionization gauge, the authors are not aware of any published work using this approach. The purpose of this paper is to report preliminary but reasonably successful results of an orbitron type ionization gauge using an external electron source.
2. Description of the gauge and experimental procedure A number of designs of electron gun were tested and the most successful, together with complete experimental tube, is shown in Figure 2. The gauge envelope is constructed from a 5 cm diameter pyrex glass tube of length 20 cm, and the positive ion collector consists of an opaque evaporated layer of gold. The 0.3 mm diameter tungsten anode was supported in the centre of the hemi-spherical ends of the envelope by 1 mm diameter tungsten rods sheathed in glass. This design was used in order to produce
Pergamon Press LtdlPrinted in Great Britain
227
R K Fitch, T d Norris and W J Thatcher: A n orbitron type ionisation gauge with an external electron s o u r c e
vo / I collector terminal
~
=---
" To vacuum pump '
Gold ion collector Gun anode
'l V~
E'egu'nOn
-I,1,1,1,11,1
Modu,ator Gouge anode
Gun orbitron circuit
Figure 3
Orbitron type ionisotion 9ouqe with external electron gun
Figure 2 favourable conditions for reflection of the electrons 2 at both ends of the cylinder. The envelope of the electron gun is made from an 18 mm diameter glass tube and is joined to the main envelope at the mid point along the cylinder. It can be seen from the figure that the electron beam was directed to a point about half way between the central anode and the cylindrical gold collector. The electron gun consists of a 0.15 mm diameter "V" shaped tungsten filament, and a nickel cylindrical modulator and gun anode, both of length 1 cm and diameter 1 cm. The modulator and gun anode have 2 mm diameter apertures and are separated by a distance of 3 ram. The tip of the tungsten filament was made to just protrude through the aperture in the modulator electrode. The complete electrode assembly was mounted onto a 4 pin glass base using tungsten leads. The voltages on the gauge anode, V, the gun anode, Vu, and the modulator, V,,, were supplied by dry batteries and a variac and 6.3V isolating transformer were used for the filament supply. The gauge anode current, i , was measured with a reflecting galvanometer, the gun anode current, ig, with a unipivot microammeter and the ion current to the collector, ip, with a dc amplifier. A diagram of the complete circuit is shown in Figure 3. It can be seen that the filament is maintained at a small positive bias voltage, lib, in order to ensure that electrons do not reach the gold ion collector which is at earth potential. Furthermore the bias lead is centre-tapped between two 22 D resistors across the transformer secondary to maintain the tip of the filament at the correct bias voltage. The modulator was always maintained at a small negative potential with respect to the filament. The gauges were evacuated and tested on a stainless steel vacuum system comprising a 5 litre/sec, sputter ion pump and a zeolite sorption pump. Calibration of the gauges was made against a Mullard Ionization Gauge, IOGI2, and the pressure was varied by admitting dry air into the system. 228
3. Results and discussion The variation of gauge sensitivity, torr -1, as a function of the gun accelerating voltage, V, is shown in Figure 4 for four values of V. All these measurements were taken at a pressure of 3 × 10 -7 torr and with io==0.5/,A, V , , : 5 V and Vb=6 V. It can be seen that the sensitivity changes smoothly with increasing V, and does not exhibit any significant ion current instabilities. However, all the characteristics show a maximum value of sensitivity for each value of Va. This optimum condition is to be expected because this implies that the electrons are being injected into the device at the most favourable average energy for the particular geometrical configuration of the electron gun relative to the gauge envelope. At V : 6 0 0 V, the maximum sensitivity is about 17,000 torr -x, but even at Vo=240 V the sensitivity is about 5000 torr-L This latter figure is comparable with those quoted for the orbitron 1."." but it can be seen that the characteristic is very stable over a large range of values of V,. With this experimental gauge the anode current was only about 10% of the emission current from the filament, the remainder being collected at the gun anode. In another design, in which the apertured disc was omitted from the gun anode this 20,000
i
Vo=6OOV 16,000
+
~. 12,000
p =3 xlO-Ttorr io=0.5/zA
/ Vo:480~
/p
I
Vo=3Sov ~ .+-
•~
8000
Vo: 240V 4000
0 Figure 4
ne~
,.~.~.-+-o-o-o. ~ . . o . . . , . ~ . . ~
20
40 60 80 Gun onode voltoge- Vq
~o== ~
I00
120
R K Fitch, T J Norris and W J Thatcher: An orbitron type Ionisation gauge with an external electron source was increased to about 35 per cent. However, this was not found to be any real advantage because it was then necessary to increase the filament temperature to obtain the same value of emission. This is on account of the weaker accelerating field between the gun anode and the filament. The effect o f varying the bias voltage was also investigated. This was performed whilst still maintaining the potential difference between V and Vb constant. However, provided Vb was greater than about 6V, the sensitivity was not critically dependent on the value of Vb. F o r Vb>6V the sensitivity decreased smoothly with increasing V and no instabilities were observed with variation in this parameter. This is not very surprising because in this device the purpose of this small filament bias potential is only to ensure that electrons do not reach the positive ion collector. This is not the case with the orbitron 5 because then the filament bias potential, and its consequent disturbance on the logarithmic electrostatic field, is necessary to cause electrons to be ejected into favourable orbits. The effect of variation of Vm was as expected and it produced no change in gauge sensitivity. It was usually maintained at IV negative with respect to the filament and its importance is in only in relation to the focusing properties of the electron gun. The full extent of the capabilities of the gauge have not yet been investigated and calibration measurements have only been made in the range 10 -s to 10 -a torr. In the range 10 -8 to 10 -4 torr the positive ion current varied linearly with pressure. However at pressures greater than 10 -4 torr the ion current did not increase linearly with pressure due to the onset of space charge limitation. At present it has not yet been possible to investigate the low pressure limit.
4. Conclusions This investigation has shown that, with a gauge of this design, sensitivities of up to 20,000 torr -~ are obtainable which are free of the serious instabilities frequently experienced with the n o r m a l orbitron gauge. This gauge design has essentially two main advantages com-
pared with the orbitron. Firstly the external gun enables the electrons to be injected into the radial field without seriously disturbing the field in the way in which the internal filament does in the orbitron. It is thought that the disturbance of the field in the region of the external gun is relatively unimportant because in this case the disturbance is at a place where the electric field is weakest. However, future designs could minimise this still further by reducing the diameter of the electron gun envelope and electrode assembly. In the second place the symmetry of this device enables the electric field to be terminated smoothly in the axial direction at both ends of the cylinder in the same manner. With the present design it has only been possible to inject about 10 per cent of the filament emission into the main gauge envelope. This is not a serious limitation but it is hoped that further work on the design of the electron gun may improve this considerably. It is perhaps rather surprising that a reasonable a m o u n t of effort appears to have been spent on the orbitron type gauge rather than on the arrangement originally proposed by G a b o r 3 for an electrostatic ion pump. The present work seems to have shown that the use of an external electron source is more appropriate. However, further work is necessary before the full potential of the device can be assessed.
5. Acknowledgements The authors would like to thank Dr C S Bull, and M r T Mulvey for their helpful discussion in this work and Mr B Cutforth and M r R Herrick for their assistance in glassblowing.
References I R B Gammon, Vtwumn, 17 (7), July 1967, 379. 2 R K Fitch and W J Thatcher, J ofPhys Ed, 1, 1968, 317. 3 D Gabor, British Patent No 887, 251, 1962. 4 j C Helmer and W H Haywood, Rev Sci hastrum, 37, 1966, 1652. 5 E A Meyer and R G Herb, J Vac Sci Technol, 2, 1965, 5. 6 p A Redhead, N.R.C. Bulletia, September, 1965, p. 44. 7 p A Redhead, J Vac Sci Technol, 4, 1967, 57. 8 R Walker and D J Pacey, Paper presented at 4th hlternational Vacuum Congress, Manchester, 1968.
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