Investigation of getter-ion pump operation in systems containing oil and mercury vapour B J Ward and J C B i l l , Ferranti Limited, Wythenshawe, Manchester

Details are given of some attempts to start a getter-ion pump which was heavily contaminated with oil vapour. Baking the pump while it was maintained under vacuum and the use of a liquid nitrogen cold trap are two methods of removing oil contamination and restoring pump operation. The effects of mercury vapour on an 8 litre/sec getter-ion pump are investigated. The getter-ion pump failed to operate with mercury vapour present at room temperature, however, in mercury vapour atmospheres less than 1 × 10 -s torr the getter-ion pump worked satisfactorily. Continuous operation of the pump was possible with vapour pressures below 1 × 10 -7 torr without any apparent deterioration in pump performance.

1. Oil vapour contamination 1.1 Introduction. In most applications of getter-ion pumps rough pumping is achieved by means of a molecular sieve absorption pump. However, there may be circumstances where it is considered desirable to use a mechanical oil pump as the fore pump, for instance on production lines where a fast process cycling time is necessary. Getter-ion pumps operating in such a system can possibly have their performances impaired, for it is known that hydrocarbon contamination has a detri- mental effect on them, particularly with regard to starting.

In this investigation a 50 litres/sec getter-ion pump (Ferranti FJDS0) was deliberately contaminated with an excessive amount of rotary pump oil vapour, and various methods used for attempting to restart the pump are reported. Such con- tamination would probably be the worst encountered in a practical getter-io n pump system containing hydrocarbon sources.

1.2 System description. The system was assembled as shown in Figure 1. The getter-ion pump was connected to a 2 in. diffusion pump body, (from which the internal jets had been removed), via a diffusion pump baffle valve. A small quantity of rotary pump oil (30 ml of Edwards No 8A) was introduced into

GETTER-ION PUMP FJ D 50

E BAFPLE-~I IF- PTRANI GAUGE HEAD

VALVE I ~ - i "

OIL DIFFUSION 1 PUMP BODY

2 STAGE ROTARY PUMP

Figure 1. System for studying oil vapour contamination

the diffusion pump, and a two stage rotary pump was connected to the outlet of the diffusion pump through a diaphragm valve. As the experiment was more concerned with the starting performance of the getter-ion pump, rather than the attainment of ultra-high vacuum, Viton "O" rings were used throughout.

1.3. Method of contamination. Although back streaming of oil vapour is likely to lead to early deterioration in the getter- ion pump performance when compared with the expected normal life of up to 50,000 hours, this deterioration may have no marked effect for several hundred hours. It was in order to accelerate this contamination that the rotary pump oil was placed in the diffusion pump body; this oil could then be heated by means of the diffusion pump heater so as to give a high oil vapour pressure in the getter-ion pump.

After allowing the oil to cool to room temperature, a pressure of 2 × 10-2 torr was recorded by the Pirani gauge. Any attempt to restart the getter-ion pump after such excess contamination was unsuccessful: operation under these conditions caused an immediate rise of pressure to 5 × 10 -2 tort and continued running caused the getter-ion pump to overheat.

1.4. Removal of contamination 1.4.1. By baking. Having contaminated the system to such an

extent, the first method adopted to remove the oil vapour was to bake the getter-ion pump while under vacuum. A heating jacket was placed round the pump and the temperature main- tained at 250°C for 24 hours while the system was evacuated by the rotary pump.

On closing the baffle valve and switching on the getter-ion pump the pressure increased to 5 × 10 -z torr. However, by cycling the pump, (ie switching off, opening the baffle valve to reduce the pressure, closing the baffle valve again and reapply- ing the voltage), satisfactory operation could be restored. After allowing the system to cool to room temperature a pressure of 1.5 × 10-6 torr was recorded. Similar results were obtained on two subsequent bake-outs following contamination of the getter-ion pump. There are indications to suggest that it may be easier to start the pump under these conditions whilst it is still hot, thus preventing traces of oil vapour from recontaminating the system.

A further method adopted to attempt to recondition the getter-ion pump was to bake in air. However, after a 48 hour bake at 250°C it was still not possible to start the pump.

Vacuum/volume 16/number 12. Pergamon Press Ltd/Printed in Great Britain 659

B d Ward and d C Bill: Investigation of getter-ion pump operation

Immediately following this the pump was baked for a further 24 hours under vacuum and successfully restarted at the end of this period.

1.4.2. By cooling. The getter-ion pump was again contamin- ated by heating the oil in the diffusion pump body for about 2 hours and a check made to ensure the pump would not start. The diffusion pump body was then surrounded by liquid nitrogen and the system evacuated with the rotary pump. After a period of 2 hours under these conditions the getter-ion pump would be restarted after a brief period of cycling and attained a pressure of 5 × 10 -6 torr within 30 minutes. This same method proved successful on two further exposures of the getter-ion pump to the oil vapour.

1.5. Conclusions. As a result of this brief examination into the use of a getter-ion pump in atmospheres containing oil vapour, it appears that two methods of improving the starting perfornmnce are practicable. The use of a liquid nitrogen cold trap had the advantage of rapidly restoring satisfactory opera- tion, although this method may not always be applicable in a practical system. Baking the pump under vacuum was also successful and this is likely to be of more general use in such systems. I f higher temperatures were attained shorter baking times would probably be sufficient.

Hall I (1959) found that with a getter-ion pump contaminated to a lesser degree with hydrocarbons, air baking applied at 400°C restored pump operation; but this procedure proved un- satisfactory after repeated hydrocarbon contamination. Thus, the reason for the ineffectiveness of the air bake at 250°C on this grossly contaminated system is more apparent.

2. M e r c u r y v a p o u r c o n t a m i n a t i o n 2.1. Introduction. A possible application for the use of a getter- ion pump has been suggested as a "keeper" pump in a mercury arc rectifier system, and with this in view the effects of mercury vapour on an 8 litrc/sec getter-ion pump were investigated. Results obtained from a more practical system in which the getter-ion pump is separated from a mercury reservoir by a vapour trap are also discussed.

2.2. Apparatus. A simple system as shown in Figure 2 was

8 LITRE/SEC PUMP

in systems containing oil and mercury vapour

assembled in which the 8 litre/sec getter-ion pump (Ferranti, FJD8) was connected by 1 in. diameter stainless steel tubing to a glass side-ann and a glass volume. The side-arm contained a small evacuated glass phial filled with mercury.

The system was first evacuated by a molecular sieve absorp- tion pump, then by the getter-ion pump, and baked at 180°C for about 80 hours to remove water vapour.

2.3. Experimental procedure and results. After breaking the glass phial by means of a small metal weight, the vapour pressure of mercury in the system could be controlled by the degree of cooling applied to the side-arm. In the following experiments the coolant consisted of a mixture of acetone and solid carbon dioxide, the resulting temperature being about - 80oc.

The variation of pump pressure with time over the range 3 × 10 -s to 3 × 10 -6 torr after removing the coolant from the side-arm is shown in Figure 3. The levelling of pressure occur-

I HEATING I COOLING t

I t

0 I0 2 0 1 410 1 I I 50 60 7~3 810 9tO I 0 0 I i 0 3 0 TIME- MINUTES.

.I. I 0 -5

uo -6

,-e

0

1.10- 7 o. 9

4

-a

- 8 1.10

// I BAKEABLE VALVE

Figure 3, Variation of pump pressure on heating and cooling the side-arm

SIDE ARM CONTAINING MERCURY CAPSULE

I" BORE STAINLESS STEEL TUBE

I LITRE F L A S K - ~ ~

Figure 2. System for studying mercury vapour contamination

ring after 10 minutes from the start of the experiment co- incided with the observed melting point of the mercury in the side-arm. The variation of pump pressure after reapplying the coolant is also shown.

If, on switching off the pump, mercury at room temperature (vapour pressure 1.3 × 10 -3 to r t ) was allowed to contaminate the system for a few hours, the pump could be re-operated satisfactorily after reapplying the coolant to the side-arm. Figure 4.1. shows the variation of pump pressure after this short duration of contamination. Figure 4.2. shows the varia- tion of pump pressure after repeating this procedure; a slower pump-down was obtained--this presumably being caused by a larger percentage of mercury being present in the system.

However, if the system was contaminated with mercury vapour at room temperature for much longer periods, the pump failed to work satisfactorily after reapplying the coolant.

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B d Ward and d C Bil l: Investigation of getter-ion pump operation in systems containing oil and mercury vapour

I I I I I I I I O IO 20 30 40 SO 60 70 SO

TIME- MINUTES.

• 1 0 - 6

i.lo -7

4

3

I I I I . I O - 8 90 IOO rio 120

Figure 4. Pump-down curves for 8 litre/sec ion pump after mercury contamination of system

Pressures lower than 1 x 10-4 torr were unattainable and con- tinued running caused the getter-ion pump to get hot. In this pressure region the power dissipated in the pttmp is increased and the temperature rises; this in turn causes the pressure to rise; more power is dissipated and consequently the pump overheats. The getter-ion pump was found to operate success- fully after baking the system at 100°C for 6 hours whilst the coolant was applied to the side arm to condense the mercury.

The variation of pump pressure after allowing the vapour pressure of mercury to increase from about 1 x 10-s tor t to that at room temperature is shown in Figure 5.1. Rapid increase in pump pressure was followed by a slower rise to ,~10 -4 tort. At this stage the pump began to overheat and

- 4 1.10

I O

2 .

I I I I S IO IS 20

TIME-MINUTES •

i.l~ s

d ,¢

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. 3

- 2

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Figure 5.1. Variation of pump pressure after removal of coolant Figure 5,2, Pump-down curve after re-applying coolant

after 30 minutes had to be switched off to prevent damage. Figure 5.2. shows the variation of pump pressure after the pump had cooled down and cooling had been restored to the side-arm.

A second system shown in Figure 6 was assembled to investi-

S I D E A R M C O N T A I N I N G

MERCURY CAPSULE

. / 8 LITRE/SEC PUMP

I'BAKEABLE VALVE ~ , , ~ PUMP MAGNET

~ ~ 1 LITRE FLASK

\ \IONISATION

GAUGE

Figure 6. Mercury vapour system

gate more practically the problems of operating a getter-ion pump for longer periods in a mercury vapour system. In this assembly the getter-ion pump was separated from the system containing the mercury by a refrigerated vapour trap. The lower part consists of a litre volume, a Bayard Alpert type ionisation gauge and the glass side-arm containing a fresh mercury capsule.

The same method, which was used in the previous experi- ment to release mercury vapour into the system, was adopted after filling the vapour trap with a cooling mixture of acetone and solid carbon dioxide. By lagging the outside of the trap temperature below --70°C could be maintained quite ado- quately for about 15 hours before it was necessary to replenish the trap with powdered carbon dioxide.

Figure 7 shows the variation in getter-ion pump and ionisa- tion gauge pressure from the time when coolant was removed from the side arm containing the mercury. After initial variations in pressures, (shown between the dotted lines) caused by air pockets being released from the mercury capsule, the pump pressure reduced to less than 1 × 10 -7 tort, whereas the pressure recorded by the ionisation gauge increased to about 2 x 10-2 torr. Over a period of 120 hours continuous operation the pump pressure remained at about 1 x 10 -a tort and a gauge reading of about 8 x 10-6 torr was recorded.

During a further 270 hours operation with the lower part of the system heated to about 45°C the pump pressure remained at about 1 x lO-S torr and the gauge reading increased to 2 × 10 -s torr.

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B J Ward and J C Bi l l : Invest igat ion of getter- ion pump operat ion n systems conta in ing oi l and mercury vapour

It must be mentioned that these ionisation gauge readings are only very approximate indications of pressure; values of the / Relative Sensitivity of an ionisation gauge to mercury vapour / have been determined2, and thus for more accurate pressure

- 3 I. IO

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GAUGE PRESSURE I

,

PUMP PRESSURE

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-3

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so too tso 200 2so 300 TIME ~ MINUTES

1.10 . 4

- 5 1.10

- 6 1.10

I , i 0 - 7

Figure 7. Variation of pump pressure and ionisation gauge reading after removal of side-arm coolant

measurements from the ionisation gauge, sensitivity factors for mercury and other gases present in the system would have to be applied. Also, the proximity of the gauge tube to the vapour trap is likely to affect these measurements, since the trap acts as a pump to mercury.

The relationship between vapour pressure and temperature for mercury is shown in Figure 83 . It can be seen that for temperatures below --72°C the vapour pressure of mercury is less than 1 x 10 -8 torr; this compares with the pump pressures obtained with the acetone-solid carbon dioxide coolant mix- ture in the vapour trap.

If the coolant was removed from the trap and the whole system contaminated with mercury vapour at room tempera- ture for a long period, similar results to those with the previous system were obtained. Before the pump could be restarted again, it was necessary to bake for a short time with the coolant replaced in the vapour trap.

Finally, the pump was deliberately run to the point of over- heating a number of times by contaminating it with mercury at vapour pressures in the region of 10 -3 to 10 4 torr. The pump was then opened and inspection of the electrodes revealed that the coating on the anode structure had a silvery colour instead of the usual dark matt appearance. Much of the film was flaking and the remainder could easily be scraped off the anode walls. Under normal conditions of operation the titanium atoms sputtered from the cathodes adhere firmly to the anode structure. It appears that the mercury atoms do not combine with titanium and are only loosely trapped on the anode by the

- 8 I I I I I I I I F I I . I 0

- 8 0 - 7 0 - 6 0 - 5 0 - 4 0 - 3 0 - 2 0 - I 0 0 I 0 2 0

TEMPERATURE o C.

Figure 8. Relationship between vapour pressure of mercury and temperature

sputtered titanium film; repeated temperature changes caused by operating the pump in this pressure region are sufficient to cause the titanium film to break away.

2.4. Conclusions. It has been found that getter-ion pump operation in mercury atmospheres is possible provided the vapour pressure is less than ~--~10 -5 torr. Prolonged use in the 10 -4 to 10 -5 torr region is likely to cause early deterioration in pump performance by producing loose flakes on the anode structure. However, reclamation may be possible by chemical cleaning and ultrasonic agitation.

Continuous operation over a period of 400 hours with mercury vapour pressures in the region 10 7 to 10 -s tort, pro- duced no apparent deterioration in pump performance.

A c k n o w l e d g e m e n t s

The authors wish to thank Messrs Ferranti Limited for per- mission to publish this paper.

R e f e r e n c e s • L D Hall , Trans 5 A VS Nat Vac Symp 1958, 158-163.

J H Leek, Pressure Measurement in Vacuum Systems, Chapman and Hall (London), 1964, 82. a R E Honig and H O Hook, RCA Review, XXIII , No 4, 1962, pages 571, 575, 578.

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