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Experimental studies of hydrogen condensation on to liquid helium cooled surfaces
Experimental studies of hydrogen condensation on to liquid helium cooled surfaces* received 16 August 1965; accepted I September 1965 J N Chubb and I E Pollard, UKAEA Research Group, Culham Laboratory, nr Abingdon, Berks
Preliminary experimental studies of condensation pumping of hydrogen by liquid hefium cooled surfaces were made using an unshielded 63 cm 2plane condensing surface. Values of sticking coefficients close to unity were obtained with no apparent dependence upon gas flow rate, in the range 1.5 × 10-3 to 17 × 10-3 litre torr cm 2 sec-t, upon the condensed gas load, up to 25 litre torr cm-2, or upon the surface temperature, in the range 3.6 to 3.9°K. Monte Carlo computer analysis of the gas flow was used to take account of pressure gradients developed in the test apparatus. The appearance of the layers of condensed hydrogen was sometimes clear and "ice"-Iike and sometimes white and "frost"-Iike, even with apparently identical experimental conditions. More accurate measurements of sticking coefficients have been made by studying the reflection of hydrogen gas molecules from liquid helium cooled surfaces. Monte Carlo analysis of probable molecular histories was used to interpret experimental measurements in terms of sticking coefficients. Studies have been made which show that the sticking coefficient for hydrogen on surfaces at 3.5 or 3.7°K varied from 0.98 to 0.99 for a gas temperature oflOO°K, to 0.91 to 0.93 for a gas temperature of 3OO°K,and down to between 0.5 and 0.7 for a gas temperature of 700 °K. Condensed hydrogen layers up to several millimetres thick have recently been grown and are observed to be quite clear and transparent. Somewhat reduced sticking coefficients are observed as layers of these thicknesses are approached, and this reduction is attributed to thermal conduction limitations in the condensed layer. 1. Introduction The process of condensation seems attractive for the very high speed contamination free vacuum pumping of hydrogen which is frequently required in plasma physics experiments. The present paper describes some experimental studies which have confirmed the technical attractions of this pumping technique, and which have also provided some accurate values for sticking coefficients during condensation. In studies of the containment and stability of energetic plasmas in magnetic field configurations known as "magnetic traps" and "magnetic wells" it is necessary for the density of background gas to be very low to minimise cooling of the plasma by radiation from impurity atoms and loss of energetic ions by collisions involving charge exchanges with neutral gas molecules1, 2, 3. Background gas densities corresponding to hydrogen pressures in the range 10-7 to 10-9 tort are required, with the total pressure of non-hydrogenous gases considerably less than the hydrogen pressure. To maintain gas pressures in the above range against the gas inflows associated with the introduction of plasma into, or formation of plasma within, a containment region, requires pumping speeds in the range 104 to 107 litres sec-1. Speeds in this range are extremely difficult to achieve by any form of external pump unit, because of the restriction of access imposed by the magnetic containment coils. The vacuum pumping of this type of plasma physics experiment must therefore be achieved by some adsorption technique on internal surfaces of the containment chamber exposed to the plasma. In the plasma physics experiments known as "Phoenix"4 and
"Ogra ''5 titanium getter films, deposited on to liquid nitrogen cooled surfaces are used to provide the vacuum pumping of the containment chamber. There are a number of technical and practical features in the use of titanium getter films which are undesirable--limited pumping duration before regeneration, inability to pump nonreactive gases6, peeling of thick films and the need to renew the getter evaporation source. The potentialities of condensation are therefore being examined as an alternative surface adsorption pumping technique, which does not suffer from these limitations. The basic requirements for effective pumping by the process of condensation are that molecules incident on the surface shall quickly and efficiently lose their incident energy to the condensing surface, so that they become bound to it, and that the total rate of evaporation of molecules from the surface is less than the adsorption rate of molecules from the gas phase. Respectively, these two requirements are for a high sticking coefficient and a vapour pressure of the condensed gas, corrected for thermal transpiration, less than the required operating pressure. The pumping speed exhibited by a condensing surface is proportional to the factor:
ps/ ] where 7 is the sticking coefficient, p the system pressure, ps the vapour pressure of the condensed layer and T and Ts respectively the temperature of the system and the condensing surface.
*This paper was presented at the conference on Cryogenicsin Relation to Vacuum organised by The Institute of Physics and The Physical Society in April 1965.
Vacuum/volume 15/number 10. Pergamon Press Ltd [ Printed in Great Britain
491
J N Chubb andl E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces After formation of the initial condensed layer the nature of the surface upon which further condensation occurs will remain constant, so that the sticking coefficient would be expected to be fairly independent of layer thickness. When the layer eventually becomes very thick the heat load to the layer, by radiation and the heat of condensation of the gas, will raise the temperature of the outermost surface, and hence increase the vapour pressure. When the vapour pressure becomes appreciable in comparison to the required system pressure, the pumping speed of the surface will fall. Calculations show, however, that the temperature difference across a layer of hydrogen even as thick as 1 mm on a surface mounted within room temperature surroundings will be only about 0.1 °K.
10-9 tort with 95 per cent efficiency. Figure 2 also shows that temperature increases from these values should be limited to within about 0.05 and 0.2°K respectively to maintain the pumping speed of the surface. Lower operating temperatures are undesirable because of the greater refrigeration cost. Condensing surface temperatures in the above range may conveniently be achieved by cooling the reverse side of the surface by direct contact with liquid helium boiling under a suitably reduced pressure. An automatic technique developed for the control of liquid helium cooled condensing surfaces over long operating periods is described elsewhere s.
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In plasma physics experiments one is only concerned with hydrogen, and its isotopes, as working gases. Figure 1 shows the vapour pressure curve for hydrogen, based on the curves of Honig and HookT, and Figure 2 shows how the pumping efficiency of a condensing surface, for any value of sticking coefficient, varies with condensing surface temperature. Figure 2 shows that surface temperatures of, for example, 3.9 and 3.0°K are required for pumping at pressures of 10 5 and
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condensing surface temperature for various system pressures. 2. I n i t i a l
experimental
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Initial experimental work on cryopumping of hydrogen by condensation on to liquid helium cooled surfaces was carried out with the apparatus shown diagrammatically in Figure 3. This work aimed to examine any marked dependance of the sticking coefficient upon the gas inflow rate, the total quantity of gas condensed, or on the condensing surface temperature. It also aimed to develop the necessary techniques for the control of the condensing surface temperature over long operating periods. The area of condensing surface exposed to the vacuum chamber was defined by a close fitting liquid nitrogen shield. This shield also reduced the total heat load to the helium container by shielding the non-pumping surfaces of the helium container from room temperature radiation. The condensing surface was made of copper, for good thermal conductivity at the operating temperature, and had an area of 63 cm 2.
J N Chubb and I E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces
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The speed of pumping of the condensing surface was calculated from measurements of the gas inflow rate to the chamber, through the small hole at the bottom, and from the reading of the ionization gauge. With the geometry of vacuum chamber used for this initial experimental work it is evident that a high pumping speed at the condensing surface will cause an appreciable pressure gradient along the length of the vacuum chamber. To calculate this pressure gradient we used a Monte Carlo computer programme, developed at Culham 9, to analyse
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probable molecular histories within a fully realistic mathematical representation of the vacuum chamber. F r o m these calculations we were able to construct the curve shown in Figure 4 relating the apparent pumping speed, observed at the ion gauge, to the sticking coefficient at the condensing surface. This figure shows that not only is the apparent speed only 70 per cent of the true pumping speed of the surface for unity sticking coefficient, but also that the ability to distinguish small changes in high sticking coefficients at the surface is impaired by less than proportional variation o f apparent pumping speed with sticking coefficient. In experimental studies of cryopumping the evaporation of gas from the condensed layer or the outgassing o f the apparatus may give a background pressure comparable to the pressure observed during gas injection. The sticking coefficient of the injected gas in this case may be calculated from the change in the observed gas pressure and the measured gas inflow rate, so long as there is no interaction within the ionization gauge between the injected gas and any component of the background gas. If the gas inflow may be switched on or off within about a second, as in our experiments, then it is easy to allow for variations in background pressure during, for example, long term studies of the variation of sticking coefficeint with quantity of gas condensed, and it is also possible to study short term variations in the vapour pressure exhibited by the condensed layer. The experiments with the apparatus in Figure 3 showed that the sticking coefficient for hydrogen condensing on to a liquid helium cooled surface was close to unity, and, within the experimental errors of perhaps 4-10 per cent, there was no dependance of sticking coefficient upon gas flow rate, in the range 1.5 × 10-3 to 17 × 10 -3 torr litres sec-1 cm-2, upon total gas load condensed up to at least 25 torr litres cm 2, or upon the condensing surface temperature in the range 3.6 to 3.9°K. Apart from the above studies of pumping performance we also tested the temperature control system 8 by shining an infrared lamp at the condensing surface, through the glass wall of the vacuum chamber. We observed that we could at least double the heat loading of the condensing surface without the surface temperature increasing more than 0.1 to 0.2°K. Some visual observations were also made of the appearance of thick hydrogen layers on the condensing surface, up to about 0.2 m m thick. It was noticed that in some experiments this was white and "frost" like, and in others clear and "ice" like. Apparently identical experimental conditions could produce either form o f structure. 3. More accurate measurements of sticking coefficients The accuracy of sticking coefficient measurements with the above type of apparatus is limited by the accuracy of calibration of the ion gauge. Ion gauges may be calibrated with nonreactive gases to an accuracy of a few per cent20, 11, 12, but with reactive gases, such as hydrogen, calibration is more difficult13, 14 and much lower accuracies may be expected. The above type of experimental apparatus was therefore n o t well suited to the accurate measurement of sticking coefficients or for studying small variations of sticking coefficient with gas flow rate, total quantity of gas condensed, or gas temperature. Since our initial studies gave sticking coefficients around unity it was apparent that sticking coefficient measurements o f much greater accuracy could be made, even using an ion gauge 493
J N Chubb andl EPoEard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces calibrated at low accuracy, by observation of the reflection coefficient of gas molecules at the condensing surface rather than by direct measurements of the sticking coefficient. The sticking coefficient (7) and the reflection coefficient (p) are related as: 7=
1 --p.
If, for example, the sticking coefficient were 0.9 and the accuracy of calibration of the ion gauge was -4-5 per cent the direct measurement would give 7 = 0.9 dz 0.045. Reflection coefficient measurements, however, would give 7 = 0.9 -4- 0.005.
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PUMPING SYSTEM Figure 5. Figure 5 shows a diagrammatic cross-section of an apparatus which we have used for the measurement of sticking coefficient through the observation of reflection coefficients. The geometry of this apparatus ensures that gas molecules may only pass from the small tubular gas inlet to the orifice connected to the ion gauge detector by undergoing at least one reflecting interaction with the condensing surface. The Monte Carlo computer programme 9 has been used to study probable molecular histories within this structure for various values of sticking coefficient. The curve shown in Figure 6 relates the reciprocal of the apparent pumping speed to the sticking coefficient at the surface. The Monte Carlo calculations on which this curve is based assumed that gas molecules which did not condense lost no energy to the condensing surface, so that their probability of sticking at subsequent impacts remained the same as for their first impact. The degree of energy accommodation will only make a
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significant difference to this curve at the lower values of sticking coefficient where an appreciable fraction of the gas molecules make several interactions with the condensing surface before adsorption. In the next phase of our experimental work we intend to make some measurements of accommodation coefficients during condensation. These measurements will then enable a more accurate sticking coefficient curve to be obtained. The temperature of the gas incident upon the condensing surface is determined in this form of apparatus by the temperature of the stub tube through which the gas is introduced. As this tube may be small its temperature is easily controlled and it is possible to study sticking coefficients for gases at quite high temperatures without imposing large radiated heat loads on the condensing surface. The temperature of the gas inlet tube may be raised above ambient, to about 1000°K, by radiation and electron bombardment heating by the filament mounted in a separately evacuated system. Alternatively, the filament may be removed and the temperature reduced below ambient by cooling, for example with a stream of liquid nitrogen. The actual apparatus used for the results reported below included a number of brass components and was sealed with indium gaskets. Only mild baking was possible, and the base pressure before introducing liquid nitrogen or liquid helium was around 5 x 10-7 torr. With liquid helium cooling of the condensing surface the pressure was in the 10-8 torr range. We have made measurements of sticking coeffÉcients by this reflection technique at a variety of gas inflow rates, gas layer thicknesses, gas temperatures and condensing surface temperatures. The ranges over which we have varied these parameters are respectively about 1.5 x 10-3 to 40 x 10-3 torr 1. sec-X cm-2, 0.6 to 200 torr 1. cm-2, 80 to 700°K, and 3.2 to 3.9 °K. The values of gas flow rate and condensed gas load are the maximum values, found at the centre of the condensing surface opposite the gas inflow tube. The type of variation of sticking coefficient with gas temperature calculated directly from our experimental measurements with hydrogen is illustrated in Figure 7. The notable fact
JN Chubb andl EPollard: Experimental studies of, hydrogen condensation on to liquid helium cooled surfaces of these observations is the very high values obtained for the sticking coefficients--for example 0.90--0.93 for hydrogen at room temperature. This is much higher than the values around 0.5 which have been reported previously15, 16, 17, and somewhat higher than the value of 0.8 reported by Mascher 18. While we offer no explanation of the discrepancy between those reported values and our present results we can find no plausible error in our techniques or equipment to make any material change in our values. As the total quantity of gas condensed becomes very large, somewhat lower sticking coefficients are observed. F o r the range of gas flow rates and temperatures studied this effect did not become significant until the condensed gas load had reached about 20 torr litre cm -2. On the basis of our present results we tentatively attribute this to the poor thermal conductivity of the solid hydrogen layer 19, which allows the heat o f condensation of the hot gas molecules to raise the vapour pressure of the outer surface appreciably during gas inflow. Because the thermal time constant of the condensed gas layer is likely to be shorter than the shut-off time constant of our gas inflow system and the response time of the ion gauge collector current amplifier we do not expect to observe this enhanced vapour pressure directly by switching off the gas inflow. A value for this increase in vapour pressure should be obtained however from detailed studies of the variations of pressure change upon gas inflow with such parameters as gas temperature, gas flow rate and the total quantity of gas condensed. If these studies show that the reduction in the observed sticking coefficient at high gas temperatures and large gas loads is in
fact determined by thermal conduction in the condensed layer, then it will be possible to obtain a value for the thermal conductivity of the layer and calculate the true variation of sticking coefficient with gas temperature. The energy of binding of hydrogen molecules to the condensed gas layer is equivalent to the thermal energy of hydrogen gas molecules at a temperature of about 100°K. The present sticking coefficient measurements thus demonstrate that a gas molecule incident upon a condensed gas layer can dissipate efficiently an energy several times the binding energy available at the surface in a time interval within the natural period of vibration of the surface atoms, about 10-13 seconds. A few sticking coefficient studies were made on argon. The results of these studies, which are presented in Figure 8, emphasise the great advantage of the present reflection technique, in comparison to pumping speed type observations, for distinguishing very small changes in sticking coefficients close to unity.
4. Conclusions The studies reported here show that liquid helium cooled surfaces in the temperature range 3.2 to 3.9 °K may be used to provide adsorption pumping surfaces with sticking coefficients between 0.90 and 0.93 for hydrogen at room temperatures. It has been shown that the sticking coefficients are not appreciably affected by condensation of quite large gas loads, up to say 20 t. I. cm-2, and that the sticking coefficient remains quite high, about 0.5, even for gas at 700°K.
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J N Chubb andl E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces
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References
1 D R Sweetman, Mirror machine experiments and related atomic crosssection measurements at ,4 HIRE Aldermaston. Proc Conf of Plasma Physics and Controlled Nuclear Fusion Research, Salzburg, Austria, 1961. Nuclear Fusion 1962 Supplement. 2 R F Post, Fast neutral particle injection into a mirror machine. International Summer Course in Plasma Physics, Riso, 1960. Riso report No 18. 3 I V Kurchatov, ,4tominaya energiiga, (,4tomic Energy), 5, 1958, 105. 4 D R Sweetman, Nuclearlnsm, 13 (3), 1962, 317. 5 V A Simonov, UCRL-Trans 736 (L), Salzburg Conference, 168. 6 R E Clausing, Sorption of gases by vopour deposited titanium films, ORNL-3481, March 1964. 7 R E Honig and H O Hook, Vapour pressure data for some common gases, RC,4 Review, Sept 1960. 8 L Gowland, I E Pollard and J N Chubb, An automatic control system for operation of liquid helium cooled cryopumps, Cryogenics in Relation to Vacuum Conference, London, Feb 1965 (to be published). 9 J N Chubb, Monte Carlo calculations of molecular gas flow. Presented at Third lnter Vac Con.[, Stuttgart, 1965. 10 W D Davis, G E Research Lab report, 63-RL-3512G, Nov 1963. 11 j R Roehrig and J C Simons, Trans 8 A V S Nat Vac Syrup, 1961, Pergamon Press, Oxford, 1962, p 511. 12 C E Normand, Trans 8 A VS Nat Vac Syrup, 1961, Pergamon Press, Oxford, 1962, p 534. 13 T W Hickmott, J ApplPhysics, 31 (1), 1960, 128. 14 R S Barton and J N Chubb, Vacuum, 15 (3), March 1965, 113. 15 von J Hengevoss and E A Trendelenburg, Z Naturforschung, 18a, 1963, 481. 16 W Bachler, G Klipping and E Mascher, Trans 9,4 VS Nat Vac Symp, 1962, Macmillan, New York, p 216. 17 G Klipping and W Mascher, Z fur angenandte Physik, 16 (6), 1963, 471. 18 W Mascher, Sticking coefficients in the cryo-condensation of gases and mixtures of gases, Cryogenics in relation to Vacuum Conf, London, Feb 1965. 19 R. W Hill and B Schneidmesser, Z P h y s Chem, 16, 1958, 257. 2o H W Wooley, R B Scott and F G Brickwedde, Journal of Research, 41, Nov 1948, 379. 2t E S Borovik, S F Grishin and E Ya Grishina, Soviet Phys Tech Phys, 5, 1960, 506.
496
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200 300 400 GAS TEMPERATURE (*K)
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Preliminary experimental studies of condensation pumping of hydrogen by liquid hefium cooled surfaces were made using an unshielded 63 cm 2plane condensing surface. Values of sticking coefficients close to unity were obtained with no apparent dependence upon gas flow rate, in the range 1.5 × 10-3 to 17 × 10-3 litre torr cm 2 sec-t, upon the condensed gas load, up to 25 litre torr cm-2, or upon the surface temperature, in the range 3.6 to 3.9°K. Monte Carlo computer analysis of the gas flow was used to take account of pressure gradients developed in the test apparatus. The appearance of the layers of condensed hydrogen was sometimes clear and "ice"-Iike and sometimes white and "frost"-Iike, even with apparently identical experimental conditions. More accurate measurements of sticking coefficients have been made by studying the reflection of hydrogen gas molecules from liquid helium cooled surfaces. Monte Carlo analysis of probable molecular histories was used to interpret experimental measurements in terms of sticking coefficients. Studies have been made which show that the sticking coefficient for hydrogen on surfaces at 3.5 or 3.7°K varied from 0.98 to 0.99 for a gas temperature oflOO°K, to 0.91 to 0.93 for a gas temperature of 3OO°K,and down to between 0.5 and 0.7 for a gas temperature of 700 °K. Condensed hydrogen layers up to several millimetres thick have recently been grown and are observed to be quite clear and transparent. Somewhat reduced sticking coefficients are observed as layers of these thicknesses are approached, and this reduction is attributed to thermal conduction limitations in the condensed layer. 1. Introduction The process of condensation seems attractive for the very high speed contamination free vacuum pumping of hydrogen which is frequently required in plasma physics experiments. The present paper describes some experimental studies which have confirmed the technical attractions of this pumping technique, and which have also provided some accurate values for sticking coefficients during condensation. In studies of the containment and stability of energetic plasmas in magnetic field configurations known as "magnetic traps" and "magnetic wells" it is necessary for the density of background gas to be very low to minimise cooling of the plasma by radiation from impurity atoms and loss of energetic ions by collisions involving charge exchanges with neutral gas molecules1, 2, 3. Background gas densities corresponding to hydrogen pressures in the range 10-7 to 10-9 tort are required, with the total pressure of non-hydrogenous gases considerably less than the hydrogen pressure. To maintain gas pressures in the above range against the gas inflows associated with the introduction of plasma into, or formation of plasma within, a containment region, requires pumping speeds in the range 104 to 107 litres sec-1. Speeds in this range are extremely difficult to achieve by any form of external pump unit, because of the restriction of access imposed by the magnetic containment coils. The vacuum pumping of this type of plasma physics experiment must therefore be achieved by some adsorption technique on internal surfaces of the containment chamber exposed to the plasma. In the plasma physics experiments known as "Phoenix"4 and
"Ogra ''5 titanium getter films, deposited on to liquid nitrogen cooled surfaces are used to provide the vacuum pumping of the containment chamber. There are a number of technical and practical features in the use of titanium getter films which are undesirable--limited pumping duration before regeneration, inability to pump nonreactive gases6, peeling of thick films and the need to renew the getter evaporation source. The potentialities of condensation are therefore being examined as an alternative surface adsorption pumping technique, which does not suffer from these limitations. The basic requirements for effective pumping by the process of condensation are that molecules incident on the surface shall quickly and efficiently lose their incident energy to the condensing surface, so that they become bound to it, and that the total rate of evaporation of molecules from the surface is less than the adsorption rate of molecules from the gas phase. Respectively, these two requirements are for a high sticking coefficient and a vapour pressure of the condensed gas, corrected for thermal transpiration, less than the required operating pressure. The pumping speed exhibited by a condensing surface is proportional to the factor:
ps/ ] where 7 is the sticking coefficient, p the system pressure, ps the vapour pressure of the condensed layer and T and Ts respectively the temperature of the system and the condensing surface.
*This paper was presented at the conference on Cryogenicsin Relation to Vacuum organised by The Institute of Physics and The Physical Society in April 1965.
Vacuum/volume 15/number 10. Pergamon Press Ltd [ Printed in Great Britain
491
J N Chubb andl E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces After formation of the initial condensed layer the nature of the surface upon which further condensation occurs will remain constant, so that the sticking coefficient would be expected to be fairly independent of layer thickness. When the layer eventually becomes very thick the heat load to the layer, by radiation and the heat of condensation of the gas, will raise the temperature of the outermost surface, and hence increase the vapour pressure. When the vapour pressure becomes appreciable in comparison to the required system pressure, the pumping speed of the surface will fall. Calculations show, however, that the temperature difference across a layer of hydrogen even as thick as 1 mm on a surface mounted within room temperature surroundings will be only about 0.1 °K.
10-9 tort with 95 per cent efficiency. Figure 2 also shows that temperature increases from these values should be limited to within about 0.05 and 0.2°K respectively to maintain the pumping speed of the surface. Lower operating temperatures are undesirable because of the greater refrigeration cost. Condensing surface temperatures in the above range may conveniently be achieved by cooling the reverse side of the surface by direct contact with liquid helium boiling under a suitably reduced pressure. An automatic technique developed for the control of liquid helium cooled condensing surfaces over long operating periods is described elsewhere s.
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In plasma physics experiments one is only concerned with hydrogen, and its isotopes, as working gases. Figure 1 shows the vapour pressure curve for hydrogen, based on the curves of Honig and HookT, and Figure 2 shows how the pumping efficiency of a condensing surface, for any value of sticking coefficient, varies with condensing surface temperature. Figure 2 shows that surface temperatures of, for example, 3.9 and 3.0°K are required for pumping at pressures of 10 5 and
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condensing surface temperature for various system pressures. 2. I n i t i a l
experimental
studies
Initial experimental work on cryopumping of hydrogen by condensation on to liquid helium cooled surfaces was carried out with the apparatus shown diagrammatically in Figure 3. This work aimed to examine any marked dependance of the sticking coefficient upon the gas inflow rate, the total quantity of gas condensed, or on the condensing surface temperature. It also aimed to develop the necessary techniques for the control of the condensing surface temperature over long operating periods. The area of condensing surface exposed to the vacuum chamber was defined by a close fitting liquid nitrogen shield. This shield also reduced the total heat load to the helium container by shielding the non-pumping surfaces of the helium container from room temperature radiation. The condensing surface was made of copper, for good thermal conductivity at the operating temperature, and had an area of 63 cm 2.
J N Chubb and I E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces
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Figure 3. Diagram of apparatus for initial cryopump studies.
The speed of pumping of the condensing surface was calculated from measurements of the gas inflow rate to the chamber, through the small hole at the bottom, and from the reading of the ionization gauge. With the geometry of vacuum chamber used for this initial experimental work it is evident that a high pumping speed at the condensing surface will cause an appreciable pressure gradient along the length of the vacuum chamber. To calculate this pressure gradient we used a Monte Carlo computer programme, developed at Culham 9, to analyse
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0:[ 012 0:3 0"4 015 0:6 017 018 0"9 I'O STICKING COEFFICIENT Figure 4. Pumping speed v stickingcoefficientfor hydrogen model B cryopump based on Monte Carlo calculations.
in
probable molecular histories within a fully realistic mathematical representation of the vacuum chamber. F r o m these calculations we were able to construct the curve shown in Figure 4 relating the apparent pumping speed, observed at the ion gauge, to the sticking coefficient at the condensing surface. This figure shows that not only is the apparent speed only 70 per cent of the true pumping speed of the surface for unity sticking coefficient, but also that the ability to distinguish small changes in high sticking coefficients at the surface is impaired by less than proportional variation o f apparent pumping speed with sticking coefficient. In experimental studies of cryopumping the evaporation of gas from the condensed layer or the outgassing o f the apparatus may give a background pressure comparable to the pressure observed during gas injection. The sticking coefficient of the injected gas in this case may be calculated from the change in the observed gas pressure and the measured gas inflow rate, so long as there is no interaction within the ionization gauge between the injected gas and any component of the background gas. If the gas inflow may be switched on or off within about a second, as in our experiments, then it is easy to allow for variations in background pressure during, for example, long term studies of the variation of sticking coefficeint with quantity of gas condensed, and it is also possible to study short term variations in the vapour pressure exhibited by the condensed layer. The experiments with the apparatus in Figure 3 showed that the sticking coefficient for hydrogen condensing on to a liquid helium cooled surface was close to unity, and, within the experimental errors of perhaps 4-10 per cent, there was no dependance of sticking coefficient upon gas flow rate, in the range 1.5 × 10-3 to 17 × 10 -3 torr litres sec-1 cm-2, upon total gas load condensed up to at least 25 torr litres cm 2, or upon the condensing surface temperature in the range 3.6 to 3.9°K. Apart from the above studies of pumping performance we also tested the temperature control system 8 by shining an infrared lamp at the condensing surface, through the glass wall of the vacuum chamber. We observed that we could at least double the heat loading of the condensing surface without the surface temperature increasing more than 0.1 to 0.2°K. Some visual observations were also made of the appearance of thick hydrogen layers on the condensing surface, up to about 0.2 m m thick. It was noticed that in some experiments this was white and "frost" like, and in others clear and "ice" like. Apparently identical experimental conditions could produce either form o f structure. 3. More accurate measurements of sticking coefficients The accuracy of sticking coefficient measurements with the above type of apparatus is limited by the accuracy of calibration of the ion gauge. Ion gauges may be calibrated with nonreactive gases to an accuracy of a few per cent20, 11, 12, but with reactive gases, such as hydrogen, calibration is more difficult13, 14 and much lower accuracies may be expected. The above type of experimental apparatus was therefore n o t well suited to the accurate measurement of sticking coefficients or for studying small variations of sticking coefficient with gas flow rate, total quantity of gas condensed, or gas temperature. Since our initial studies gave sticking coefficients around unity it was apparent that sticking coefficient measurements o f much greater accuracy could be made, even using an ion gauge 493
J N Chubb andl EPoEard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces calibrated at low accuracy, by observation of the reflection coefficient of gas molecules at the condensing surface rather than by direct measurements of the sticking coefficient. The sticking coefficient (7) and the reflection coefficient (p) are related as: 7=
1 --p.
If, for example, the sticking coefficient were 0.9 and the accuracy of calibration of the ion gauge was -4-5 per cent the direct measurement would give 7 = 0.9 dz 0.045. Reflection coefficient measurements, however, would give 7 = 0.9 -4- 0.005.
t.~
i,i J O
I.N
o x T n'I-
9-0
LtJ
8.0
uJ 7'0
u.J
z _~-
I-VAPOUR PRESSURE THERMOMETER SITUATED IN COPPER SURFACE
LIQUID HELIUM~
0"003
~-
F-
6'0 z 0'002
40 30 2.0 1.0
>_. _J
e~
0"001
o er" n p-0 IK
0"1 0'2 0'5 014 O"S 016 017 018 019 I0 STICKING COEFFICIENT~ ' Figure 6. 1/(Apparent speed) v sticking coefficient for hydrogen in model C cryo pump.
/
I
IONISATION
q
J
FILAMENT
" ~
GAUGE
~GLASS VIEWING WINDOW
~ ~ GAS
INLET
FILAMENT LEADS~ t
;-
'1 1' TO
PUMPING SYSTEM Figure 5. Figure 5 shows a diagrammatic cross-section of an apparatus which we have used for the measurement of sticking coefficient through the observation of reflection coefficients. The geometry of this apparatus ensures that gas molecules may only pass from the small tubular gas inlet to the orifice connected to the ion gauge detector by undergoing at least one reflecting interaction with the condensing surface. The Monte Carlo computer programme 9 has been used to study probable molecular histories within this structure for various values of sticking coefficient. The curve shown in Figure 6 relates the reciprocal of the apparent pumping speed to the sticking coefficient at the surface. The Monte Carlo calculations on which this curve is based assumed that gas molecules which did not condense lost no energy to the condensing surface, so that their probability of sticking at subsequent impacts remained the same as for their first impact. The degree of energy accommodation will only make a
494
er"
0-004 o_ E o
,,, 5'0 r-,
LIQUID NITROGEN~_~_
THERMOCOUPLE-~ I
I0,0
significant difference to this curve at the lower values of sticking coefficient where an appreciable fraction of the gas molecules make several interactions with the condensing surface before adsorption. In the next phase of our experimental work we intend to make some measurements of accommodation coefficients during condensation. These measurements will then enable a more accurate sticking coefficient curve to be obtained. The temperature of the gas incident upon the condensing surface is determined in this form of apparatus by the temperature of the stub tube through which the gas is introduced. As this tube may be small its temperature is easily controlled and it is possible to study sticking coefficients for gases at quite high temperatures without imposing large radiated heat loads on the condensing surface. The temperature of the gas inlet tube may be raised above ambient, to about 1000°K, by radiation and electron bombardment heating by the filament mounted in a separately evacuated system. Alternatively, the filament may be removed and the temperature reduced below ambient by cooling, for example with a stream of liquid nitrogen. The actual apparatus used for the results reported below included a number of brass components and was sealed with indium gaskets. Only mild baking was possible, and the base pressure before introducing liquid nitrogen or liquid helium was around 5 x 10-7 torr. With liquid helium cooling of the condensing surface the pressure was in the 10-8 torr range. We have made measurements of sticking coeffÉcients by this reflection technique at a variety of gas inflow rates, gas layer thicknesses, gas temperatures and condensing surface temperatures. The ranges over which we have varied these parameters are respectively about 1.5 x 10-3 to 40 x 10-3 torr 1. sec-X cm-2, 0.6 to 200 torr 1. cm-2, 80 to 700°K, and 3.2 to 3.9 °K. The values of gas flow rate and condensed gas load are the maximum values, found at the centre of the condensing surface opposite the gas inflow tube. The type of variation of sticking coefficient with gas temperature calculated directly from our experimental measurements with hydrogen is illustrated in Figure 7. The notable fact
JN Chubb andl EPollard: Experimental studies of, hydrogen condensation on to liquid helium cooled surfaces of these observations is the very high values obtained for the sticking coefficients--for example 0.90--0.93 for hydrogen at room temperature. This is much higher than the values around 0.5 which have been reported previously15, 16, 17, and somewhat higher than the value of 0.8 reported by Mascher 18. While we offer no explanation of the discrepancy between those reported values and our present results we can find no plausible error in our techniques or equipment to make any material change in our values. As the total quantity of gas condensed becomes very large, somewhat lower sticking coefficients are observed. F o r the range of gas flow rates and temperatures studied this effect did not become significant until the condensed gas load had reached about 20 torr litre cm -2. On the basis of our present results we tentatively attribute this to the poor thermal conductivity of the solid hydrogen layer 19, which allows the heat o f condensation of the hot gas molecules to raise the vapour pressure of the outer surface appreciably during gas inflow. Because the thermal time constant of the condensed gas layer is likely to be shorter than the shut-off time constant of our gas inflow system and the response time of the ion gauge collector current amplifier we do not expect to observe this enhanced vapour pressure directly by switching off the gas inflow. A value for this increase in vapour pressure should be obtained however from detailed studies of the variations of pressure change upon gas inflow with such parameters as gas temperature, gas flow rate and the total quantity of gas condensed. If these studies show that the reduction in the observed sticking coefficient at high gas temperatures and large gas loads is in
fact determined by thermal conduction in the condensed layer, then it will be possible to obtain a value for the thermal conductivity of the layer and calculate the true variation of sticking coefficient with gas temperature. The energy of binding of hydrogen molecules to the condensed gas layer is equivalent to the thermal energy of hydrogen gas molecules at a temperature of about 100°K. The present sticking coefficient measurements thus demonstrate that a gas molecule incident upon a condensed gas layer can dissipate efficiently an energy several times the binding energy available at the surface in a time interval within the natural period of vibration of the surface atoms, about 10-13 seconds. A few sticking coefficient studies were made on argon. The results of these studies, which are presented in Figure 8, emphasise the great advantage of the present reflection technique, in comparison to pumping speed type observations, for distinguishing very small changes in sticking coefficients close to unity.
4. Conclusions The studies reported here show that liquid helium cooled surfaces in the temperature range 3.2 to 3.9 °K may be used to provide adsorption pumping surfaces with sticking coefficients between 0.90 and 0.93 for hydrogen at room temperatures. It has been shown that the sticking coefficients are not appreciably affected by condensation of quite large gas loads, up to say 20 t. I. cm-2, and that the sticking coefficient remains quite high, about 0.5, even for gas at 700°K.
STICKING COEFFICIENT OF HYDROGEN V. GAS TEMPERATURE
I'0
T
0:9
Z
CONDENSING SURFACE TEMPERATURES FOR
b.-
o
~7
DIFFERENT EXPERIMENTS
(1) 3"5°K A 3.7°K [] 3.77°K V 3.7°K X 3-9°K
0"8
z
o I-'-
m
0"7
0-6
I00 ' 200 ' 3b 0 400 ' 500 ' 6 0"76 0 800 ' 900 ' I000 ' GAS TEMPERATURE (°K)
Figure 7. 495
J N Chubb andl E Pollard: Experimental studies of hydrogen condensation on to liquid helium cooled surfaces
STICKING COEFFICIENT OF ARGON V. GAS TEMPERATURE I'00 0'999 0"998
~0'997 IJJ
U0-996 t.l_
~0"995 0 U0.994 Zo.993
~0"992
SURFACE
TEMPERATURE
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if)
0"991 0"990 Figure
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I
I00
I
,
l
8.
References
1 D R Sweetman, Mirror machine experiments and related atomic crosssection measurements at ,4 HIRE Aldermaston. Proc Conf of Plasma Physics and Controlled Nuclear Fusion Research, Salzburg, Austria, 1961. Nuclear Fusion 1962 Supplement. 2 R F Post, Fast neutral particle injection into a mirror machine. International Summer Course in Plasma Physics, Riso, 1960. Riso report No 18. 3 I V Kurchatov, ,4tominaya energiiga, (,4tomic Energy), 5, 1958, 105. 4 D R Sweetman, Nuclearlnsm, 13 (3), 1962, 317. 5 V A Simonov, UCRL-Trans 736 (L), Salzburg Conference, 168. 6 R E Clausing, Sorption of gases by vopour deposited titanium films, ORNL-3481, March 1964. 7 R E Honig and H O Hook, Vapour pressure data for some common gases, RC,4 Review, Sept 1960. 8 L Gowland, I E Pollard and J N Chubb, An automatic control system for operation of liquid helium cooled cryopumps, Cryogenics in Relation to Vacuum Conference, London, Feb 1965 (to be published). 9 J N Chubb, Monte Carlo calculations of molecular gas flow. Presented at Third lnter Vac Con.[, Stuttgart, 1965. 10 W D Davis, G E Research Lab report, 63-RL-3512G, Nov 1963. 11 j R Roehrig and J C Simons, Trans 8 A V S Nat Vac Syrup, 1961, Pergamon Press, Oxford, 1962, p 511. 12 C E Normand, Trans 8 A VS Nat Vac Syrup, 1961, Pergamon Press, Oxford, 1962, p 534. 13 T W Hickmott, J ApplPhysics, 31 (1), 1960, 128. 14 R S Barton and J N Chubb, Vacuum, 15 (3), March 1965, 113. 15 von J Hengevoss and E A Trendelenburg, Z Naturforschung, 18a, 1963, 481. 16 W Bachler, G Klipping and E Mascher, Trans 9,4 VS Nat Vac Symp, 1962, Macmillan, New York, p 216. 17 G Klipping and W Mascher, Z fur angenandte Physik, 16 (6), 1963, 471. 18 W Mascher, Sticking coefficients in the cryo-condensation of gases and mixtures of gases, Cryogenics in relation to Vacuum Conf, London, Feb 1965. 19 R. W Hill and B Schneidmesser, Z P h y s Chem, 16, 1958, 257. 2o H W Wooley, R B Scott and F G Brickwedde, Journal of Research, 41, Nov 1948, 379. 2t E S Borovik, S F Grishin and E Ya Grishina, Soviet Phys Tech Phys, 5, 1960, 506.
496
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,
200 300 400 GAS TEMPERATURE (*K)
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500 ,
L
600