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The use of diffusion pumps for obtaining ultraclean vacuum environments
Classified abstracts 1292--1300 starting a n d p u m p d o w n are described. T Snouse, J Vac Sci Technol, 8 (1), Jan~Feb 1971, 283 285. 21 1292. Analysis of accommodation pumps. (USA) Studies on the basic m e c h a n i s m of a c c o m m o d a t i o n p u m p i n g have been continued. T w o geometrical replicas of the 28-stage p u m p were constructed, one with all surfaces of n o r m a l s m o o t h Pyrex, the other with all surfaces of leached Pyrex. Step functions of helium gas pressure were applied at each end of all three samples with a n d without cooling. Analysis of the pressure changes showed that the model of an a c c o m m o d a t i o n p u m p as a pipe with unequal conductances in two directions was valid. a P Hobson, J Vac Sci Technol, 8 ( I ), Jan/Feb 197 I, 290-293. 21 1293. The use of diffusion pumps tbr obtaining ultraclean vacuum environments. (USA) Most of the v a c u u m systems in the world are c o n t a m i n a t e d with h y d r o c a r b o n s . In s o m e instances, it has become popular to blame this c o n t a m i n a t i o n on oil-diffusion p u m p s . This paper will first analyze the various other sources of h y d r o c a r b o n s which are frequently present in v a c u u m s y s t e m s in terms of their m a g n i t u d e and elimination. T h e effectiveness of different types of traps will be considered in terms of their ability to stop different types of p u m p - g e n e r a t e d gases. As a result of this analysis, it will be s h o w n that properly designed a n d properly operated oil-diffusion-pumped systems can produce ultraclean v a c u u m e n v i r o n m e n t s as d e m o n s t r a t e d by base pressures in the 10 a* torr range. D 3 Santeler, J Vac Sci Technol, 8 (1), Jan~Feb 197 I, 299 307. 21 i 294. Backstreaming from cryogenic traps. ( U SA ) An tmusual m e c h a n i s m of gross b a c k s t r e a m i n g has been recently observed in diffusion-pump systems with liquid-nitrogen traps. It occurs during or following the charging of the trap with liquid nitrogen a n d c a n be recognized by the appearance of small droplets u p s t r e a m from the trap and by a c c o m p a n y i n g pressure fluctuations. T h e a m o u n t o f p u m p i n g fluid which m a y reach the baseplate of a typical bell-jar v a c u u m system due to this m e c h a n i s m m a y be few orders of m a g n i t u d e greater t h a n the normally expected steady-state b a c k s t r e a m i n g rate. T h e p h e n o m e n o n is produced by fracture of the frozen p u m p i n g fluid film due to the unequal thermal expansion coefficients between it and the metal surfaces on which it has been previously deposited. T h e elastic energy stored in the film is sufficient to impart high velocity to the fragments resulting from the fracture. T h e paper describes the observations a n d suggests m e t h o d s of prevention. M H Hablanian and R F Herrick, J Vac Sci Technol, 8 ( I ), Jult/l.'eb 1971, 317 318. 21 1295. A clean cryo-vacuum system with high pumping speeds for all gas species. (USA) Calibration of the H a r v a r d College Observatory spectrometers that are to fly aboard the Apollo telescope m o u n t ( A T M ) required a c o m pletely clean v a c u u m system having the capability for p u m p i n g hydrogen, helium, argon, and other inert gases at high speeds between l0 ~ a n d 10 ~ torr. T h e spectrometer performance would be degraded if c o n t a m i n a t e d by exposure to h y d r o c a r b o n s , electromagnetic radiation, or metallic films. For this reason, a cryogenically p u m p e d system was selected. This paper describes the concept, development, and operation of the v a c u u m system which was both c r y o - c o n d e n s a t i o n a n d cryosorption p u m p e d in the molecular flow range. Molecular sieve, bonded to liquid helium-cooled panels, provided the clean, fast, and economical p u m p i n g device. Data presented show p u m p i n g speeds measured, adsorbent capacity of molecular sieve, liquid helium c o n s u m p t i o n rates, a n d cryogenic performance of the system. T h i s data permits analysis o f the c r y o - v a c u u m p u m p i n g technique for other applications, R J Powers and R M Chambers, .I Vac Sci Technol, 8 ( 1), Jan/["eb 197 I. 319 323. 21 1296. Millitorr pumpdown anomaly relationships with large pipe chamber design tradeoffs. (USA) P u m p d o w n predictability of large pipe-type v a c u u m c h a m b e r s is improved t h r o u g h design tradeoff reevaluation based on checkout a n o m a l y analysis. T r u n c a t e d - c o n e pipe c h a m b e r s up to 2000 ft k)ng a n d 200,000 ft 3 in volume, which served as u n d e r g r o u n d nuclear effects testbeds, were analyzed. Inconsistencies in evacuations to a 562
base pressure of I mtorr are identified v, ith the sorbing time ,,ariablc of the chamber wall and experiment support materials. Variations ill pressure levels along the pipe chambers are shown related to configuration factors and to design tradeoffs on allowable inleakage from concrete shrouding, sand stemming, and tunnel atmospheres. Cascading graphs innovated for tradeoffs include two recently derived flow conductance equations. C A Sehalla, J Vac Sci Technol, g ( I ), Jan,,'l-~4~ 197 I, 344 347. 21 : 32 1297. Vacuum system lbr Stanford storage ring S P E A R . (USA) D u e to its unusual geometry a n d gas load, the S P E A R ~acuum system incorporates several unique design features. The system consists of an a l u m i n i u m c h a m b e r in the form of a 200-ft-diam ring which m u s t achieve a pressure in the 10 '~ torr range with a large gas load due to synchrotron-radiation-induced desorption. Since the system is c o n d u c t a n c e limited, it was necessary to develop distributed sputter-ion p u m p s located within the c h a m b e r proper. A large n u m b e r of alurninium stainless-steel interfaces required the development and testing of various flange a r r a n g e m e n t s which would withstand a bakeout to 200 C. U Cummings et al, J Vac Sci Teclmol, 8 ( I ), Jan/Fel~ 197 l, 348 35 I. 21:47 1298. Residual gas analysis with a quadrupole mass filter. (Japan) T w o series o f residual gas analyses by q u a d r u p o l e mass filter were performed for two independent v a c u u m systems: one a 4-inch oil diffusion p u m p with silicone DC-705 as the working fluid; the other two 2.5-inch oil diffusion p u m p s using a naphthyl derivative ( m o n o eicosyl). T h e results indicate the existence of a non-negligible a m o u n t of diffusion oil vapour in the v a c u u m system. Characteristic mass peaks were obtained for each diffusion p u m p oil a n d tentative a s s i g n m e n t s are m a d e for the structures of these fragment ions. T h e analyses show that the system of naphthyl derivative diffusion punlps is more suitable for electron impact experiments. Y Koyama and K Maeda, J Vac Soc Japan, 14 (4), 1971, 140 147 (m Japanese).
22. G A U G E S 1299. Thermistor thermo-electriegauges. (USSR) T h e head of the developed gauge consists of pressure m e a s u r i n g and t h e r m o - c o m p e n s a t o r parts, in which thermistors of V.,O4 with phase iunction, platinum leads, a n d nichrome heaters serve as thermoelectric converters at temperature of 6 T C . T h e head is connected to the m e a s u r i n g circuit which m a i n t a i n s the temperature of both thermistors constant a n d equal to 67 C. Calibration has been performed with the aid of a static m e t h o d using claibrated volunles. The error of the m e t h o d did not exceed 10 per cent. The calibrating cur~.e deviates from the theoretical one. Sensitivity of the instrument does not a p p r o a c h zero in the region I 760 torr due to enhanced convective losses of the pressure m e a s u r i n g part of the gauge as a resuh of mechanical vibrations of the heater heated by' 25 Hz ac current. Violation of linearity of the characteristic in the pressure region below 10 '-' torr is explained by decrease of the temperature coefficient of the thermistor resistance. T h e m a x i m u m time constant of the gauge is 3 seconds. T h e m e a s u r i n g accuracy evaluated theoretica 15' in the pressure range 10 '-' to 10 a torr is in the range :k20 to 45 per cent at the temperature change fiom 15 to 20 C and is 5 times better than in the case of a thermocouple gauge. Experimentally determined error of the gauge indication for a temperature change of 12 C at pressure of 6 × I 0 a tort is 50 per cent. G E Novikov, lzv Lenin~,r Elektrotekhn htst, 94, I~,U(), 18 20 (m Russian). 1300. Mass spectrometer with ion separation by time-ofllight in retarding electric field. (USSR) New' type of time-of-flight mass spectrometer is described which usc~ ion separation by time-of-flight in a region with a retarding electric field. C o m p a r i s o n is m a d e with the conventional time-of-flight mas~ spectrometer with fieldless drift region. It is theoretically shown thal with equal geometrical forms of the ion sources and the applied voltages (pulsed and stead},) the resolution is the same for both instruments if the length of the rctarding region is one halt" of the fieldless drift region and the ion losses due to the angular spread are smaller in the retarding field region than in the drift region. With equal length of flight a n d total ion energy,, the resolution of the instrument with the retarding field region is considerably higher, the
base pressure of I mtorr are identified v, ith the sorbing time ,,ariablc of the chamber wall and experiment support materials. Variations ill pressure levels along the pipe chambers are shown related to configuration factors and to design tradeoffs on allowable inleakage from concrete shrouding, sand stemming, and tunnel atmospheres. Cascading graphs innovated for tradeoffs include two recently derived flow conductance equations. C A Sehalla, J Vac Sci Technol, g ( I ), Jan,,'l-~4~ 197 I, 344 347. 21 : 32 1297. Vacuum system lbr Stanford storage ring S P E A R . (USA) D u e to its unusual geometry a n d gas load, the S P E A R ~acuum system incorporates several unique design features. The system consists of an a l u m i n i u m c h a m b e r in the form of a 200-ft-diam ring which m u s t achieve a pressure in the 10 '~ torr range with a large gas load due to synchrotron-radiation-induced desorption. Since the system is c o n d u c t a n c e limited, it was necessary to develop distributed sputter-ion p u m p s located within the c h a m b e r proper. A large n u m b e r of alurninium stainless-steel interfaces required the development and testing of various flange a r r a n g e m e n t s which would withstand a bakeout to 200 C. U Cummings et al, J Vac Sci Teclmol, 8 ( I ), Jan/Fel~ 197 l, 348 35 I. 21:47 1298. Residual gas analysis with a quadrupole mass filter. (Japan) T w o series o f residual gas analyses by q u a d r u p o l e mass filter were performed for two independent v a c u u m systems: one a 4-inch oil diffusion p u m p with silicone DC-705 as the working fluid; the other two 2.5-inch oil diffusion p u m p s using a naphthyl derivative ( m o n o eicosyl). T h e results indicate the existence of a non-negligible a m o u n t of diffusion oil vapour in the v a c u u m system. Characteristic mass peaks were obtained for each diffusion p u m p oil a n d tentative a s s i g n m e n t s are m a d e for the structures of these fragment ions. T h e analyses show that the system of naphthyl derivative diffusion punlps is more suitable for electron impact experiments. Y Koyama and K Maeda, J Vac Soc Japan, 14 (4), 1971, 140 147 (m Japanese).
22. G A U G E S 1299. Thermistor thermo-electriegauges. (USSR) T h e head of the developed gauge consists of pressure m e a s u r i n g and t h e r m o - c o m p e n s a t o r parts, in which thermistors of V.,O4 with phase iunction, platinum leads, a n d nichrome heaters serve as thermoelectric converters at temperature of 6 T C . T h e head is connected to the m e a s u r i n g circuit which m a i n t a i n s the temperature of both thermistors constant a n d equal to 67 C. Calibration has been performed with the aid of a static m e t h o d using claibrated volunles. The error of the m e t h o d did not exceed 10 per cent. The calibrating cur~.e deviates from the theoretical one. Sensitivity of the instrument does not a p p r o a c h zero in the region I 760 torr due to enhanced convective losses of the pressure m e a s u r i n g part of the gauge as a resuh of mechanical vibrations of the heater heated by' 25 Hz ac current. Violation of linearity of the characteristic in the pressure region below 10 '-' torr is explained by decrease of the temperature coefficient of the thermistor resistance. T h e m a x i m u m time constant of the gauge is 3 seconds. T h e m e a s u r i n g accuracy evaluated theoretica 15' in the pressure range 10 '-' to 10 a torr is in the range :k20 to 45 per cent at the temperature change fiom 15 to 20 C and is 5 times better than in the case of a thermocouple gauge. Experimentally determined error of the gauge indication for a temperature change of 12 C at pressure of 6 × I 0 a tort is 50 per cent. G E Novikov, lzv Lenin~,r Elektrotekhn htst, 94, I~,U(), 18 20 (m Russian). 1300. Mass spectrometer with ion separation by time-ofllight in retarding electric field. (USSR) New' type of time-of-flight mass spectrometer is described which usc~ ion separation by time-of-flight in a region with a retarding electric field. C o m p a r i s o n is m a d e with the conventional time-of-flight mas~ spectrometer with fieldless drift region. It is theoretically shown thal with equal geometrical forms of the ion sources and the applied voltages (pulsed and stead},) the resolution is the same for both instruments if the length of the rctarding region is one halt" of the fieldless drift region and the ion losses due to the angular spread are smaller in the retarding field region than in the drift region. With equal length of flight a n d total ion energy,, the resolution of the instrument with the retarding field region is considerably higher, the