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1237. Scaling laws for experimental determination of pumpdown time in the molecular flow range
Classified abstracts 1235-1244 series of lines with a spacing of 190 MHz, and some frequency instabilities were observed. M S Borisova and A M Pyndyk, Radiotekh Elektron, 13 (4), April 1968, 754-756 (in Russian). 19 : 31 1235. Frequency effects in a laser with a nonlinearly-absorbing gas. (USSR) From a theoretical discussion of frequency effects in a laser with a nonlinearly-absorbing gas cell inside the resonator, it is shown that autostabilization of the generated frequency is possible. For an absorbing cell of 100 cm at pressure of 10-4 to 10-3 torr, the theoretical value of the frequency stability was found to be 10-la. V S Letokhov, Zh Eksper Teor Fiz, 54 (4), April 1968, 1244-1252
(in Russian). 19 : 31 1236. Theory of gas lasers with nonlinear absorption. (USSR) Theoretical considerations are presented for a gas laser with an absorbing cell, filled by a suitable gas at a determined pressure, and located inside the laser resonator. The system may be used for frequency stabilization, optical spectroscopy or, since hysteresis of ignition was observed, as an optical memory device. A P Kazantsev et al, Zh Eksper Teor Fiz, 54 (5), May 1968, 1409-1421
(in Russian).
II. V a c u u m apparatus and auxiliaries 20. P U M P I N G SYSTEMS 20 1237. Scaling laws for experimental determination of pumpdown time in the molecular flow range. (USA) The pumpdown time and ultimate pressure of a vacuum system cannot be accurately calculated since the outgassing rate of the walls, which is the determining factor, is a complex function of time and pressure. Scaling laws are derived for dimensions of a scaled-down model of a large vacuum system made of the same materials which has the same pressure-time behaviour. The results are generalized and scaling laws are given for a system consisting of several lumped volumes with outgassing surfaces connected in series through conductances with or without outgassing. The same ratios apply in systems with local temperature variations if the same relative temperature distribution is maintained in both systems and if particle speed and particle conductance are substituted for pump speed and flow conductance. The influence of wall thickness on the outgassing rate is also discussed. G Lewin, J Vac Sci Technol, 5 (3), May-June 1968, 71-74. 20 : 14 1238. Gas flow in vacuum systems with local temperature variations for the molecular flow range. (USA) Gas flow is conventionally measured in torr. litres/sec and the pressure distribution calculated from relations expressing the gas flow as a product of conductance and pressure change and as a product of pumping speed and pressure. These relations require that the gas flow is the same everywhere in the system. If temperature varies locally, the gas flow varies also but the particle flow (in molecules/sec) remains constant throughout. A procedure analogous to the conventional method of calculation but based on particle flow, is proposed for systems with local temperature variations and a particle speed and particle conductance are substituted for the pumping speed and conductance. Examples are given to illustrate the method. G Lewin, J Vac Sci Technol, 5 (3), May-June 1968, 75-78. 20 1239. Application of the pulse method to a specific heat and densityindependent measurement of thermal conductivity; expansion of the method to very small specimens. (Great Britain) The pulse method is discussed, and the experimental apparatus is described. It consists of a vacuum furnace and its associated vacuum and heating systems, a flash circuit and a temperature measuring circuit. The vacuum furnace was specifically designed for use with extremely small samples (diameters 0.144 inch, length 0.03-0.04 inch). (USA) R A di Noel, J Sci Instrum, Ser 2, 1 (4), 1968, 379-382.
21. P U M P S AND P U M P FLUIDS 21 1240. Experimental ultrahigh vacuum pumping system. (France) The system described allows the testing of different pump combinations. A special roughing line permits comparison of zeolite pumps with a trapped rotary pump. The performance of Penning pump cathodes was investigated using this system, and improved results were obtained by covering the cathodes with a mesh of expanded metal. J Vaumoron et al, Le Vide, 23 (134), March-April 1968, 93-96 (in
French). 21 : 32 1241. Attachment of an ion pump to an electron multiplier without increase in background noise. (France) In the assembly of an in-line mass spectrometer in an accelerator for the study of short period isotopes arising from nuclear reactions, evacuation by turbomolecular pumps results in 10-7 torr pressure after several hours. Optimal conditions for operation of the electron multiplier require rapid attainment of 2 × 10-8. This is achieved by a 15 litre/sec ion pump attached to the electron multiplier chamber. Straight-tube connections resulted in a background noise of 5000 c/s and shielding by a 90 ° curve in the connection reduced the noise to 2000 c/s which was still inadequate. The introduction of a 90 ° angle section in the connection reduced the noise to the same level as that without the ion pump. R Fergeau, Le Vide, 23 (134), March-April 1968, 97-98 (in French). 21 1242. Beam line liquid helium eryopump. (USA) A beam line liquid helium cryopump has been designed and built for Alice (a neutral-injection, controlled thermonuclear research machine). The pump prevents large quantities of hydrogen and other residual gases from entering into the ultrahigh vacuum environment chamber for plasma. The beam line cryopump consists of an annular tube through which the neutral energetic beam passes. The annular volume is connected to a helium reservoir above it by two-well insulated and shielded lines. The thermal syphoning effect causes the circulation of liquid helium from the annular pumping surface to the liquid helium reservoir and eliminates the presence of gas pockets and hot spots on the pumping surface. This pump has a 4800 cm 2 bare metal cryogenic pumping surface that can be subcooled to the Lambda point or lower. The pumping speed for hydrogen was measured at 2°K to be 100.000 litres/sec at a system pressure of 8 × 10-1° torr. The heat leak was measured to be 0.75 watt. B S Denhoy, Rep UCRL-70590, Sept 1967, 19 pages (Sci Tech Aerospace Reps, 6 (5), 668, N68-14744).
22. GAUGES 22 1243. Construction and performance of a liquid nitrogen cooled, titanium sublimation pump, the "SubUmvide 8000". (France) Sublimed titanium is deposited on a liquid nitrogen cooled torus where reactive gases form stable titanium compounds. Under test this pump has provided continuous pumping at a speed of about 20,000 litres/sec for two hours at below 5 × 10-8 tort. The titanium film is regenerated by a resistance heated filament on the aluminium bronze cold wall. The design of the pump allows change of the filaments without breaking the vacuum. Performance curves indicate that dynamic high speed pumping is possible in permanent vacuum when gas is being introduced. Static vacuum may also be obtained by cycled titanium evaporation. P Beaufils and R Geller, Le Vide, 23 (134), March-April 1968, 67-74
(in French). 22 1244. Measurements with Bayard-Alpert gauges at the soft x-ray limit. (France) Groszkowski's modification of the Bayard-Alpert gauge, designed to measure pressures below 10-1° torr, is described. In this gauge, the collector is exterior to the anode space and is protected, by a glass shield, from soft x-ray impact. A theoretical study of the x-ray limit, involving the position and diameter of the shield, shows that it should be possible to read pressures lower than 10-x4 torr, by means of geometrical modification. Experimental comparisons with an Ultravac Bayard-Alpert gauge indicate lowest pressures slightly higher than the theoretical limit but these have not as yet been experimentally verified. The construction of the Groszkowski gauge
533
(in Russian). 19 : 31 1236. Theory of gas lasers with nonlinear absorption. (USSR) Theoretical considerations are presented for a gas laser with an absorbing cell, filled by a suitable gas at a determined pressure, and located inside the laser resonator. The system may be used for frequency stabilization, optical spectroscopy or, since hysteresis of ignition was observed, as an optical memory device. A P Kazantsev et al, Zh Eksper Teor Fiz, 54 (5), May 1968, 1409-1421
(in Russian).
II. V a c u u m apparatus and auxiliaries 20. P U M P I N G SYSTEMS 20 1237. Scaling laws for experimental determination of pumpdown time in the molecular flow range. (USA) The pumpdown time and ultimate pressure of a vacuum system cannot be accurately calculated since the outgassing rate of the walls, which is the determining factor, is a complex function of time and pressure. Scaling laws are derived for dimensions of a scaled-down model of a large vacuum system made of the same materials which has the same pressure-time behaviour. The results are generalized and scaling laws are given for a system consisting of several lumped volumes with outgassing surfaces connected in series through conductances with or without outgassing. The same ratios apply in systems with local temperature variations if the same relative temperature distribution is maintained in both systems and if particle speed and particle conductance are substituted for pump speed and flow conductance. The influence of wall thickness on the outgassing rate is also discussed. G Lewin, J Vac Sci Technol, 5 (3), May-June 1968, 71-74. 20 : 14 1238. Gas flow in vacuum systems with local temperature variations for the molecular flow range. (USA) Gas flow is conventionally measured in torr. litres/sec and the pressure distribution calculated from relations expressing the gas flow as a product of conductance and pressure change and as a product of pumping speed and pressure. These relations require that the gas flow is the same everywhere in the system. If temperature varies locally, the gas flow varies also but the particle flow (in molecules/sec) remains constant throughout. A procedure analogous to the conventional method of calculation but based on particle flow, is proposed for systems with local temperature variations and a particle speed and particle conductance are substituted for the pumping speed and conductance. Examples are given to illustrate the method. G Lewin, J Vac Sci Technol, 5 (3), May-June 1968, 75-78. 20 1239. Application of the pulse method to a specific heat and densityindependent measurement of thermal conductivity; expansion of the method to very small specimens. (Great Britain) The pulse method is discussed, and the experimental apparatus is described. It consists of a vacuum furnace and its associated vacuum and heating systems, a flash circuit and a temperature measuring circuit. The vacuum furnace was specifically designed for use with extremely small samples (diameters 0.144 inch, length 0.03-0.04 inch). (USA) R A di Noel, J Sci Instrum, Ser 2, 1 (4), 1968, 379-382.
21. P U M P S AND P U M P FLUIDS 21 1240. Experimental ultrahigh vacuum pumping system. (France) The system described allows the testing of different pump combinations. A special roughing line permits comparison of zeolite pumps with a trapped rotary pump. The performance of Penning pump cathodes was investigated using this system, and improved results were obtained by covering the cathodes with a mesh of expanded metal. J Vaumoron et al, Le Vide, 23 (134), March-April 1968, 93-96 (in
French). 21 : 32 1241. Attachment of an ion pump to an electron multiplier without increase in background noise. (France) In the assembly of an in-line mass spectrometer in an accelerator for the study of short period isotopes arising from nuclear reactions, evacuation by turbomolecular pumps results in 10-7 torr pressure after several hours. Optimal conditions for operation of the electron multiplier require rapid attainment of 2 × 10-8. This is achieved by a 15 litre/sec ion pump attached to the electron multiplier chamber. Straight-tube connections resulted in a background noise of 5000 c/s and shielding by a 90 ° curve in the connection reduced the noise to 2000 c/s which was still inadequate. The introduction of a 90 ° angle section in the connection reduced the noise to the same level as that without the ion pump. R Fergeau, Le Vide, 23 (134), March-April 1968, 97-98 (in French). 21 1242. Beam line liquid helium eryopump. (USA) A beam line liquid helium cryopump has been designed and built for Alice (a neutral-injection, controlled thermonuclear research machine). The pump prevents large quantities of hydrogen and other residual gases from entering into the ultrahigh vacuum environment chamber for plasma. The beam line cryopump consists of an annular tube through which the neutral energetic beam passes. The annular volume is connected to a helium reservoir above it by two-well insulated and shielded lines. The thermal syphoning effect causes the circulation of liquid helium from the annular pumping surface to the liquid helium reservoir and eliminates the presence of gas pockets and hot spots on the pumping surface. This pump has a 4800 cm 2 bare metal cryogenic pumping surface that can be subcooled to the Lambda point or lower. The pumping speed for hydrogen was measured at 2°K to be 100.000 litres/sec at a system pressure of 8 × 10-1° torr. The heat leak was measured to be 0.75 watt. B S Denhoy, Rep UCRL-70590, Sept 1967, 19 pages (Sci Tech Aerospace Reps, 6 (5), 668, N68-14744).
22. GAUGES 22 1243. Construction and performance of a liquid nitrogen cooled, titanium sublimation pump, the "SubUmvide 8000". (France) Sublimed titanium is deposited on a liquid nitrogen cooled torus where reactive gases form stable titanium compounds. Under test this pump has provided continuous pumping at a speed of about 20,000 litres/sec for two hours at below 5 × 10-8 tort. The titanium film is regenerated by a resistance heated filament on the aluminium bronze cold wall. The design of the pump allows change of the filaments without breaking the vacuum. Performance curves indicate that dynamic high speed pumping is possible in permanent vacuum when gas is being introduced. Static vacuum may also be obtained by cycled titanium evaporation. P Beaufils and R Geller, Le Vide, 23 (134), March-April 1968, 67-74
(in French). 22 1244. Measurements with Bayard-Alpert gauges at the soft x-ray limit. (France) Groszkowski's modification of the Bayard-Alpert gauge, designed to measure pressures below 10-1° torr, is described. In this gauge, the collector is exterior to the anode space and is protected, by a glass shield, from soft x-ray impact. A theoretical study of the x-ray limit, involving the position and diameter of the shield, shows that it should be possible to read pressures lower than 10-x4 torr, by means of geometrical modification. Experimental comparisons with an Ultravac Bayard-Alpert gauge indicate lowest pressures slightly higher than the theoretical limit but these have not as yet been experimentally verified. The construction of the Groszkowski gauge
533