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The Ferranti oscillogyro: a miniature vacuum system
Classified
abstracts
5902-5913
duration and the rise time of the temperature must be less than the lifetime of the adspecies. Jui-teng Lin and Thomas F George, J Appl Phys, 54 (l), 1983, 382-387. IL
5902. uv laser triggering of high-voltage gas switches. (USA) Two different techniques are discussed for uv laser triggering of highvoltage gas switches using a KrF laser (248 nm) to create an ionized channel through the dielectric gas in a spark gap. One technique uses an uv laser to induce breakdown in SF,. For this techniaue. we uresent data that demonstrate a l-a jitter of f 150ps for a 0.5MV switch at 80% of its selfbreakdown voltage using a low-divergence KrF laser. The other scheme uses additives to the normal dielectric gas, such as tripropylamine, which are selected to undergo resonant two-step ionization in the uv laser field. (USA) J R Woodworth et al, J Appl Phys, 53, (7) 1982, 47344739. 12 5903. High-efficiency high-energy flashlamp-pumped dye laser. (USA) A flashlamp-pumped dye laser using barium sulfate or polytetrafluoroethylene powders as diffuse reflectors in a close-coupled pumping geometry is described. Long pulse pumping ( - 15 ns) and use of a vacuumwalled dye cell gave an efficiency of 0.8%. J Jethwa et al, Appl Optics, 21 (15) 1982, 2778-2781. 5904. Infrared optogalvanic measurements on helium and helium-hydrogen discharges. (GB) A tunable cw F-centre laser has been used for optogalvanic detection of the helium 4’,3F61,3G transition at 2.624 pm. From a fully quantitative analysis, including measurement of the line absorption coefficient, we have determined a value for the ionization efficiency per laser photon absorbed using a steady-state model of the positive column. This value agrees well with the efficiency calculated from the linear steady-state rate equations. Also, n=4 to 6 transitions of helium and hydrogen have been optogalvanically detected in He-H, mixtures. Observations for various mixture ratios are used to explain the quenching effect of H, on the lines. S A M AlChalabi et al, J Phys D: Appl Phys, 16, (2) 1983, 115-124. 12 5905. Density perturbations induced by a discharge in a laser cavity. (GB) A linearized hydrodynamic analysis of the density perturbations induced in a laser cavity by short, sustained and exponentially decaying discharges is described. The results obtained are compared with, and found to differ from, those of Longaker and Litvak. Our results are discussed, special emphasis being placed on those features which are characteristic of the essentially two-dimensional radial propagation of sound waves in the cavity and illustrative numerical results are presented. R J A Tough et al, J Phys D: Appl Phys, 15 (12), 1982, 2433-2442. 12 5906. A quantitative account of the mechanisms of the positive column HeCd’ laser (441.6 mm). (GB) Excitation rates of the stepwise excitation process via the Cd(B) ground state, Penning excitation by He excited atoms and other processes, are calculated for the upper and lower states of the 441.6 nm line under actual discharge conditions for laser action. The calculation uses previously measured values of particle densities, electron energy distribution functions and excitation cross-section data, and population densities of Cd(lI) excited states measured more accurately in this experiment. The excitation mechanism of the 441.6 nm laser is clarified on a quantitative basis. (Japan) T Goto and T Sakurai, J Phys D: Appl Phys, 15 (12) 1982, 2413-2421. 1‘
5907.
On magnetic shielding in hydrogen masers. (GB) A set of five concentric cylindrical magnetic shields made of Moly Permalloy is studied. The results obtained are applied to the case of the hydrogen maser. It is found that the longitudinal and transverse shielding factors are respectively 100 and 125 dB. The improvement over the more standard three shields approach is 33 dB for the longitudinal shielding factor and 40 dB for the transverse shielding factor. (Canada) R Kunski and J Vanier, J Phys E: Sci Instrum, 15 (ll), 1982, 120771209. 12 5908. A high-power polarized coherent TE Nz laser. (GB) A new design of an oscillator-amplifier system in the TE N, laser is described. Three laser channels: an oscillator, preamplifier and main amplifier, are connected to a common spark gap, and the discharge timing of these channels is adjusted by varying the pressure of the gas in the 328
cavities. The characteristics of the laser light obtained from this laser system are an output power of 400 kW, a pulse width of 5 ns, a single transverse mode and a beam divergence of 1 mrad. Completely polarized laser light is extracted by inserting a polarizing prism. K Kagawa et al, J Phys E: Sci Instrum, 15 (1 l), 1982, 1192-l 197. 12 5909. New operation mode of a microchannel plate for the detection of lowenergy positive ions. (USA) The gain characteristics of a microchannel plate (MCP) operated in the reflection mode are measured for positive ions with energies between 100 and 300 eV. Singly charged alkali ions emitted thermionically are used as the incident ions, The gain in the reflection mode is found to be higher than that in the conventional transmission mode. A preacceleration for the penetrating ions into the channels is responsible for the high gain obtainable. This result suggests the potential usefulness of a MCP operated in the reflection mode for the detection of low-energy positive ions. (Japan) Nobuyoshi Koshida and Norio Nagashima, Reo Sci Instrum, 54 (I), 1983, 6264.
II. Vacuum 21. PUMPS
apparatus
AND PUMP
and auxiliaries
FLUIDS
21 5910. Recent developments in cryopumping. (GB) This review covers small cryopumps cooled by refrigerators, sorption pumping of hydrogen and helium, and very large cryopumps for nuclear fusion experiments. B A Hands, I/acuum, 32 (lO/ll). 1982, 603612. 21 5911. The Ferranti oscillogyro: a miniature vacuum system. (GB) The Ferranti oscillogyro is described from the point of view of the vacuum engineer, illustrating the tenet that small vacuum systems can provide the vacuum engineer with problems as severe as those presented by large ones. The oscillogyro is a mechanical instrument, about 40 mm diameter and 45 mm long, which, with suitable electronics, can detect rotation as slow as 0.01’ h- i. The gyroscopic sensitive element (the ‘beam’) rotates at 10,500 rpm and as it must not be subject to air damping, it is enclosed in an evacuated volume. The whole vacuum tank rotates with the beam. The vacuum required is only modest, but there can be no pumping after initial manufacture. The total vacuum volume is 3 ml, the design lifetime 5 yr and hence the maximum permissible total outgassing and leakage is about lo-r3 mbar I s-i. The choice of materials is limited by the need for thermal expansion matching and high mechanical tolerances. Evacuation is through a tube conductance 0.04 I s-r. J D Nuttall, Vacuum, 32 (lO/ll), 1982, 645649. 21 5912. Cryopump regeneration studies. (USA) A cryopump (APCI Model HV202-8C) connected to a pump speed test dome was used to evaluate the time and effectiveness of its regeneration using alternate methods of purging and heating the pump. Regeneration effectiveness was determined by measuring the pumping speed and helium adsorption capacity of the charcoal after cool down. The pump was loaded with 8 g of water and 10 std. L of argon prior to regeneration. For all tested regeneration methods the helium speeds were within 80% of the maximum speed. Warm up and evacuation without extra heat or a purge followed by cooling can be done in 4 h and 15 min. Regeneration was accomplished in 2 h and 45 min using a heater on the outside of the housing and a nitrogen purge to bring the cryopump to 310 K, followed by evacuation with a mechanical pump to 100 mtorr without heat or purge, then closing the roughing valve and starting the cryopump. An initial water vapor pressure of 100 mtorr does not increase the cool-down time compared to a lower starting pressure. Tests with other gases showed that argon and nitrogen have to be pumped below 20 mtorr and hydrogen and helium have to be pumped below 4 mtorr to avoid long delays in cool down. R C Longworth and G E Bonney, J Vuc Sci Tech&, 21 (4), 1982, 1022-1027. 21 5913. Large turbomolecular pumps for fusion research and high-energy physics. (GB) Progress in high-energy physics and nuclear fusion research requires among other things an ultra-clean, hydrocarbon-free vacuum environment. The use of large turbomolecular pumps as roughing and high vacuum pumping systems for particle accelerators and for the torus and
abstracts
5902-5913
duration and the rise time of the temperature must be less than the lifetime of the adspecies. Jui-teng Lin and Thomas F George, J Appl Phys, 54 (l), 1983, 382-387. IL
5902. uv laser triggering of high-voltage gas switches. (USA) Two different techniques are discussed for uv laser triggering of highvoltage gas switches using a KrF laser (248 nm) to create an ionized channel through the dielectric gas in a spark gap. One technique uses an uv laser to induce breakdown in SF,. For this techniaue. we uresent data that demonstrate a l-a jitter of f 150ps for a 0.5MV switch at 80% of its selfbreakdown voltage using a low-divergence KrF laser. The other scheme uses additives to the normal dielectric gas, such as tripropylamine, which are selected to undergo resonant two-step ionization in the uv laser field. (USA) J R Woodworth et al, J Appl Phys, 53, (7) 1982, 47344739. 12 5903. High-efficiency high-energy flashlamp-pumped dye laser. (USA) A flashlamp-pumped dye laser using barium sulfate or polytetrafluoroethylene powders as diffuse reflectors in a close-coupled pumping geometry is described. Long pulse pumping ( - 15 ns) and use of a vacuumwalled dye cell gave an efficiency of 0.8%. J Jethwa et al, Appl Optics, 21 (15) 1982, 2778-2781. 5904. Infrared optogalvanic measurements on helium and helium-hydrogen discharges. (GB) A tunable cw F-centre laser has been used for optogalvanic detection of the helium 4’,3F61,3G transition at 2.624 pm. From a fully quantitative analysis, including measurement of the line absorption coefficient, we have determined a value for the ionization efficiency per laser photon absorbed using a steady-state model of the positive column. This value agrees well with the efficiency calculated from the linear steady-state rate equations. Also, n=4 to 6 transitions of helium and hydrogen have been optogalvanically detected in He-H, mixtures. Observations for various mixture ratios are used to explain the quenching effect of H, on the lines. S A M AlChalabi et al, J Phys D: Appl Phys, 16, (2) 1983, 115-124. 12 5905. Density perturbations induced by a discharge in a laser cavity. (GB) A linearized hydrodynamic analysis of the density perturbations induced in a laser cavity by short, sustained and exponentially decaying discharges is described. The results obtained are compared with, and found to differ from, those of Longaker and Litvak. Our results are discussed, special emphasis being placed on those features which are characteristic of the essentially two-dimensional radial propagation of sound waves in the cavity and illustrative numerical results are presented. R J A Tough et al, J Phys D: Appl Phys, 15 (12), 1982, 2433-2442. 12 5906. A quantitative account of the mechanisms of the positive column HeCd’ laser (441.6 mm). (GB) Excitation rates of the stepwise excitation process via the Cd(B) ground state, Penning excitation by He excited atoms and other processes, are calculated for the upper and lower states of the 441.6 nm line under actual discharge conditions for laser action. The calculation uses previously measured values of particle densities, electron energy distribution functions and excitation cross-section data, and population densities of Cd(lI) excited states measured more accurately in this experiment. The excitation mechanism of the 441.6 nm laser is clarified on a quantitative basis. (Japan) T Goto and T Sakurai, J Phys D: Appl Phys, 15 (12) 1982, 2413-2421. 1‘
5907.
On magnetic shielding in hydrogen masers. (GB) A set of five concentric cylindrical magnetic shields made of Moly Permalloy is studied. The results obtained are applied to the case of the hydrogen maser. It is found that the longitudinal and transverse shielding factors are respectively 100 and 125 dB. The improvement over the more standard three shields approach is 33 dB for the longitudinal shielding factor and 40 dB for the transverse shielding factor. (Canada) R Kunski and J Vanier, J Phys E: Sci Instrum, 15 (ll), 1982, 120771209. 12 5908. A high-power polarized coherent TE Nz laser. (GB) A new design of an oscillator-amplifier system in the TE N, laser is described. Three laser channels: an oscillator, preamplifier and main amplifier, are connected to a common spark gap, and the discharge timing of these channels is adjusted by varying the pressure of the gas in the 328
cavities. The characteristics of the laser light obtained from this laser system are an output power of 400 kW, a pulse width of 5 ns, a single transverse mode and a beam divergence of 1 mrad. Completely polarized laser light is extracted by inserting a polarizing prism. K Kagawa et al, J Phys E: Sci Instrum, 15 (1 l), 1982, 1192-l 197. 12 5909. New operation mode of a microchannel plate for the detection of lowenergy positive ions. (USA) The gain characteristics of a microchannel plate (MCP) operated in the reflection mode are measured for positive ions with energies between 100 and 300 eV. Singly charged alkali ions emitted thermionically are used as the incident ions, The gain in the reflection mode is found to be higher than that in the conventional transmission mode. A preacceleration for the penetrating ions into the channels is responsible for the high gain obtainable. This result suggests the potential usefulness of a MCP operated in the reflection mode for the detection of low-energy positive ions. (Japan) Nobuyoshi Koshida and Norio Nagashima, Reo Sci Instrum, 54 (I), 1983, 6264.
II. Vacuum 21. PUMPS
apparatus
AND PUMP
and auxiliaries
FLUIDS
21 5910. Recent developments in cryopumping. (GB) This review covers small cryopumps cooled by refrigerators, sorption pumping of hydrogen and helium, and very large cryopumps for nuclear fusion experiments. B A Hands, I/acuum, 32 (lO/ll). 1982, 603612. 21 5911. The Ferranti oscillogyro: a miniature vacuum system. (GB) The Ferranti oscillogyro is described from the point of view of the vacuum engineer, illustrating the tenet that small vacuum systems can provide the vacuum engineer with problems as severe as those presented by large ones. The oscillogyro is a mechanical instrument, about 40 mm diameter and 45 mm long, which, with suitable electronics, can detect rotation as slow as 0.01’ h- i. The gyroscopic sensitive element (the ‘beam’) rotates at 10,500 rpm and as it must not be subject to air damping, it is enclosed in an evacuated volume. The whole vacuum tank rotates with the beam. The vacuum required is only modest, but there can be no pumping after initial manufacture. The total vacuum volume is 3 ml, the design lifetime 5 yr and hence the maximum permissible total outgassing and leakage is about lo-r3 mbar I s-i. The choice of materials is limited by the need for thermal expansion matching and high mechanical tolerances. Evacuation is through a tube conductance 0.04 I s-r. J D Nuttall, Vacuum, 32 (lO/ll), 1982, 645649. 21 5912. Cryopump regeneration studies. (USA) A cryopump (APCI Model HV202-8C) connected to a pump speed test dome was used to evaluate the time and effectiveness of its regeneration using alternate methods of purging and heating the pump. Regeneration effectiveness was determined by measuring the pumping speed and helium adsorption capacity of the charcoal after cool down. The pump was loaded with 8 g of water and 10 std. L of argon prior to regeneration. For all tested regeneration methods the helium speeds were within 80% of the maximum speed. Warm up and evacuation without extra heat or a purge followed by cooling can be done in 4 h and 15 min. Regeneration was accomplished in 2 h and 45 min using a heater on the outside of the housing and a nitrogen purge to bring the cryopump to 310 K, followed by evacuation with a mechanical pump to 100 mtorr without heat or purge, then closing the roughing valve and starting the cryopump. An initial water vapor pressure of 100 mtorr does not increase the cool-down time compared to a lower starting pressure. Tests with other gases showed that argon and nitrogen have to be pumped below 20 mtorr and hydrogen and helium have to be pumped below 4 mtorr to avoid long delays in cool down. R C Longworth and G E Bonney, J Vuc Sci Tech&, 21 (4), 1982, 1022-1027. 21 5913. Large turbomolecular pumps for fusion research and high-energy physics. (GB) Progress in high-energy physics and nuclear fusion research requires among other things an ultra-clean, hydrocarbon-free vacuum environment. The use of large turbomolecular pumps as roughing and high vacuum pumping systems for particle accelerators and for the torus and