1262. Ionization and breakdown in oxygen

Classified abstracts 1258-l 267 18 1258. Comparison between 50 Hz ac and dc vacuum breakdown. (GB) The dc breakdown voltage of an ultrahigh vacuum gap is measured using a silver alloy and found to increase to the level of the peak ac (50 Hz) voltage after ‘conditioning’ by repeated sparkings of both electrodes. The conditioning of the electrodes in high vacuum is done in two separate experiments with ac sparkings and using both polarities of the dc applied voltage. The equality of the dc and ac dielectric strength of the vacuum gap is confirmed over a wide range of gap distances. R Hackman, J Phys D: Appl Phys, 8 (4), 1975, L53-L55. 18 1259. Surface wave propagation along a current-carrying warm plasma column. (GB) The propagation of high-frequency surface waves along a plasma column was investigated theoretically and experimentally. From the equations of the one-fluid hydrodynamics, expressions were obtained in a quasistatic approximation for the dispersion of axially symmetric and dipole modes of high-frequency surface waves, propagating along a current-carrying warm plasma surrounded by a dielectric and a vacuum. The detailed experimental investigations of the dispersion and the space damping rate of the surface waves in a neon gas discharge plasma, realized on the basis of the phase shift measurement of the applied and accepted signals and by using a time-space diagram method, was effected at different discharge currents and gas pressures in a wide frequency range and showed very good agreement with the theoretical results. (Bulgaria) A Shivarova et al, J Phys D: Appl Phys, 8 (4), 1975, 383-393. 18 1260. Coronas in gases and vapours. (GB) The light from coronas at point electrodes in gases and vapours has been photographed. The radius of curvature of the boundary of the luminous region has been measured. Values of the local field E at the corona boundary have been deduced for the range 2-760 torr for air, helium, hydrogen, sulphur hexafluoride, n-pentane, n-hexane and water vapour. Values of E for n-pentane and n-hexane vapours at 100 torr are comparable with those previoulsy obtained for corresponding liquids at atmospheric pressure. Both the values of E and the voltages applied to the point electrodes to obtain coronas are greater for n-pentane, n-hexane and water vapour than for air under similar conditions. It is suggested that this difference may contribute to explosions following the cooling of quantities of flammable vapours in an air environemnt. C Sorapipatana et al, J Phys D: Appl Phys, 8 (4), 1975, 394-399. 18 1261. Interferometric measurements of the gas density distribution in the dark space of an abnormal glow discharge in xenon. (GB) Measurements of the spatial dependence of the gas density distribution near the cathode of an abnormal glow discharge on the current density are described. The technique uses a pulsed discharge in xenon with a mercury cathode, placed in one arm of a Michelson interferometer so that the changing density influences the optical path length. The sensitivity of the system is equivalent to approximately 0.04 torr, at 300 K. Considerable changes in gas density are observed in the dark space, particularly at high current densities. The proposed theoretical model for this non-uniform gas density agrees well with the experimental results. A J T Holmes and J R Cozens, J Phys D: Appl Phys, 8 (4), 1975, 400-404. 18 1262. Ionization and breakdown in oxygen. (GB) Measurements are reported of uniform-fleld breakdown voltages and of effective primary and secondary ionization coefficients in oxygen in therange0.825 x 10” I Ni 9.90 x 1018cm-3, 1.1 X lo-” 2 E/ N 5 2.1 x IO-l5 V cme2, where N is the gas number density and E is the electric field strength. The results are in general agreement with those of previous authors. Oscillographic observations show that, over the range covered in these measurements, the secondary electrons are produced only by photoelectric processes. Also reported are theoretical values of the secondary ionization coefficient, calculated using a one-dimensional model that takes account of the effects of unstable negative ions on the discharge growth. The results show that for N = 0.825 x lOi cmM3 the secondary process can be described in terms of a single radiation component with an absorption coefficient of 38 cm- I (at N = 25.1 x 10” cm-‘): this radiation

component has been investigated by previous workers. In the range of E/N covered in the present work, the secondary process appears to be mainly cathode photoemission, but photo-ionization of the gas also plays a significant part. For N = 3.30 x 10” crne3, on the other hand, the calculated and measured secondary coefficients differ considerab!v, and additional radiation components must be assumed to be aciive. D T A Blair and H W Whittington, J Phys D: Appl Phys, 8 (4), 1975, 405-415.

II. Vacuum 20. PUMPING

apparatus

and auxiliaries

SYSTEMS

20 1263. Fully autoclavable peristaltic pumping system using modified Watson-Marlow ‘MHRE delta’ series pump modules. (GB) A Watson-Marlow 2 20-tube peristaltic pump is used in an apparatus for controlling the flow of nutrient and other media to a battery of bacterial growth cells. Before the start of an experiment the complete apparatus is sterilized in a high pressure steam autoclave. As the pump modules on the MHRE series are easily detached from their drive units, it is convenient to leave them joined to the main apparatus, with each tube wrapped slackly around the rotor for autoclaving. Some moditications to the pump modules are described. G H Diblin and R P Shellis, Lab Practice, 26 (4), 1975, 246. 21. PUMPS

AND

PUMP FLUIDS

21 1264. The memory effect for Argon in triode type sputter-ion pumps. (Germany) The memory effect of sputter-ion pumps is mainly noticeable when pumping noble gases because chemical sorption of these gases does not occur and pumping speed for these gases is relatively low. Partial pressure measurements on triode type sputter-ion pumps show that after pumping of pure Argon and of air the memory effect for Argon is insignificantly low. This applies to both the residual gas composition and the partial pressure composition when operating the pump at high pressures whereby considerable warming-up of the electrodes takes place. H Henning, Vakuum-Tech, 24 (2), 1975, 37-43 (in German). 1265. Design of a water-activated switch The design of a simple and inexpensive the overheating of oil in a water-cooled water supply failure is described. D Chopra and H Babb, Rev Sci Instrum,

for a diffusion pump. (US:; switch to safeguard against diffusion pump, caused by 46 (4), 1975,491-492.

21 1266. Simple device for measurements of the sticking coefficient of cryopumps. (USA) An apparatus is described which allows direct measurements of the reflection coefficient of cryopumps, the complement of the sticking coefficient. The reflected fraction of a gas flow directed towards a cryopump is measured directly. This gives accurate results especially for high sticking coefficients. Measured values 20.995 of the sticking coefficient of nitrogen gas on a 16 K cryopump are presented as a runction of the gas flow per unit area of the pump. The density of the condensed layer is nearly the same as for solid nitrogen ice. (Netherlands) A H M Habets et al, Rev Sci Instrum, 46 (5), 1975, 613-616. 21 1267. A magnetic electrical-discharge pump. (USSR) A magnetic electrical-discharge pump designed to increase the starting pressure and ensure stable operation at pressures below 10m3 torr by improved anode-cooling efficiency is described. The anode is made in the form of a spiral tube with a coolant passing through it. This arrangement enables the discharge current to be increased, and the sorption of gases by the cooled films of active metal formed on the anode is thereby intensified. This system is suitable for use in the evacuation of electrical-vacuum devices, such as low-noise travellingwave tubes and parametric amplifiers, since as well as giving a high limiting vacuum it facilitates variations in pumping rates. B G Bekker and S V Solov’ev, Soviet Parent, class H 01 j, 41 /OO,NO. 371633, claimed 15 April 1968, publd 22 February 1973 (in Russian). 455