Avalanche breakdown in n-hexane

Classified abstracts 4X52-4363 be created only in a plasma where the floating potential is close to zero or positive with respect to the grcunded support of the Si sample. (Czechoslovakia) J Musil et al, J P/rvs D: Appl Plrys, 12 (5), 1979, L61-L63. 12 4352. Avalanche breakdown in n-hexane. (GB) Time lags to breakdown in n-hexane have been measured in high over-voltages, using - 100 ns width rectangular pulses. At a constant field, the time lags are divided into two types, N and B, at a ‘critical distance’ d,. Above d,, Laue plots have almost similar kinked-line characteristics; below d,, a straight line is obtained. From the prebreakdown current estimated from the voltage waveforms, the ionization coefficient, which increases with increasing applied fields, is estimated to be 6.4 x 10” cm-’ at 3.5 MV cm-‘, provided a single-electron avalanche is occurring. (Japan) K Arii et al, J Phys D: Appl Phys, 12 (5). 1979, 787-796. 12 4353. Electrode heating by the cathode spot of a vacuum arc. (GB) It is shown how the results of Daalder (1977) can be adequately explained using the previous theory of Beilis (Beilis Ed al 1972, Beilis 1974) for currents exceeding - 100 A. (USSR) M P Zektser and G A Lyubimov, J PI,& D: Appl P/IJ~s,12 (5), 1979, 761-763. 12 4354. Coherent non-linear coupling of ionization waves in a neon discharge. (GB) Coherent non-linear coupling of ionization waves, where the beatwave is out of resonance, is treated experimentally by using space correlation functions. The observed non-resonant interaction of ionization waves results in an onset of turbulent wave motion. (Czechoslovakia) J Krasa et al, J Phys D: Appl Phys, 12 (5). 1979, 723-728. IL

4355. Observations of electric double layers in a magnetized plasma column. (G B) Strong elcclric double layers are produced in a low-density plasma column confined by an axial magnetic field and maintained by singlcended inflow of plasma along the magnetic Field. The double layer evolves from an anode sheath, and ionization \vithin the sheath is shown to be a significant process in this conversion. Once the double layer has been formed, its axial position can be controlled by the external electric circuit. The layer exhibits an axial motion back and forth with amplitudes somcwhat larger than the layer thickness. (Sweden) S Torvcn and D Andersson,JP/rl*s D: Appl P/~J~s,12 (5). 1979, 717-722. 12 4356. Energy dependence of excitation in sputtering. (GB) Bombardment of Si by 50-500 keV Ar+ ions results in the emission of optical lines in the spectral range of the visible and vacuum ultraviolet. We present results of the measurements on the energy dependence of various lines emitted by sputtered atoms and ions. The results indicate that the photons are emitted by those atoms which are sputtered in a few collisions from the near-surface region. (Germany) R S Bhattacharya et al, J Phys D: Appl Phys, 12 (5). 1979, L55-L59. 12 4357. On sputtering in vacuum arcs. (GB) The effective condensation coefficient for the ion flux coming from the cathode spot region of a vacuum arc will be influenced by sputtering. Calculations for a copper vacuum arc indicate that sputtering alone will limit the effective ion condensation coefficient to, at most, 0.94 or 0.80 for copper surfaces at anode or cathode potential, respectively. (USA) H C Miller, J Phys D: Appl Phys, 12 (8), 1979, 1293-1298. 12 4358. Photographic observations of impulse breakdown in short vacuum gaps. (GB) An image intensifier and image converter with a maximum sweep speed of 1 ns mm-’ have been used to observe the development of impulse breakdown in short vacuum gaps of up to 1 mm. Electrodes of stainless steel, copper and aluminium have been used and it has been found that in each case, metal vapour is always produced first at the cathode surface and later at the anode surface: the final arc

discharge develops by propagation of the anode vapour. Time delays between voltage application and the appearance of the anode vapour have been measured in the range 10-50 ns and their dependence upon electrode separation and electrode material has been explained using a simple model involving anode heating by the emission current from a vapourized emitting site at the cathode. I D Chalmers and B D Phukan, J Phys D: Appl Phys, 12 (8), 1979, 1285-1292. 12 4359. On the role of the electron impact ionization and electron scattering cross-section in the breakdown strength of dielectric gases. (GB) In this paper we discuss the role of the electron impact ionization cross-section O,(E) and the electron scattering cross-section O,=(C) as a function of electron energy E in the breakdown strength of gases/ mixtures. Four gases (Ne, Ar, N1 and SF6) were chosen for which U,(E) and o.~(E) at low energies are known. Direct-current breakdown strength measurements on Ne, Ar, (Ne + SF& (Ar + SF6), (N, + Ne + SF6) and (N, + Ar + SFB) were made and are reported, using sphere-on-sphere to sphere and square-rod to plane electrode geometries. On the basis of these measurements and the data on u,,(c) and o,(c) it was concluded that the magnitude of o,=(c) at subionization (c < I) and especially at subexcitation (c < energy of lowest excited electronic state) energies is much more significant in effecting a high dielectric strength than is u,(c). The effect of perfluorination and static polarizability on O,(E) is also discussed. (USA) L G Cbristophorou et al, J Phys D: Appl Phys, 12 (8). 1979, 12231236. 4360. Plasma-wall interaction in flashtubes. (GB) A quasi-stationary arc has been established in a Rashtube filled with a rare gas. The current intensity was varied in the range 1-3 kA and the arc lasted about 1 ms. The plasma parameters were determined spectroscopically and are typically N, - lOI cme3 and T - 1 eV. This allows the study of non-ideal plasma effects. We have investigated how the plasma fills the tube by means of an ultra-rapid camera either in streak or in framing mode. Two types of phenomena are found: (a) oscillations of the luminous boundary at the end of the expansion (this is usually ascribed to shock wave interaction); (6) dark contractions from place to place, restricting the current cross-section to a narrow channel. From the velocity measurements, and from the influence of the initial pressure, the energy and the nature of the gas, some conclusions can be made on the physical origin of these contractions. For the experimental determination of the electrical conductivity, the stability of the plasma must be carefully analysed. (France) Y Vitel et al, J Plzys D: Appl Plr~rs, 12 (7), 1979, 1125-l 128. 12 4361. Electron swarm characteristics in an argon discharge. (GB) A numerical study is applied to the argon gas discharge by solving Boltzmann’s equation with two other limiting conditions. The gas parameters are obtained and compared with available experimental and theoretical data. The study suggests that at moderate values of E/P. i.e. less than 30 V cm-’ torr-I. a little more than one half of the total excitation energy, and at higher values of E/P about 0.3 of it, is expended in the excitation of the metastable states by the electron collisions in the gas. (Saudi Arabia) I A Ismail and A A Garamoon, J Phys D: Appl Phys, 12 (7), 1979, 1117-1123. .e.

4362. He-Cd hollow-cathode oscillations in an axial magnetic flerd: (GB) The influence of the axial magnetic field on the output of a hollowcathode Cd-He laser oscillating on the 533.7 and 537.8 nm lines is studied for various discharge conditions. The application of the axial magnetic field causes a decrease in the laser intensity. This behaviour can be explained by taking into account the action of the magnetic field both on the atomic transition and on the plasma inside the hollow cathode. (Romania) C P Cristescu et al, J Phys D: Appl Phys, 12 (6). 1979, 845-851. 1L

4363. Some characteristics of tungsten filaments operated as cathodes in a gas discharge. (USA) Some physical properties of tungsten filaments, when operated as cathodes in a gas discharge, are presented. For a 0.1524 cm dia 239