7640. A double layer induced ionisation instability

Classified abstracts 7640-7648 40 7640. A double layer induced ionisation instability Low-frequency ( ~ 50(~ ~ 2000 Hz) oscillations are detected in a doubleplasma device in association with double layers in which the potential drop somewhat exceeds the ionisation potential of the gas. The oscillations are interpreted as ion-acoustic waves excited by an ionisation instability. A simple theory of instability is presented and its results are compared with observations. J C Johnson et al, J Phys D : Appl Phys, 23, 1990, 682-685. 40 7641. Plasma parameters in noble-gas narrow-tube and capillary-tube discharge, positive column plasmas The plasma parameters in noble-gas narrow-tube and capillary-tube plasmas have been studied experimentally over the pressure (p) range from 0.1 to 10 torr. Tube inner diameters, 2R, of 0.5 and 0.8 m m (capillary) and 3 m m (narrow) were used in the present investigation, together with axially movable anodes and cylindrical electrostatic double probes for the electric field and the electron temperature measurements, respectively, in noble gases, i.e. He, Ne, At, Kr and Xe. The results show that the axial electric field Ez/p and plasma density decrease with increasing pR and the atomic weight of noble gases. No significant tube radius effect has been observed for the axial electric field if we analyse results in terms of pR. The electron temperature decreases significantly with increasing pR for He and Ne, and depends slightly on pR for At, Xe and Kr. T Kaneda et al, J Phys D : Appl Phys', 23, 1990, 50(~503. 4O 7642. Departure from quasilinear beating in a cross-field ion beam experiment Ion heating by cross-field current-driven ion acoustic turbulence is investigated in a double-plasma device. The spatial heating rate OU/Ox of an ion beam is measured as a function of the integrated noise energy density T of broadband noise. At the lower values o f t the observations approximately follow the quasilinear prediction that OU/Ox (or OU/Ot) oc Tzc where zc is the autocorrelation time. As T approaches its highest observed values, ~U/Ox reaches saturation. At the transition between the quasilinear and saturation regimes, zc is of the order of the particle trapping time. R G Greaves and P J Barrett, Plasma Phys Controlled Fusion, 32, 1990, 213 219. 40 7643. Electron assisted glow discharges for conditioning fusion tokamak devices Glow discharge conditioning of tokamaks with graphite plasma-facing surfaces has been used to reduce impurities and obtain density control of the plasma discharge. However, a major operational disadvantage of glow conditioning is the high pressure required to initiate the glow discharge, e.g. ~ 70 mtorr for helium in DIII-D, which requires isolating auxiliary components that can not tolerate the high pressure. An electrongun-assisted glow discharge can lower breakdown pressure, possibly eliminating the necessity of isolating these auxiliary systems during glow discharge conditioning and allowing glow discharge operation at lower pressures. An electron-assisted glow discharge experiment has been carried out in a small v a c u u m vessel to evaluate whether such a system can be employed in the DIII-D tokamak. With an electron gun to produce an initial source of electrons, the pressure at which a helium glow discharge can be initiated has been decreased by two orders of magnitude. The glow was produced in a 0.40 m 3 Inconel test chamber (3.5 m z surface area) and was pumped with a 3301 s I turbomolecular pump. The electron gun consists of a tungsten filament and grid assembly. A n electron current of up to 18 m A with energies up to 1.6 kV has been used. The pressure is measured with a capacitance manometer, the gas composition with a residual gas analyzer and the electron temperature, density, and plasma floating potential with a Langmuir probe. With the addition of an electron current of 10 mA, the initiation pressure was reduced from 165 to 1.6 mtorr. The lowest sustaining pressure dropped from 3.0 mtorr in the absence of electron assist to 1.5 mtorr with 10 m A of electron current. K M Schaubel and G L Jackson, J Vac Sci Technol, A8, 1990, 3063 3066. 40 7644. Monte Carlo simulation of ionization in a magnetron plasma A Monte Carlo simulation of electrons emitted from the cathode of a planar magnetron is tested against experiments that were reported by Wendt, Lieberman, and M e u t h [J Vac Sci Technol, A6, 1827 (1988)] and by G u and Lieberman [J Vac Sci Technol, A6, 2960 (1988)]. Comparing 690

their measurements of the radial profile of current and the axial profile of optical emission to the ionization profiles predicted by the model, we find good agreement for a typical magnetic field strength of 456 G. We also find that at 456 G the product of the average n u m b e r of ionizations (N~) and the secondary electron emission coefficient ? is ~ 1. This indicates that secondary emission contributes significantly to the ionization that sustains the discharge. At 171 G, however, (N~) ? << 1, revealing that cathode emission is inadequate to sustain a discharge at a low magnetic field. J E Miranda et al, J Vac Sei Teehnol, A8, 1990, 1627 1631. 40 7645. Compressibility effects of the gravitational instability of a plasma-vacuum interface The stability of gravitational compressible surface modes of a p l a s m a vacuum interface is investigated. Stratified equilibrium profiles of density and magnetic field in the plasma are considered. The corresponding boundary conditions for magnetohydrodynamic linear perturbations are deduced. Three types of surface normal modes (slow, intermediate and fast) m a y appear. The slow mode is unstable below a critical wavenumber. The stability criterion is affected in general by the compressibility. The growth rate of the unstable modes is increased by compressibility. It is shown that for 'flute' or pure interchange modes with respect to the v a c u u m magnetic field (but not with respect to the equilibrium magnetic field in the plasma) the compressibility enlarges the instability domain of surface modes. The interval of existence of unstable surface perturbations, as well as their growth rates, are larger than those of the internal unstable modes. Intervals of non-existence of surface modes appear for the flute perturbations with respect to the plasma magnetic field. The critical values of the wavenumber below which the modes are unstable, are discussed in general. A G Gonzalez and J Gratton, Plasma Phys Controlled Fusion, 32, 1990, 3-19. 41. P L A S M A D I A G N O S T I C S 41 7646. Plasmas with negative ions--probe measurements and charge equilibrium Extensions of the electrostatic probe technique to situations with negative ions, both theoretical and experimental, are described. Charged particle distributions and the p l a s m a - s h e a t h boundary are discussed in relation to probe measurements. Plasmas with negative ions, their occurrence and interest, particularly situations where the negative ion density becomes comparable to or exceeds the electron density are described. Some data in negative ion containing plasmas are shown and discussed. H Amemiya, J Phys D : Appl Phys, 23, 1990, 99~1014. 41 7647. Determination of the velocity profile of a streaming highly ionized plasma by laser-induced fluorescence spectroscopy The velocity profile of a streaming hydrogen plasma with a temperature of 15 eV and a density of 3 x 1026 m 3 is measured by means of laserinduced fluorescence spectroscopy (LIFS). The LIFS method takes advantage of the fact that the wavelength of an argon fluoride excimer laser coincides with a transition within the singlet system of neutral carbon. The Doppler shift and width of the fluorescence radiation at 247.9 n m provide local values of the plasma velocity and the ion temperature. The measurements confirm earlier assumptions that the flow profile is determined by equilibrium of electrical forces and viscosity which allows one to extract information on the ion viscosity. J Giilker and A Stampa, Plasma Phys Controlled Fusion, 32, 1990, 163171.

V. Analysis of materials and surfaces 50. R U T H E R F O R D REACTIONS

BACKSCATTERING AND NUCLEAR

5O 7648. Computer based visualization of channelling directions at single crystal surfaces A qualitative geometric model is applied to the penetration of particles into single crystal surfaces where the substrate atoms are represented by hard spheres. F r o m the study of a large n u m b e r of model trajectories an opaqueness function Bhkt is determined yielding the probability of a