New developments in processing cathodic arc plasmas

Filtering of plasmas by curved solenoidal ducts is well established as a method of removing macroparticles. By analyzing the interactions of planar probes with the drifting plasma of the cathodic arc, new insights have been obtained into the operation of these ducts. Theoretical modeling of these interactions suggests, and experiment confirms, that the use of a separate biased electrode on the inside of the duct gives enhanced transmission without drawing excessive electron current. Theoretical modeling of a negatively biased planar electrode lying parallel to the drift velocity as well as experiment both show that ions are captured effectively onto the electrode producing a macroparticle free film at good deposition rates. The application of pulsed high voltage to the substrate placed at the exit of the duct is treated theoretically, and a model is proposed which gives a good agreement with the experimental concentration profile for a silicon surface coated and simultaneously implanted with titanium     

Quasi-monochromatic measurements of homogeneous arc plasmas

The refined diagnostic information obtainable by high-order spectrometry is illustrated by the results of quantitative measurements of a few rotational lines of OH in the ultraviolet spectrum of water-vapor plasmas generated in a wall-stabilized arc. Because of the high spectral and spatial resolution achieved in end-on measurements, the emission and also the absorption coefficients pertaining to homogeneous arc regions were obtained directly from measured line spectra—although the absorption was not measured explicitly—leading to the occupation of the upper and the lower state for the transition. The gas temperature was determined from the halfwidth of the Doppler-broadened rotational lines. The measured resolving power of the spectrometer was of the order of 400,000 in these measurements.     

Langmuir probe measurements in the sheath of a highly charged body

Summary We perform a model study of the electrical characteristics of a Langmuir probe moving in the field of a highly charged central body. The study is mainly motivated by a recent proposal, in the context of the Tethered Satellite System project, where the use of probes to study the space charge region surrounding a highly charged satellite is foreseen.

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Riassunto Si presenta un’analisi delle caratteristiche elettriche di una sonda di Langmuir nel campo elettrico di un corpo centrale ad alto stato di carica. Lo studio è soprattutto motivato da una recente proposta di esperimento per il progetto Tethered Satellite System dove si prevede l’uso di sonde per studiare la regione di carica spaziale attorno da un satellite a elevati potenziali.

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Измерения с помошью Ленгмюровского зонда в оболочке сильно заряженного тела

Резюме Предлагается анализ Электрических характеристик Ленгмюровского зонда, движушегося в поле центрального сильно заряженного тела. Йсследование, в основном, было обусловлено недавно предложенным Экспериментом в связи с проектом ? привязанной спутниковой системы ? в котором предполагается использование зондов для исследования области пространственного заряда, окружаюшей сильно заряженный спутник.

     

On the lowering of the electronic energy in model insulators due to surface reconstruction

Shubnikov-de Haas oscillations, piezoresistance, Hall mobility, and transverse “Hall” field due to mobility anisotropy have been studied on n-channel (111) Si inversion layers. The valley degeneracy was found to be 2 between 1.7 and 300 K. Under uniaxial mechanical stress the initially isotropic conductivity became strongly anisotropic. All results can be described by the existence of domains in the inversion layer.     

Modeling of impedance collapse in high-voltage diodes

Electron-beam diodes driven by fast-rising, high-voltage pulses often operate with cold cathodes for which the presence of a plasma adjacent to the cathode surface is essential to obtain adequate electron emission. A consequence of such surface plasma, however, is closure of the interelectrode gap by plasma motion. Resistive heating of the plasma competes with work performed in expanding the plasma and heat transfer to the cold-cathode boundary. The resulting closure speed is calculated, using an MHD code, and found to agree well with results of experiments using organic-cloth cathodes at 35 kV. Computed plasma speeds are typically 8-12 km/s, and are relatively insensitive to the applied voltage. Gap closure due to the plasma motion calculated numerically corresponds to estimates based on impedance collapse in the experiments     

A DC probe diagnostics for fast electron temperature measurements in tokamak edge plasmas

The tunnel probe is a new kind of Langmuir probe for use in the tokamak scrape-off layer. It provides simultaneous measurements of electron temperature and parallel ion current density with high frequency at the same point in space. We describe ongoing work to characterize the ion flows within the probe, and to calibrate the diagnostics using 2D kinetic simulations. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreus, Seitzerland, June 23–24, 2002.     

Surface arcing of insulators due to bentonite contamination

Impulse, alternating and direct voltage tests together with optical observations have been done under clean and polluted surface conditions with respect to bentonite, which is treated as a pollutant. The insulator materials tested were polymethyl methacrylate and polythene. Bentonite pollution may affect several surface flashover characteristics such as voltage at breakdown and 50% breakdown voltage under negative lightning impulse (1.2/50 μs) and the time to breakdown under wet condition for both negative and positive impulse voltage. There are four types of paths taken by the discharge channels, based on which the degree of degradation of the material surface may vary.     

Ultrafast relaxation of electrons and selfconsistent electrical charging in insulators

Low energy electron scattering in insulators is described by a Monte Carlo program based on acoustic and optical phonon scattering as well as on impact ionization of valence band electrons, especially aimed to the scattering of ballistic and drifting electrons in wide gap dielectrics. There is a rapid relaxation of excited electrons within the conduction band of wide gap insulators like SiO₂ and Al₂O₃ over femtoseconds. The field-dependent transport and trapping parameters allow us to model the selfconsistent charge transport and charging-up of insulating samples during electron irradiation. The resulting spatial and time-dependent distributions of all currents j(x,t), charges ρ(x,t), the electric field F(x,t) and the potential V(x,t) are obtained and compared with experimental measurements.     

Infrared absorption in ionic insulators due to multiphonon processes

The absorption β in ionic insulators due to multiphonon processes is examined for frequencies higher than several times the reststrahl frequency. The frequency dependence and temperature dependence of β are calculated for simple models which include approximations appropriate to ionic insulators: anharmonic potential and an estimate of the influence of a small quadratic term in the dipole moment operator. The frequency dependence of β is found to be approximately exponential with a slope given by the log of the ratio of atomic displacement to a distance characterizing the onset of the anharmonicity in the potential. The calculated values of β are found to be in good agreement with the experimental results for reasonable values of the parameters in the models.     

Dynamics of sheath evolution in magnetized charge-fluctuating dusty plasmas

The evolution of sheath in plasma contaminated with varying dust charges under the effect of an external magnetic field is studied. Study of Sagdeev potential through pseudoptential approach has been attempted with a view to deriving the sheath equation. Numerical analysis has been carried out to study the potential variation with sheath-ward distance for various plasma parameters. A unique finding of the study is that the presence of dust particles as well as the magnetic field drastically modifies the Bohm sheath criterion for plasma sheath formation as obtained earlier in unmagnetised two-component plasma. The results have more realistic interpretation in showing explicitly the interaction of magnetic field and impurity caused by dust charge variation, with the possibility of its impact in various technological applications including plasma-material interaction, material processing and electro-mechanical devices.     

Ion-sound turbulence due to shock gradients in collisionless plasmas

The dispersion relation for ion sound waves generated in a perpendicular shock is derived and the energy density of ion-sound turbulence is obtained using quasilinear theory. The result is compared with the lower hybrid turbulence generated under similar conditions. It is shown that ion-sound turbulence is a better candidate for the generation of type-I radio bursts in the solar corona.     

Calculation of radiative heat transfer in argon arc plasmas

Radiation emission and absorption in arc plasmas are important energy transfer processes. Exact calculations, though possible in principle, are usually impossible in practice because of the need to treat a large number of spectral lines and also the continuum radiation in the whole spectrum range. Recently, we have used an approximate method of partial characteristics to evaluate the radiation intensities, radiation fluxes and the divergence of radiation fluxes for SF₆ arc plasma with cylindrical symmetry. In this paper, we have extended our calculations toargon arc plasmas for the plasma pressures of 0.1, 0.5 and 1.0 MPa. We have calculated the coefficients of absorption for Ar plasmas at temperatures from 300 to 35 000 K, and have used these coefficients to calculate the partial characteristics. Both the continuum and the line spectra have been included in calculations. We have taken into account the radiative photo-recombination and bremsstrahlung for the continuous spectrum, and over 500 spectral lines for the discrete spectra.The method of partial characteristics has been applied to three-dimensional calculations of radiative heat transfer — i.e. radiation intensity, radiation flux and its divergence — in simplified temperature profiles. Conclusions have been made concerning validity and utilization of the method of partial characteristics in general gas dynamics problems.     

Spatio-temporal development of the filaments due to the thermal self-focusing instability near the critical surface in ionospheric plasmas

We study fully nonlinear spatio-temporal development of the thermal self-focusing instability of high-power radio waves near the critical surface of the ionosphere. These simulations improve on our earlier work by including an evolution equation for the density instead of using the assumption of constant pressure to determine the perturbed density connected with the known temperature perturbation. Using our two-dimensional nonlinear code we analyze the time scale and associated velocity for the development of the field-aligned irregularities as they spread from the critical surface both in the underdense as well as the overdense regions. The scaling of this velocity as a function of the radiated power of the heater electromagnetic wave (ERP) is determined. We also study the characteristic size of the self-focused filament as a function of ERP. Finally, the spectrum of the density and temperature fluctuations as well as modifications in the equilibrium values of these parameters for different values of ERP are presented. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 7, pp. 670–681, July 1999.     

Theory of the cathode sheath in a vacuum arc

A previous theory of the plasma sheath transition starting from the charge exchange model for ion collisions is extended to account for ionization and recombination. It is applied to the quasi-neutral boundary layer (presheath) in front of the cathode sheath of a vacuum arc. An essential potential and density difference between the sheath edge and cathodic plasma ball is found. This difference is accounted for in a unified theory of the arc cathode based on G. Ecker's (1971) existence diagram method, which indicates possible areas of arc operation in the T_cj plane, where T_c is the spot temperature and j is the current density. A numerical evaluation for Cu gives the results which are qualitatively similar to Ecker's theory. The existence areas are quantitatively enlarged and shifted to lower current densities     

Influence of vacuum space on formation of potential sheath in plasmas

Properties of potential sheaths developed in plasmas are investigated in terms of the plasma Debye length and the dimension of vacuum space. Biased plasma potential and the potential profile depend very sensitively on the geometrical configuration of plasma and vacuum space. The potential sheath is never developed near electrodes in high-density plasmas where the Debye length is much less than the dimension of the vacuum space. In this case, most of the potential drops occur in the vacuum space and almost no electric field exists inside the plasma. Parametric investigation of the potential sheath in terms of the vacuum-space and plasma dimensions is carried out.     

双麦克斯韦分布尘埃等离子体中尘埃粒子的充电研究

根据充电方程和电荷守恒条件,导出了双麦克斯韦分布的弱电离尘埃等离子体充电频率(电荷弛豫速率),给出了充电电流的计算公式. 对结果分析表明,定向运动速度大小对充电电流和充电频率有一定的影响,充电频率随着定向速度增加而减小,当定向速度远远小于电子的热速度时,充电频率与文献给出的表达式一致. 关键词： 尘埃等离子体 双麦克斯韦分布 充电频率