M. Stanojević

Fluid and kinetic parameters near the plasma-sheath boundary for finite Debye lengths

In a recent paper by Kuhn et al. [Phys. Plasmas 13, 013503 (2006)], it has been demonstrated that in a plasma, the polytropic coefficient γ is a spatially varying quantity rather than a global constant as usually assumed in fluid theory. Assuming cold ion sources and using the asymptotic two-scale approximation (in which the ratio of the Debye length over the characteristic presheath length, e, is set to zero), it was found that the γ profile exhibits a sharp peak (with values roughly between 6 and 8) at the plasma-sheath boundary. In the present paper, it is shown that in a finite e approach, this sharp peak is smoothed to a regular maximum, which for increasing e (e.g., decreasing plasma density) decreases and finally disappears. In any case, assumptions like γ=1 and/or γ=3, which are customarily encountered in the context of fluid approaches, are disproved. Although the present results were obtained for collisionless plasmas, it is reasonable to assume that the behavior uncovered holds qualitatively fo...

An investigation of the electrode current oscillations caused by the potential relaxation instability in a weakly magnetized discharge plasma

Oscillations of the electrode current, plasma density and plasma potential, caused by the potential relaxation instability in a weakly magnetized discharge plasma are studied. The oscillations appear when the electrode bias exceeds the plasma potential. As long as the difference between the electrode bias and the plasma potential is below approximately 2 V, that is approximately kTe/e0 in our experiment, the oscillations are periodic. When the electrode bias is increased above this value, the oscillations become irregular. The limit cycle in the phase space disappears, the power spectrum becomes broader and the autocorrelation time shorter. Presence of f- alpha noise is observed in the power spectra. Distribution of Fourier phases for electrode current, plasma density and plasma potential oscillations is shown. Dimensional analysis of the signals is performed using the Grassberger-Procaccia algorithm. The correlation dimension is found to be between 1.2 and 2.0 for low electrode biases and no saturation of the In(C(r)) versus In(r/r0) plots is found for higher electrode biases. Entropy K2 is plotted versus r/r0. The largest Lyapunov exponent is calculated using two different methods, one developed by Rosenstein and co-workers and the second by Wolf and co-workers. It appears that the largest Lyapunov exponent is positive and close to zero. An attempt to calculate the complete Lyapunov spectrum with the method of Kruel and co-workers is also made.

Fluid model of the magnetic presheath in a turbulent plasma

A fluid model of the magnetic presheath in a turbulent boundary plasma is presented. Turbulent transport corrections of the classical three-dimensional fluid transport equations, which can be used to study magnetic presheaths in various geometries, are derived by means of the ensemble averaging procedure from the statistical theory of plasma turbulence. Then, the magnetic presheath in front of an infinite plane surface is analysed in detail. The linearized planar magnetic presheath equations are applied to the plasma-presheath–magnetic-presheath boundary (i.e. the magnetic presheath edge), whereas the original non-linear planar magnetic presheath equations are used for the entire magnetic presheath, allowing for various sets of experimentally relevant free model parameters to be applied. Important new results of this study are, among others, new expressions for the fluid Bohm criterion at the Debye sheath edge and for the ion flux density perpendicular to the wall. These new results, which exhibit corrections due to the turbulent charged particle transport, can qualitatively explain the fact that whenever the angle between the magnetic field and the wall is very small (i.e. several degrees) or zero, electric currents, measured by Langmuir probes in the boundary regions of nuclear fusion devices and in various low-temperature plasmas, are anomalously enhanced in comparison with those expected or predicted by other theoretical models.

Mode suppression of a two-dimensional potential relaxation instability in a weakly magnetized discharge plasma

Abstract A potential relaxation instability (PRI) is modulated by an external signal using an additional grid to modulate the radial plasma potential profile in a magnetized plasma column in a linear magnetized discharge plasma device. It is observed that the electrode current oscillations follow the van der Pol equation with an external forcing term, and the linear growth rate of the instability is measured.

Determination of plasma parameters from plane-probe characteristics in a current-carrying plasma

The authors present an approximate method of determining the temperature and the drift velocity of electrons in a current-carrying plasma. An approximate expression for the determination of plasma density is also derived. The authors consider the probe characteristics obtained by a one-sided plane probe in a plasma in which the electron distribution may be assumed to be a shifted Maxwellian. The method they propose for analysing such characteristics can be regarded as a generalization of the standard probe method for the determination of plasma parameters in a Maxwellian plasma. Application of the method is illustrated by experimental examples. A confirmation of the method is performed by comparing the results with those obtained by other techniques.

Conditions for the existence of strong double layers

We present the results of a theoretical investigation of the conditions for the existence of a stationary, strong, monotonic double layer. A model that includes finite temperatures both of the particle species accelerated by the double layer and of those reflected by it is developed on the basis of the general theory of double layers due to Andrews and Allen. A numerical solution of the model equations is presented and analysed. Explicit approximate formulae for the electron-to-ion current ratio (i.e. the Langmuir factor) and for the initial velocities (i.e. directional energies of the accelerated particles at the double-layer boundaries before acceleration) are also derived that fit almost exactly the exact numerical solution over a wide range of double-layer potentials. It is shown that in the cases in which the standard results of Langmuir and Bohm are an oversimplification, approximate formulae may be used instead of exact numerical solutions to obtain quite accurate results in a simple manner.