Experimental investigation of the plasma-wall transition

We report on measurements of the ion velocity distribution as a function of distance to a target immersed in a magnetized argon plasma. Two situations are investigated: (a) practically the whole plasma streams onto a large target, and (b) the size of the target is significantly smaller than the plasma diameter. The Mach number M=u/c_{s} decreases from M=1 at the target surface to values around 0.5 at a typical scale of lambda_{a}=30 mm and lambda_{b}=5 mm, respectively. In order to explain these small decay lengths, the measurements of case (a) are compared with a source-diffusion model and those of (b) to Hutchinson's model. In (a) good agreement between modeling and experiment is obtained assuming a low neutral gas temperature. The data in (b) also agree excellently with modeled profiles, although the large fitting parameter D=20 m;{2}/s indicates that other processes than diffusion contribute significantly to the transport.     

Phase transition in the collisionless damping regime for wave-particle interaction

Gibbs statistical mechanics is derived for the Hamiltonian system coupling a wave to N particles self-consistently. This identifies Landau damping with a regime where a second order phase transition occurs. For nonequilibrium initial data with warm particles, a critical initial wave intensity is found: above it, thermodynamics predicts a finite wave amplitude in the limit N-->infinity; below it, the equilibrium amplitude vanishes. Simulations support these predictions providing new insight into the long-time nonlinear fate of the wave due to Landau damping in plasmas.     

Numerical study of plasma-wall transition using an Eulerian Vlasov code

A one-dimensional Eulerian Vlasov code is used to study the self-consistent solution of a plasma facing a floating collector, in the absence of an external magnetic field. Both electrons and ions are treated with a kinetic equation. A Bhatnagar-Gross-Krook (BGK) collision term is used to describe the collisions. Acceleration of the ion flow at the Debye sheath entrance is observed together with the formation of a stable steep negative electric field in front of the floating collector. This negative electric field acts to accelerate the positive ions towards the plate, pushing back the negative electrons, such that at steady state the total current collected at the plate is zero. The codes are run for a sufficiently long time on the ions time scale to ensure the ions (argon) distribution function is reaching a steady state. For the different parameters used, the solution shows the existence of persistent regular oscillations of constant amplitude when the electron collisions are very small or negligible. These oscillations will be studied. The increase in the electron collisions damps these oscillations and helps the system reach an equilibrium.Received: 16 October 2003, Published online: 26 May 2004PACS: 52.65.Ff Fokker-Planck and Vlasov equation     

Analysis of the evolution of perturbations in a magnetized collisionless plasma with an integral-equation technique

Summary A technique recently proposed to study the classical problem of the evolution of small perturbations in a collisionless unmagnetized plasma is extended to a magnetized plasma. A time-convolutive integral equation for the plasma density is obtained from the Vlasov equation for a homogeneous plasma in a uniform, stationary magnetic field. The equation can be solved by means of simple numerical algorithms and, in some cases, analytical solutions can be obtained. The procedure proves to be analytically simpler than the classical one and is more convenient from a numerical point of view. Techniques of solution are presented and analytical and numerical results for electrostatic perturbations are discussed.     

Cauchy problem for the hydrodynamic equations of a collisionless cold plasma

A general solution is obtained for the nonstationary hydrodynamic equations of a cold plasma, containing as arbitrary functions the initial distributions or boundary values of particle velocities and densities. The evolution of initial perturbation density in an unbounded medium, and the mechanism of formation and discharge reversal, as well as dynamics of the flow injected into the plasma gap, are investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 53–58, October, 1988.The author is grateful to A. V. Zharinov and A. A. Rukhadze for their continuing interest in this study and for a number of useful comments.     

Exact solution of the problem of quasineutral expansion into vacuum of a localized collisionless plasma with cold ions

An analytical solution of the Vlasov equation for the electrons and the hydrodynamic equations for the ions in a self-consistent electric field is constructed in the quasineutral approximation. This solution is valid for a finite electron-to-ion mass ratio. It permits describing the expansion into vacuum of a collisionless plasma with cold ions and arbitrary initial electron velocity distribution, forming a plasmoid that is bounded and, in the general case, spherically asymmetric in space. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 543–547 (25 April 1998)     

Relaxation of heavy ions in collisionless shock waves in cosmic plasma

We report on the results of hybrid particle-in-cell simulation of shock waves (SWs) in the cosmic plasma with admixture of heavy weakly charged ions. The dependence of ion relaxation and the SW structure on the angle between the magnetic field and the normal to the wavefront is analyzed. The conditions for invariability of the anisotropic ion velocity distribution behind the front of quasi-transverse SWs are indicated on scales substantially exceeding the width of the collisionless SW front (up to the Coulomb relaxation length). The obtained results are essential for determining the effectiveness of heating of heavy ions and observation diagnostic of collisionless SWs in the cosmic plasma.     

Turbulent heating of electrons and ions in a collisionless shock wave

Turbulent heating of a nonisothermal plasma by a collisionless shock wave is analysed in the situation when a small-scale high-frequency instability occurs at the wave front. Effective time of electron and ion heating is estimated.     

The dielectric tensor and field equations in the inhomogeneous cold collisionless magnetized drift plasmas with elliptical cross sections

In this Letter the dielectric permittivity tensor and field equations in multi layer cold collisionless magnetized inhomogeneous drift plasma columns with confocal elliptical cross sections are investigated. It will be shown that the dielectric tensor of each region can be written as non-operational Hermitian and pure spatial operational parts.     

Accurate solution of the boundary problem for collisionless gas in the field of a plane gravitational wave

An accurate solution of the Cauchy problem is found for a general-relativity collisionless kinetic equation against the background of the metric of a nonlinear plane gravitational field with an arbitrary law of gas-particle reflection from a boundary of any specified form. It is shown that in the field of a gravitational wave, interaction of the gas with the boundary necessarily leads to the appearance of shock waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 41–45, December, 1985.It remains to thank the participants of the gravitation seminar at Kazan' State University for discussions of the work.     

Formation of steep gradients with plasma detachment at grazing B-field incidence at a plasma-wall transition

We use an Eulerian Vlasov code to study the problem of the formation of a plasma sheath with plasma detachment (i.e. no plasma transport to the wall) at a plasma-wall transition, in front of an absorbing wall. The ions are described by a kinetic equation in the full velocity space which integrates exactly the ion orbits. We consider a one-dimensional slab geometry, where the inhomogeneous direction is perpendicular to the wall. The electrons, assumed to only move along the magnetic field lines, are described by a parallel-B kinetic equation. The magnetized electrons are restricted to move along the magnetic field because the surface-parallel diamagnetic and electric drifts are equal and opposite. The present study shows how an ambipolar flow in the direction normal to the wall is established. For very small values of the angleα between the magnetic field and the plane of the wall, steep density and potential gradients are established in front of the plate surface with almost no plasma transport towards the wall. Presented at 5th Workshop “Role of Electric Fields in Plasma Confinement and Exhaust”, Montreux, Switzerland, June 23–24, 2002.     

Interference simulation in a cold collisionless moving magnetized plasma slab and (free surface of plasma slab)

Abstract

The pattern of intensity due to the interference in a cold collisionless magnetized moving plasma slab is investigated. Theoretically, it is assumed the mentioned layer has been located as a thin layer in an etalon Fabry–Perot interferometer surrounded by vacuum. The direction of external magnetic field is normal to the plasma surface and the plasma slab moves parallel with external constant magnetic field. By taking into account the relativistic considerations, the functions of transmitted intensity are presented coincident with the Airy function form in laboratory and plasma slab frames, respectively. The effects of plasma frequency, cyclotron frequency, thickness of plasma slab, and velocity of the plasma slab on band width, finesse factor, and visibility are simulated. Finally with the assumption that there are two wavelengths near together in incident electromagnetic beam the power resolution for this configuration are analyzed. All studies mentioned above have been done for S-polarized and P-polarized electromagnetic beams separately.     

Ion acoustic solitary waves with adiabatic ions in magnetized electron-positron-ion plasmas

Linear and nonlinear ion acoustic waves in the presence of adiabatically heated ions in magnetized electron-positron-ion plasmas are studied. The Sagdeev potential approach is employed to obtain the energy integral equation in such a mulitcomponent plasma using fluid theory. It is found that electron density humps are formed in the subsonic region in magnetized electron-positron-ion plasmas. The amplitude of electron density hump is decreased with the increase of hot ion temperature in electron-positron-ion plasmas. However, the increase in positron concentration and obliqueness of the wave increases the amplitude of nonlinear structure. The increase in positron concentration also reduces the width of the nonlinear structure in a magnetized multicomponent plasma. The numerical solutions in the form of solitary pulses are also presented for different plasma cases. The results may be applicable to astrophysical plasma situations, where magnetized electron-positron-ion plasma with hot ions can exist.