Most plasmas have a very thin sheath compared with the plasma dimension. This necessitates separate calculations of the plasma and sheath. The Bohm criterion provides the boundary condition for calculation of plasma profiles. To calculate sheath properties a value of electric field at the plasma-sheath interface has to be specified in addition to the Bohm criterion. The value of the boundary electric field and robust procedure to approximately patch plasma and collisionless sheath with a very good accuracy are reported.
In this paper, the central concern is to analyze the influence of the forces that controls the ion dynamics inside a magnetized plasma sheath under collisionless conditions. The ionization effects are considered within the sheath. The magnetic field is tilted in x-z plane and makes an angle with the x-axis. The motivation of the paper is to see the effect of both field strength variation and the variation of the inclination angle on the force fields inside the sheath. The pitch length and pitch angle for the particle velocity field is also calculated and has been found to vary widely with the …
Ion acceleration due to the interaction between a short high-intensity laser pulse and a moderately overdense plasma target is studied using Eulerian Vlasov-Maxwell simulations. The effects of variations in the plasma density profile and laser pulse parameters are investigated, and the interplay of collisionless shock and target normal sheath acceleration is analyzed. It is shown that the use of a layered-target with a combination of light and heavy ions, on the front and rear side respectively, yields a strong quasi-static sheath-field on the rear side of the heavy-ion part of the target. Thi…
A review of the theoretical and computational aspects of plasma-wall transition is presented. The conditions for the existence of plasma sheaths in front of a solid surface are established. Various regimes are analyzed -collisionless and collisional. fluid and kinetic, with and without ionization. The partition of the plasma-wall transition into distinct regions (Debye sheath, quasineutral presheath. magnetic presheath, etc...) is also discussed. Applications to the theory of electrostatic probes are illustrated by means of a recently developed kinetic model. The influence of the probe on ion …
Collisionless Bulk Electron Heating in Resonant Low-Voltage Capacitively Coupled Plasmas
Institute for Plasma Research（等离子体研究所）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Princeton Plasma Physics Laboratory（普林斯顿等离子体物理实验室）；Department of Physics, Indian Institute of Science（印度科学学院物理系）；School of Physical Sciences, UM-DAE Centre for Excellence in Basic Sciences, University of Mumbai（孟买大学基础科学卓越中心物理科学学院）
We propose a theoretical ground for emissive capacitively coupled radio-frequency plasma sheath under low pressure. The rf sheath is assumed to be collisionless, and oscillates with external source. A known sinusoidal voltage instead of current is taken as prerequisite to derive sheath dynamics. Kinetic studies are performed to determine mean wall potential as a function of secondary emission coefficient and applied voltage amplitude, with which the complete mean DC sheath is resolved. Analytical analyses under homogeneous model and numerical analyses under inhomogeneous model are conducted to…
A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction ($T_{e\parallel}$) is lower and drops towards the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field ($T_{e\perp}$) not only is higher but also spikes up in the sheath. This abnormal behavior of $T_{e\perp}$ spike …
A new type of proton acceleration stemming from large-scale gradients, low-density targets, irradiated by an intense near-infrared laser is observed. The produced protons are characterized by high-energies (with a broad spectrum), are emitted in a very directional manner, and the process is associated to relaxed laser (no need for high-contrast) and target (no need for ultra-thin or expensive targets) constraints. As such, this process appears quite effective compared to the standard and commonly used Target Normal Sheath Acceleration technique (TNSA), or more exploratory mechanisms like Radia…
The sheath formation in a weakly magnetized collisionless electronegative plasma consisting of electrons, negative and positive ions has been numerically investigated using the hydrodynamic equations. The electrons and negative ions are assumed to follow Boltzmann relation. A sheath formation criterion has been analytically derived. The paper focuses on studying the sheath structure by varying the electronegativity. It has been observed that the presence of negative ions has a substantial effect on the sheath structure. The observations made in the present work have profound significance on pr…
A careful study of plasma-material interactions is essential to understand and improve the operation of devices where plasma contacts a wall. Key contributions of this work include (i) novel continuum kinetic algorithms with novel boundary conditions that directly discretize the Vlasov/Boltzmann equation using the discontinuous Galerkin method, (ii) fundamental studies of a plasma sheath physics with collisions, ionization, and physics-based wall emission, and (iii) theoretical and numerical studies of the linear growth and nonlinear saturation of the kinetic Weibel instability, including its …
A high energy density plasma embedded in a neutral gas is able to launch an outward-propagating nonlinear electrostatic ionization wave that traps energetic electrons. The trapping maintains a strong sheath electric field, enabling rapid and long-lasting wave propagation aided by field ionization. Using 1D3V kinetic simulations, we examine the propagation of the ionization wave in the presence of a transverse MG-level magnetic field with the objective to identify qualitative changes in a regime where the initial thermal pressure of the plasma exceeds the pressure of the magnetic field ($β>1$).…
When an unmagnetized plasma comes in contact with a material surface, the difference in mobility between the electrons and the ions creates a nonneutral layer known as the Debye sheath (DS). However, in magnetic fusion devices, the open magnetic field lines intersect the structural elements of the device with near grazing incidence angles. The magnetic field tends to align the particle flow along its own field lines, thus counteracting the mechanism that leads to the formation of the DS. Recent work using a fluid model [P. Stangeby, Nucl. Fusion {\bf 52}, 083012 (2012)] showed that the DS disa…
The method of matched asymptotic expansions is applied to the problem of a collisionless plasma generated by UV illumination localized in a central part of the plasma in the limiting case of small Debye length $λ_D$. A second-approximation asymptotic solution is found for the double layer positioned at the boundary of the illuminated region and for the un-illuminated plasma for the plane geometry. Numerical calculations for different values of $λ_D$ are reported and found to confirm the asymptotic results. The net integral space charge of the double layer is asymptotically small, although in t…
The thin-shell instability has been named as one process, which can generate entangled structures in astrophysical plasma on collisional (fluid) scales. It is driven by a spatially varying imbalance between the ram pressure of the inflowing upstream plasma and the downstream's thermal pressure at a non-planar shock. Here we show by means of a particle-in-cell (PIC) simulation that an analogue process can destabilize a thin shell formed by two interpenetrating, unmagnetized and collisionless plasma clouds. The amplitude of the shell's spatial modulation grows and saturates after about ten inver…
The results of an analysis of the physics of levitation of negatively charged dust particles over a surface (wall) in an inverse sheath are reported. It is shown that under suitable parameter regime, the ion-drag force may balance the combined electrostatic and gravitational force on the dust particles owing to its hollow profile as one moves away from the surface. Our analysis shows that the parameter regimes in which such a situation may result is realizable in laboratory and space plasma environments, particularly the near-surface dayside lunar plasma. The lunar surface and dust grains are …
Transport in collisionless plasmas is usually called anomalous, being due to the interaction between the particles and the self-generated turbulence by their collective interactions. Because of its relevance for astrophysical and space plasmas, we explore the excitation of turbulence in current sheets prone to component- or guide-field reconnection, a process not well understood, yet. We analyze the anomalous transport properties by using 2.5D Particle-in-Cell (PiC) code simulations. We split off the mean, slow variation (in contrast to the fast turbulent fluctuations) of the macroscopic obser…
