Magnetized Plasma Sheath Research Articles

The ion density distribution in a magnetized plasma sheath

The ion density distribution of plasma sheath in an oblique magnetic field is investigated with a fluid model. We performed numerical simulations of the sheath. The results reveal that the magnetic field has significant effects on the plasma sheath, including ion density distribution and space charge density distribution. Two cases of ion incidence are considered here. Under suitable conditions, Lorentz force induces fluctuations in the ion density. And the magnetic field parallel to the board is responsible for these changes. The action states of ions are more complicated while the ions enter the sheath with an oblique incidence angle. Ions could gather in some regions, so that it leads to small peaks of the density curve. Also the space charge density in such regions is slightly higher.

Studies on waves and instabilities in a plasma sheath formed on the outer surface of a space craft

Using the normal mode analysis, the number of possible modes is obtained in a magnetized inhomogeneous plasma sheath formed during the motion of a space craft which consists of negative ions (due to dust) along with the positive ions and the isothermal electrons. In addition to three propagating modes with phase velocities λ1, λ2, and λ3 such that λ1<λ2<λ3, two types of instabilities with growth rates γ1 and γ2 also occur in such a plasma sheath. The growth rate γ1 is increased with the negative to positive ion density ratio r0, ion temperature T, and obliqueness θ of the magnetic field B0. The growth rate γ2 of the other instability gets lower with the density ratio r0 but gets higher with the temperature T. The growth rate γ2 is sensitive to the temperature T, whereas the growth rate γ1 gets prominently changed with the density ratio r0. The increase in the growth rate γ1 with the obliqueness θ is more pronounced under the effect of stronger magnetic field. On other hand, the phase velocity λ1 shows weak dependence on r0 and T (though it gets larger) but it gets significantly changed (increased) for the larger obliqueness θ. The phase velocity λ2 gets larger with r0, B0, and θ and becomes lower for the higher temperature T. The phase velocity λ3 is decreased for the higher values of r0 and B0 and is increased for the larger values of T and θ.

A magnetized plasma sheath where the ion collision frequency depends on ion flow velocity

Recently some correlative works have been done to investigate the effects of collisions between ion and neutral gas atoms on the characteristics of plasma sheath in an external magnetic field. In general, the momentum transferring cross section (and thus the ion collision frequency) has a power law dependence on the ion flow velocity in the depth direction. Usually in the literature, constant collision frequency and constant mean free path are treated. Here, by using a collisional fluid model, we investigate the characteristics of a magnetized plasma sheath where the ion cross section depends on the ion flow velocity. The numerical calculations show that the effects of collisions on ion characteristics are more powerful when the momentum transferring cross section is constant. However, some parameters such as the electron density distribution are independent of this dependence.

Lateral motion of a dust particle in magnetized plasma sheath

In this paper, lateral mobilization of a dust particle through application of an oblique magnetic field in a plasma sheath with infinitely long lateral dimensions is studied. A weakly collisional two-fluid model is implemented to simulate the plasma. A single submicrometer-sized particle is released at the sheath edge with zero initial velocity and is tracked in Lagrangian frame, taking into account various forces. Various characteristics of the particle lateral motion such as time history of its horizontal position, velocity, and acceleration components are presented and discussed. Through investigation of power spectra of plasma and particle variables, the particle drift in the horizontal plane is explained. While varying parameters such as magnetic field intensity, particle density, and its radius, the particle horizontal velocity components are calculated. The variation of these velocity components is then explained by considering the ion velocity components at the particle location.

The active magnetized plasma-sheath over a wide range of collisionality

Recent work by Sternberg and Poggie (2004 IEEE Trans. Plasma Sci. 32 2217), concerned with the structure of an active collisionless magnetized plasma-sheath, is extended to include the effect of collisions on the ion motion. In the collisionless case an expression for the eigenvalue of the problem analogous to that obtained by Franklin and Ockendon (1970 J. Plasma Phys. 4 371) for an active unmagnetized plasma is obtained. As is to be expected with increasing collisionality, increased transport of ions across the magnetic field lines means that the effect of the magnetic field is diminished. Special attention is given to the case when the magnetic field is nearly parallel to the wall.

Dust particle dynamics in magnetized plasma sheath

In this paper, the structure of a plasma sheath in the presence of an oblique magnetic field is investigated, and dynamics of a dust particle embedded in the sheath is elaborated. To simulate the sheath, a weakly collisional two-fluid model is implemented. For various magnitudes and directions of the magnetic field and chamber pressures, different plasma parameters including the electron and ion densities, ion flow velocity, and electric potential are calculated. A complete set of forces acting on the dust particle originating from the electric field in the sheath, the static magnetic field, gravity, and ion and neutral drags is taken into account. Through the trapping potential energy, the particle stable and unstable equilibria are studied while the particle is stationary inside the sheath. Other features such as the possibility of the dust levitation and trapping in the sheath, and the effect of the Lorentz force on the charged dust particle motion are also examined. An interesting feature is captured for the variation of the particle charge as a function of the magnetic field magnitude.

Characteristic behaviour of dust grains in a magnetized plasma sheath

Based on quasipotential analysis, the Sagdeev potential equation has been derived to study the sheath characteristics in magnetized plasma. Theoretical studies have tried to determine if there are new features in the sheath region. Because of the ubiquitous presence of dust in every plasma environment, our motivation is to estimate the interaction of charged dust grains levitated into the magnetized plasma sheath. We have calculated the size of dust grains, the generation of surface potential and the net forces acting on it inside the sheath, for some chosen laboratory plasma parameters. The estimation of dust dynamics in the sheath should take into account that it is controlled by its properties like size, surface potential and the force acting on it, besides the ambient plasma properties. The overall observations of the forces on different dust sizes enable us to obtain information about the formation of dust atmosphere.

Dynamics of dust particles in a magnetized plasma sheath in a fully ionized space plasma

The dynamics of negatively charged dust particles in sheath formed in plasma similar to those observed in moons, asteroids, and other atmospheres in planetary system have been studied. The ambient conditions are based on the consideration of a static magnetic field in the dynamical system. It is assumed that the dust has been injected into the sheath by the impact of meteorites. Some charged dust particles of very small size might be drawn into the sheath by the electrostatic levitation also. The forces on the dust particles are assumed to be from the electric field in the sheath, the static magnetic field and from gravitation. It is expected that the charged dust particles would describe a slanted helix with variable pitch in the sheath. The size of the dust particles is likely to influence the progress of the ejected dust particles in the sheath. The size of the charged dust particles should determine whether they escape the sheath or fall back to the ground or form a cloud in the sheath.

Effects of energetic electrons on magnetized electrostatic plasma sheaths

The influence of energetic electrons on magnetized plasma sheaths is studied for different current regimes and different secondary-electron emission coefficients. (Here, the term “plasma sheath” denotes the collisionless region consisting of the non-neutral Debye sheath and the quasi-neutral magnetic presheath.) It is shown that the presence of even a small population of energetic electrons can significantly influence the potential drop across the sheath and the energy flux to the wall. For example, for plasma parameters typical of contemporary tokamaks, the presence of a fast-electron population with density smaller than 0.1% (!) can double the potential drop across the sheath and the energy flux to the wall, and the presence of a few percent of fast electrons can enhance these values by up to one order. The effect of fast electrons decreases with increasing secondary-electron emission coefficient and increasing current to the wall. Analytical results obtained are checked against particle-in-cell (PIC) simulations for different current regimes and different secondary-electron emission coefficients, showing good agreement except in some cases where the simulation results exhibit strong fluctuations.

Theory of magnetized, coupled, rf-driven plasma sheaths in one dimension

A set of equations for magnetized plasma sheaths in rf capacitance discharges is developed within the adiabatic electron, fluid ion framework. It differs from previous approaches in that (a) ion demagnetization, i.e., the detachment of the ion flow from the magnetic lines caused by electrostatic gradients, is introduced self-consistently, (b) the ion injection velocity from the presheath is reevaluated to ensure the ion-electron flux balance, and (c) two symmetrically opposed coupled sheaths are driven by a sinusoidal driving voltage instead of a sinusoidal current. It is found that the sheath potential and thickness increase considerably with increasing magnetic inclination \ensuremath{\theta} relative to the electric field compared to the unmagnetized results; the latter are recovered at the parallel magnetic field limit. Also, the ion injection velocity along the magnetic lines is subsonic and depends on the magnetic inclination. Finally, a sinusoidal ac voltage drives an anharmonic sheath current, while a sinusoidal current drives anharmonic voltage.

Unstable particle drift across a magnetic field caused by oblique electric field gradients

The presence of a positive electric field gradient e dE/dx>0 oblique to a uniform magnetostatic field is shown to cause unstable particle motion across the magnetic lines. Both the drift velocity along ∇E and the E×B drift velocity exponentiate in time with growth rate proportional to |dE/dx|1/2 sin θ, where π/2−θ is the angle between E and B. Thus the cross-B transport due to ∇E is more severe than the ∇B and ∂E/∂t effects, that cause constant drift velocity under uniform field variation. The result has implications to a variety of situations involving oblique electric gradients, such as magnetized plasma sheaths, large amplitude drift waves, and possibly tokamak edge transport.

Particle simulation of the magnetized rf plasma sheath

In the sheath regions strong electric fields can develop that accelerate ions to sufficient energy to cause sputtering and increase the plasma impurities to an intolerable level. Simulations of these sheaths are performed with a one-and-two-half-dimensional (y,vx,vy,vz) electrostatic particle model with a homogeneous magnetic field tilted in relation to y; guiding-center electrons and full dynamic ions are used. The plasma is taken to be bounded by metallic walls in which a time-dependent potential difference V(t) is applied and the average energies and flux of ions hitting the walls are computed. The results show that the magnetic fields inhibits the sputtering caused by D atoms. Comparisons of the present model with previous work, which used a Vlasov model and fluid approach, are also made with good agreement in the appropriate regime.

Radiation through an inhomogeneous, magnetized plasma sheath

Radiation through an inhomogeneous, magnetized plasma sheath