Session JP16: Poster Session: Fundamental Plasmas: Sheaths (2:00pm - 5:00pm)

Simulations of Thermionic Emission, Collisions and Ion Trapping in Multidimensional Plasma Systems, and Illustration of a Technique to Identify the Sheath Regime Around Emissive Probes

Using a recently developed 2D-2V kinetic continuum code designed to study effects of strongly emitting surfaces on plasmas with collisions, we have shown that multiple dimensions introduce new sheath effects [1] that are not captured in previous 1D simulations of emitting surfaces. Analysis of the sheath physics, current flow and potential distributions in various 2D configurations including floating and biased emissive probes, filament discharges, and surfaces with nonuniform emission have demonstrated multidimensional extensions of inverse and space-charge limited sheath regimes. Using emissive probe simulations, we introduce a methodology that one could use in experiments to differentiate classical, space-charge limited and inverse regimes by the probe's current response. [1] G. R. Johnson, M. D. Campanell, and M. V. Umansky ``Effects of emitting surfaces and trapped ions on the sheath physics and thermionic current flow in multidimensional plasma systems'', submitted   

The effect of a collisional presheath on the Bohm criterion

The plasma exit flow speed at the sheath entrance is constrained by the Bohm criterion. The so-called Bohm speed regulates the plasma particle and power exhaust fluxes to the wall, and it is commonly deployed as a boundary condition to exclude the sheath region in modeling. Traditional Bohm criterion analyses invoke equation of state, and thus ignore transport physics of both collisionless and collisional origin. Previously we have found that the electron parallel heat flux has a critical role in setting the Bohm speed. Here we perform both collisional VPIC simulation and theoretical analysis to decipher the subtle and important roles of thermal force and energy exchange between the parallel and perpendicular degrees of freedom. Although thermal force enters the Bohm criterion analysis straightforwardly since it is a gradient of temperature, similar to heat flux, the collisional energy exchange between parallel and perpendicular temperatures is less obvious. The collisionality dependence of electric field at the sheath entrance makes this physics important even when the collisionality is lowered. This work was supported by the U.S. DOE Office of Science through the Tokamak Disruption. Simulation (TDS) SciDAC project.   

Effects on sheath dynamics of electron reflection from a material boundary

When a plasma sheath forms next to a dielectric wall, material properties influence electron absorption and reflection from the surface, impacting the sheath formation and structure. This interaction may be modeled from quantum mechanical first principles. This work implements the resulting reflection function as a boundary condition in continuum kinetic sheath simulations using direct discretization of the Vlasov-Maxwell equations. The discontinuous Galerkin method is used with the Gkeyll code in this work. The resulting sheath dynamics are examined in parameter regimes relevant to Hall thruster and fusion applications using multiple wall materials.   

Kinetic Analysis of the Collisional Layer

To understand plasma behaviour in the scrape-off layer, we need to know the boundary conditions for the plasma and electromagnetic fields near a divertor. At the boundary, in the direction normal to the wall, there are four length scales of interest, the Debye length $\lambda_D$, the ion gyroradius $\rho_i$, the projection of the collisional mean free path in the direction normal to the wall $L_N$ and the device size $L$. Assuming that the plasma near the divertor satisfies $\lambda_D\ll\rho_i\ll L_N\ll L$, we can split the plasma-wall boundary into three layers\footnote{K-U Riemann, \textbf{J. Phys. D: Appl. Phys.} 24:493, 1991}. At distances of order $\rho_i$ from the wall the plasma is collisionless and the distribution is far from Maxwellian. At distances much greater than $L_N$ from the wall, Braginskii fluid equations are used to model the plasma, since collisionality is high and the distribution is close to Maxwellian\footnote{P Ricci et al., \textbf{PPCF} 54:124047, 2012}. We focus on the collisional layer of width $L_N$ that connects these two regions. We use a Galerkin method to numerically solve the ion drift kinetic equation in one spatial dimension, with the full Fokker-Planck collision operator, and the quasineutrality equation with adiabatic electrons.   

Influence of Oblique Magnetic Angle on ExB drift in the Magnetic Presheath

Magnetically confined plasmas will always be subject to ExB drifts at physical boundaries due to sheath potentials. While electrostatic sheaths are often on the order of millimeters, the magnetic presheath, predicted by Chodura, can extend centimeters further into the plasma. This presheath supplies a potential which couples to the background magnetic field to give rise to an ExB drift. In this work we measured this effect as a function of incident magnetic field angle by inserting a rotatable plate into a helicon plasma. Using laser induced fluorescence and diagnostic probes we measure ion and electron temperatures, flows, and densities at different oblique magnetic field angles and distances from the plate. Our measurements demonstrate a magnetic field angle dependence of the magnitude of the ExB drift. The measurements also show the extent the magnetic presheath penetrates into the plasma. The location of the magnetic presheath boundary has been disputed in theory, but drift speed measurements provide a new backing for predictions.   

Afterglow Plasma with Adiabatic Electrons

We study, by numerical and analytical means, the evolution of a collisionless plasma initiated between absorbing walls. The ensuing flow is described by rarefaction waves that travel inward from the boundaries, interact, and eventually vanish after crossing through, leading up to the asymptotic stage of the decay. Particle simulations indicate that the kinetic evolution strongly resembles one found in isentropic gas dynamics. Namely, a very gradual density profile forms in the expanding central region where the rarefaction waves interact, with an accompanying linear velocity profile. Asymptotically, the density falls off as $1/t$. The density and the flux at the boundary show little variation over the period when rarefaction waves still exist. Plasma potential, on the other hand, drops quite rapidly (on the underlying ion-acoustic time scale) to less than $T_e$ when over 70\% of the particles still remain in the system. This is due to electron kinetics being governed by conservation of adiabatic invariant in a slowly varying potential well. Analytical model of the velocity distribution is presented to explain the simulations. The results have implications for afterglow plasmas used in material processing and also for ion-extraction devices.