Three-dimensional cross-field flows at the plasma-material interface in an oblique magnetic field

Abstract

This article describes experimental evidence that the magnetic presheath is a fully three-dimensional structure modified by ion–neutral collisions. Velocity distributions of both ions and neutrals, obtained via laser-induced fluorescence, show that cross field ion drifts do not result from entrainment of ions in a flowing neutral background. Ion flows parallel to E×B arise and accelerate to as much as 0.2cs within several ion gyroradii of the boundary surface, where cs is the sound speed. Within measurement resolution, the onset of the E×B aligned flow occurs at the same distance to the surface that ions begin to deflect from travel along magnetic field lines. Collisional fluid and particle-in-cell simulations of the boundary region are compared to the experimental measurements. We find that, in contrast to the classical collisionless Chodura model, collisional effects between the ions and the non-flowing neutral population are essential to quantitatively predict the observed ion drift velocities. No momentum coupling between ions and neutrals, separable from noise and other effects, is observed in either signal. We discuss several explanations and implications of this observation.

Two-dimensional collisional particle model of the divertor sheath with electron emissive walls

“…From subplot (a), it can be seen that the required ion Mach number for a stable pre-sheath diminishes due to the effect of the ionneutral collisions, while electron collisions and wall-emission have negligible effects, except very close to the wall. This ion Mach number reduction has been observed, with quantitative similarities, by Thompson et al [8] in a recent publication, and predicted by collisional pre-sheath theory [5]. Regarding the electric potential, on the other hand, collisions have the effect of increasing the plasma potential by around 15 V. Electronwall emission due to both SEE and TEE, however, reduces this potential drop by a larger amount, so that, when taking all effects into consideration, the potential drop decreases with respect to the case with no effects.…”

“…Such distributions are particularly relevant for the induced wall sputtering and can deviate significantly from the optical (impact along the magnetic field direction) or fluid (impact along the average ion velocity) approximations, even in collisionless scenarios, as also predicted by ion orbit studies [7]. A recent work by Thompson et al [8] includes the effects of ion/neutral collisions in a similar 1D PIC code.…”

RF antenna helicity dependent particle heating in a helicon source

“…PHASMA is designed to study space plasma-relevant phenomena, including particle heating and acceleration at kinetic scales during magnetic reconnection, [17,18] electromagnetic instabilities driven by ion temperature anisotropy and plasma pressure [19], ion acceleration in expanding plasmas [20], and cross-field ion flows near plasma-material interfaces immersed in a magnetized, high-density plasma [21]. A key feature of PHASMA is the availability of volumetric, nonperturbative, laser diagnostics for ion and electron velocity distribution function measurements with spatial resolution at the kinetic, gyroradius, scale (∼mm for electrons) on both sides of the RF antenna.…”

“…A probe will collect electrons along the magnetic field lines it intersects in addition to electrons transported across field lines to the probe surface. If the electron gyroradius of the system is close to the probe dimensions, given by ρ e ∼ r p ln l p /r p where r p and l p are probe radius and length respectively, then current collection by the probe can be approximated by the Druyvesteyn method as detailed in previous work [21]. The Druyvesteyn method does not assume a particular distribution for the electrons when calculating n e and T e .…”