Particle Simulation Study of Dust Particle Dynamics in Sheaths

Abstract

Dynamics of dust particles in a divertor plasma is simulated using numerical solutions of dust momentum and charging equations in an electrostatic sheath and an ionizing presheath, where an electrostatic force and a drag force due to absorption of plasma ions are taken into account. Spatial distributions of plasma parameters in a divertor are obtained with particle simulations including ionization by electron-impact. We found that the critical dust radius exists. Dust particles with a larger radius than this critical one are incapable to come off a divertor plate because the ion drag force is stronger compared to the electrostatic force. Control of the plate potential can suppress the dust motion from the plate. The radii of heavy dust particles are classified with respect to equilibrium positions. Considering dust particles started at the plate with radii smaller than the critical one, two types of dust motion were found. Dust particles with the first type of motion come back to the plate before approaching their equilibrium positions. The second type of motion is presented by smaller dust particles which oscillate around their equilibrium positions.

Dust in magnetic fusion devices

“…Thus, the distribution of dust mobilization speeds along with dust composition and size plays a crucial role in the dust ability to deliver its mass deep into the plasma [135,223]. The initial speed needed for micrometer dust to leave a plasma contacting surface and the adjacent plasma sheath region was estimated as a few m s −1 [106,226]. A dust grain can attain such initial speed either due to specific production mechanisms releasing internal stresses in dust forming material or due to acceleration of dust by plasma along the wall with subsequent scattering on wall surface edges and roughness.…”

Modelling of dynamics and transport of carbon dust particles in tokamaks

“…The heating power due to the absorption of neutral particles can be obtained from (10) or (11) by substituting χ = 0. As previously in [11], in the dust heat balance (9) we also take into account neutralization of ions at the dust surface, backscattering of incident plasma particles, cooling energy fluxes associated with dust ablation and with thermal radiation. We assume that neutralization probability for ions on the dust surface approaching 100% [18].…”

“…Efforts are ongoing to understand the behaviour of dust in tokamaks during discharges. The launching of dust from plasma exposed surfaces and the mechanisms of dust acceleration in sheaths have been analysed in [9,10], and further aspects of the dynamics of dust in edge plasmas of fusion devices have been simulated in [11]. Recently, experiments on the tracking of dust particle trajectories during discharges in tokamaks have begun [12].…”

Gravitational Effect on Release Conditions of Dust Particle from Plasma-Facing Wall

“…In this study we theoretically investigate the effect of the gravitational force on release conditions of the spherical dust particles attached on the plasma-facing wall, which is important to understand the behavior of the dust in the divertor plasma. We have estimated the release conditions of the spherical dust particles from the plasma-facing wall without the gravitational effect [4], where the threshold wall potential and the critical dust radius were discussed. In this analysis, the effects of the gravitational force directed toward or from the wall are discussed.…”