Stable potential formation with neutral and dust particles in expanding magnetic field near divertor plate

The effects of neutral and dust particles for the stable or monotonically changing potential formation in an expanding magnetic field toward the divertor plate was investigated in a quasi-neutral plasma by using one-dimensional kinetic analysis. Unless there are plasma ion sources, such as ionization of neutral particles, the required ion flow velocity for injected ions to form the stable potential has to be larger than an ion sound velocity, i.e. the generalized Bohm’s criterion. It was clarified that the plasma ion sources mitigate this requirement. On the other hand, since the dust particles decrease the plasma ion density due to absorption to them, the required ion flow velocity for injected ions increases. The numerical values of these effects are presented. The decreasing magnetic field in a plate direction also increases this required velocity

Particle Simulation Study of Dust Particle Dynamics in Sheaths

"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.