Rotational motion of spherical dust in plasmas with magnetic fields

Abstract

The rotational dynamics of spherical, conducting dust grains in collisionless plasmas due to asymmetric momentum and current collection in the presence of a uniform magnetic field was modeled using the Monte Carlo code DiMPl. The dependence of the torque, equilibrium rotation rate, and equilibration timescale on the strength of the magnetic field and the surface potential was determined. The direction and magnitude of the rotational velocity were found to depend on the sign and magnitude of the surface potential. At a fixed positive charge, dust rotated anti-parallel to the magnetic field due to accumulation of angular momentum from electron impact while at the floating potential, dust rotated parallel to the magnetic field. This model was used to explain existing experimental observations of isolated dust grain rotation in low temperature discharges. In isothermal tokamak plasmas, equilibrium rotation rates of the order 105 Hz are anticipated to occur over time-scales of 20 ms which will have important consequences for dust dynamics.

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“…The net torque, consisting of competing electron and ion contributions which can also feature a different dust radius dependence of the collected particle flux, depends non-monotonically on R d and crosses zero for an intermediate R Le < R d < R Li . For collisionless plasmas, Monte-Carlo simulations of the net torque, also including contributions due to asymmetric current collection, have been carried out (Simons and Long 2021). It is also worth noting that for an appreciable effect on dust lifetime, the characteristic spin-up must be sufficiently small to allow multiple splitting events as the per event R d reduction is merely by the factor of (1∕2) 1∕3 .…”