PIC modeling of the sheath-within-a-sheath problem for adhered tokamak dust

The emergence of dust - condensed matter particulates with sizes ranging from few nm to hundreds of μm - is a relevant operational and safety issue for ITER and future nuclear fusion reactors. Among the various dust release mechanisms, dust remobilization is defined as the mobilization towards the tokamak vessel of dust residing on a plasma-facing component location away from its original production site. In particular, the present understanding of dust remobilization under normal tokamak operation upon the action of plasma-induced forces is still insufficient. The major uncertainties arise from the quantitative assessment of plasma forces exerted on adhered dust, since the main assumptions of conventional analytical theories are expected to be violated due to overlapping inhomogeneities in the density and electrostatic potential profiles - the so-called sheath-within-a-sheath problem. We present a numerical approach capable of reliably quantifying plasma-induced forces and torques on adhered dust, based on Particle-In-Cell simulations with CPIC, a code developed at the Los Alamos National Laboratory, which is precisely suited for multiscale plasma-object interaction studies. New numerical diagnostics that post-process the solution of the Vlasov-Poisson system in order to calculate plasma forces and torques acting on arbitrarily shaped dust grains are implemented. A series of benchmarking tests based on the comparison with existing analytical or semi-analytical results for certain idealized scenarios are presented. Then, the code is applied to the problem of a single perfectly spherical and conducting dust grain attached to a perfectly planar and conducting wall under the assumptions of unmagnetized plasma, smooth surfaces, perfectly absorbing boundaries and non-emissive dust. It is revealed that plasma forces can tend to lift up grains with radius much smaller than the electron Debye length, but are not expected to overcome adhesion. In contrast, larger dust is compressed towards the wall by the plasma.