A Hybrid Particle Method for the Kinetic Treatment of Magnetized Plasma Sheaths using Full Orbit to Guiding Center Conversion

The guiding center approximation is commonly adopted in fusion particle codes to resolve the motion of charged particles in the core and at the plasma edge of a magnetic fusion device. However, the guiding center approximation does not hold at regions where the field gradients are large, such as the plasma sheath and presheath formed in front of a material surface. Here we present a hybrid particle method, which converts the guiding center particles into finite-orbit particles once they reach a region close to the boundary, where the field gradients become large. The domain is divided in a guiding center region and a full-orbit region, and the conversion of the particle is performed both ways. The conversion requires the adoption of a ring-charge method to avoid discontinuities in the potentials. We present numerical results from an implementation of this new scheme within the hPIC Particle-in-Cell developed at Illinois. We show that the approach can accurately resolve the full-orbit physics of the plasma sheath, still allowing to treat the vast majority of the plasma bulk with the usual guiding-center approximation.