Simulations of the plasma-wall transition with oblique magnetic field, collisions, and electron emission

Secondary, thermionic and photoelectron emission are predicted to alter the sheath potential, heat transport and sputtering at plasma-facing surfaces such as tokamak divertor plates and probes. Although a number of publications have described studies of emitting sheaths in unmagnetized plasmas, only a few studies have included oblique magnetic fields, typically without collisions. Unmagnetized sheath simulations show that collisions cause ion trapping in the reversed-field region near an emitting surface. Thus it is important to consider how collisions affect the magnetized, emissive plasma sheath, where electron and ion gyromotions near the surface play a key role. By adapting the WarpX particle-in-cell simulation framework developed jointly by LBNL and LLNL we developed a simulation code that captures the combined effects of collisions, magnetic fields and electron emission on the plasma-wall interaction.  Different regimes of sheath behavior are observed as the governing parameters (such as particle temperatures, B field inclination angle, collision mean free paths, and emitted flux) are varied.