Dependence of the pickup-like ion effective heating on the poloidal and toroidal magnetic fields during magnetic reconnection

Abstract

The dependence of the ion effective heating on the poloidal (reconnection) and toroidal (guide) magnetic fields during magnetic reconnection in the presence of a guide magnetic field is investigated by means of particle simulations, which mimic merging plasmas in a spherical tokamak. In previous works, our simulations demonstrated that the ion temperature perpendicular to the magnetic field grows mainly in the downstream, in which ring-shaped velocity distributions are formed. This means that ions are effectively heated. The basic theory explains that the ring-shaped distribution is formed by the ions which rotate around the guide magnetic field while E × B drifting. In this work, the basic theory is extended to a more general theory including not only a ring-shaped distribution, but also a circular-arc-shaped distribution. The generalized theory explains that the effective temperature changes by the radius and the central angle of the arc-shaped velocity distribution and conjectures the dependence of the ion effective heating on the poloidal and toroidal magnetic fields. The simulations show that the ion heating energy is proportional to the square of the poloidal magnetic field, whereas the ion temperature decreases as the toroidal field is larger, but the toroidal field dependence becomes small for the regime of high toroidal field. These tendencies are consistent with those observed in experiments.