Property of an X-point generated velocity-space hole in a diverted tokamak plasma edge

Abstract

Due to vanishingly small poloidal magnetic field in the vicinity of a divertor X point in a tokamak geometry, there exists a significant ion velocity space hole at the thermal energy level in the absence of a radial electric field. Physical property of the loss orbits and loss hole is studied in a lower single null geometry using a Hamiltonian guiding center particle code. The loss occurs through both high and low field sides of the X point, and in both trapped and passing velocity regions in the conventional sense. Loss holes between forward and backward delB drift cases are compared to find that there is little difference in velocity space. However, the loss hole in the forward delB plasma occupies a much larger real space volume due to flux expansion near the X point. A radial electric field shifts the velocity hole to a higher energy and, thus, reduces the orbit loss flux from the plasma near the X point (in the forward delB case) or at the vertically opposite side (in the backward delB case) by the same amount. On the midplane, however, the orbit loss hole disappears to an extremely high energy. The loss hole boundary shows a modification by an electrostatic fluctuation. (C) 2004 American Institute of Physics.