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

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 ∇B drift cases are compared to find that there is little difference in velocity space. However, the loss hole in the forward ∇B 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 ∇B case) or at the vertically opposite side (in the backward ∇B 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.