Radial Current across the separatrix due to direct ion orbit loss of a fast ion population

In all tokamak experiments with the necessary diagnostic equipment the L-H transition has been observed to be accompanied by a strong, non-uniform radial electric field. According to the most successful model for the L-H transition, this electric field is generated by the radial current due to the direct ion orbit loss of energetic ions. In this work we investigate the magnitude of the radial current due to fast ions using a particle following code that takes into account diffusion both in the velocity space and in real space. Also, a prescribed, non-uniform radial electric field is included. The numerical model simulates the DIII-D tokamak. The radial currents evaluated for L- and E-mode type plasmas have quite similar values with E-mode showing slightly larger current. A larger H-mode current is consistent with the transition model and the observation of a stronger radial electric field in H-mode. The introduction of a prescribed, non-uniform radial electric field increases the radial current dramatically. This seems to imply that a non-uniform radial electric field can, under some circumstances, increase the width of the banana orbits. (orig.)