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

Understanding and controlling the plasma confinement is crucial for the development of a fusion reactor. In a so-called L-H transition the plasma confinement suddenly increases when the auxiliary heating exceeds a critical value. Experimentally, an L-H transition seems to be always accompanied with the generation of a strong, sheared radial electric field near the edge. Moreover, such a transition can be triggered even in the absence of any auxiliary heating by externally imposing an edge radial electric field. It is the shear in the poloidal rotation corresponding to this radial electric field that is believed to suppress the microturbulence, thus reducing anomalous transport. Therefore, it is very important to understand how the radial electric field is generated. According to the most successful theory so far, the radial current due to high energy ions leaving the plasma produces the radial electric field and the poloidal rotation necessary for the suppression of microturbulence. The critical quantity in the original theory was the collisionality of the edge plasma - as the edge plasma gets collisionless, the high energy ions with wide banana orbits are able to leave the plasma through direct ion orbit loss. Therefore this model ran into difficulties when the L-H transition was achieved on the JFT-2M tokamak under very collisional circumstances. According to a revision of the original theory, this problem is removed when the radial current is caused by a non-thermal hot ion component in the edge plasma. A Monte Carlo simulation, in which a group of fast ions scatters off a stationary background plasma, should be an appropriate tool for evaluating the ion orbit loss current of the revised theory. In this paper we study the radial current across the tokamak separatrix that is generated by the direct ion orbit loss of fast ions. (author) 12 refs., 4 figs