Fast electrons generated in front of lower hybrid (LH) grills can cause damage to tokamak divertor and first wall components. It is therefore of interest to identify possible edge electron acceleration mechanisms and suggest measures to limit these. One possible acceleration process described here depends on mode overlap in the antenna electric field spectrum and on the presence of high- modes, resonant with the ambient ( eV) edge electrons. These electrons then diffuse throughout the globally stochastic region of -space whose upper bound is given by the last overlapping mode. It is shown that the described acceleration is seriously aggravated by the presence of higher order waveguide modes generated at sharp waveguide-plasma transitions and that rounding-off the waveguide septa can considerably reduce the fields and electron acceleration associated with these higher order modes. Additional antenna field non-uniformities arising from a grill modular structure and random fields generated at the plasma edge through parametric decay and scattering of the LH slow wave from plasma density fluctuations can further enhance the edge electron energy.
Fast Alfven eigenmodes in a cylindrical inhomogeneous plasma