Power spectrum of the three waveguide grill for the T-7 tokamak

Numerical studies are presented in an effort to optimize the power spectrum of the three wave grill for the T-7 tokamak, with special emphasis on the lower hybrid range of frequencies. The theoretical basis is the work by Brambilla et al on the radiation from the slow-down structures of the grill type. Here it is assumed that the height of the waveguide is infinite in one direction, while a slab geometry is adopted for the edge plasma density gradient. Coupling between fast and slow waves is neglected. First results are given for the case when the grill is retracted from the corrugated wall. The efficiency dependence on plasma density gradient is determined. Efficiency is found to increase when simple metallic walls are inserted in the end parts of wave guides, which partition them off, thereby obtaining a six wave guide grill. An optimum phase shift of 60 degrees between odd and even numbered wave guides is found. 6 refs, 11 figs