On optimization of the multijunction grill for the lower hybrid current drive

The design of a multijunction grill which is supposed to be used in current drive experiments, calls for the optimization over three most important parameters: the phase shift ΔΦ among the adjacent waveguides, the length of the multijunction grill Lg (from the junction to the mouth) and the surface density n0. This was demonstrated by the results of a numerical investigation of the four-waveguide grill designed for a small tokamak. It was shown that current drive efficiency (or the directivity) can reach 50% for practically arbitrary ΔΦ (≠0deg, 180deg), if Lg is dosen adequately. Enhanced current drive efficiency at ΔΦ≠90deg follows from an unevenly distributed power among separate waveguides and from the self-adaptive adjustment of the phases of the incident waves. The risk of power overload of the waveguides grows with decreasing n0. A brief description of the author's variant of the theory of wave diffraction on the junction is given in the Appendix. (author). 13 figs., 17 refs