QUASI-OPTICAL DIFFRACTION GRILL FOR EXCITATION OF LOWER-HYBRID WAVES IN TOKAMAKS

A theoretical investigation of infinitely periodic quasi-optical grill structures for excitation of lower-hybrid waves in toroidal plasmas h as been undertaken for cylinder grill elements with an arbitrary cross section. The basis for investigation is the expansion of the scattere d RF field into the infinite series of spatial harmonics which makes i t possible to obtain the integral equation for the distribution of mag netic current over the surface of grill elements and to calculate the efficiency of LH wave excitation depending upon the LH wave slowing-do wn and parameters of the grill structure and of the plasma layer. The numerical simulation performed for the single-layer grill, with elemen ts of elliptical cross section irradiated from vacuum by infinite plan e waves, demonstrates the possibility of achieving a high efficiency o f up to 25-30% of the unidirectional LH wave excitation with the slowi ng-down N-parallel to less than or equal to 2, which is relevant to la rge fusion installations. The calculations have also been performed fo r the grill structure irradiated by quasi-optical RF beams with finite cross section; it is shown that the coupling efficiency degrades insi gnificantly if the RF beam aperture (which may be of the order of the size of the RF port in the direction of the toroidal magnetic field) e ssentially exceeds the vacuum wavelength of radiation. The results obt ained seem to be encouraging for construction of a double-layer grill with high efficiency close to unity and low Q-value of the quasi-optic al cavity, resulting in a comparatively low value of RF field intensit y near the grill elements and in a weak dependence of grill efficiency upon plasma parameters.