A new concept of launchers for lower hybrid waves in tokamaks: Quasi-optical grills

There is an urgent need to develop highly simplified rf launchers in the 4 - 10 GHz range for future large tokamaks operating with with fields on the order of 5 T, The quasi-optical launcher (QOG) for lower hybrid waves offers one possible solution to this problem, A short survey of existing QOG theories is given as well as their intrinsic limitations, A more promising approach is our theory of QOG mounted in a hyperguide in which the rods are situated in an oversized waveguide and irradiated obliquely by the wave emerging (in a form of a higher mode) from an auxiliary oversized waveguide. To illustrate our theory, numerical results are presented for several large structures tentatively proposed for TORE-SUPRA tokamak, A sixteen rods structure, operating at 3.7 GHz, can be used as a proof-of-principle QOG for LHCD in large tokamaks. We find that it is easy to design the transmission line exciting LSE(14) mode in the auxiliary hyperguide at 99% efficiency. This simple one-row structure has good coupling (R(tot) < 9%) and acceptable directivity (delta(CD)(w) = 46%). We also comment on the original QOG design for TORE-SUPRA at 8 GHz. It is shown how an alternate QOG design would lead to good results for either 1-or 2-row structures of 31 rods.