CONCEPTUAL STUDY OF A REFLECTOR WAVE-GUIDE ARRAY FOR LAUNCHING LOWER-HYBRID WAVES IN-REACTOR GRADE PLASMAS

A new concept - the reflector waveguide array - is proposed to improve and simplify the design of continuous wave lower hybrid (LH) launchers for steady state reactor applications. Mechanical robustness of the antenna and efficient heat removal are provided by a thick wall waveguide structure that can accommodate a large number of cooling ducts. The plasma facing mouthpiece could be made of the same material as the reactor first wall and could be easily replaceable through remote handling. In order to compensate for the increased horizontal distance between adjacent waveguides, the front ends of the thick septa are grooved to form short (congruent to lambda/4) passive waveguides that act as reflectors between the radiofrequency powered waveguides (drivers). Then, for an adequate phasing between the active waveguides, the total electric field at the reflector waveguide apertures varies coherently with the one in the drivers to launch a highly directional slow wave. It is shown that the coupling properties of such an array and the directivity of the radiated power spectrum are similar to those of present day launchers. Their dependences upon the depth of the reflector waveguides, and the electron density and its gradient are investigated. The effect of changing the phase between the drivers is also studied. The proposed reflector LH antenna would provide enough flexibility to vary the Nil spectrum for plasma control purposes in a steady state fusion reactor.