Antenna

Abstract

A fast wave current drive antenna was developed for LHD, motivated by the need to provide a capability for rotational transform profile control by noninductively driven current. Stability calculations suggest that it is possible to increase the beta limit and obtain access to the second stability regime by controlling the rotational transform profile. Current drive by the ICRF fast wave (magnetosonic wave) is suitable fo r such a purpose. The "fishbone" antenna [1] built for LHD is equivalent to two "combline" antennas [2] stacked vertically, but has only one input and one output. Such a design enables high power operation even with limited port space. This antenna will be placed on the large major radius side of the torus where the plasma is elongated in the vertical direction. The antenna is divided into 10 nearly identical modules, each consisting of a stainless steel half-wavelength resonant structure approximately 1 meter long, grounded at the midplane (T-bar current strap), a water-cooled stainless steel backplate, and a U shaped molybdenum Faraday shield. These modules are placed side by side in the toroidal direction, following the helical shape ofthe plasma surface. The whole assembly is surrounded by carbon protection tiles arranged in a "picture frame" configuration to reduce the plasma density at the Faraday shield. The spacing between adjacent straps (center to center) is 0.11 m, which corresponds to a wavenumber of 14 m 1 when the phase difference between adjacent current straps is 90°. The operation frequency is