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(FT/P2-05) Development of a Fishbone Travelling Wave Antenna for LHD

Y. Takase¹⁾, A. Ejiri¹⁾, S. Shiraiwa¹⁾, N. Kasuya²⁾, H. Wada²⁾, H. Kasahara¹⁾, T. Taniguchi²⁾, K. Yamagishi²⁾, N. Takeuchi³⁾, T. Seki³⁾, R. Kumazawa³⁾, T. Mutoh³⁾, T. Watari³⁾, K. Saito³⁾, H. Torii³⁾, T. Yamamoto³⁾, M. Saigusa⁴⁾, C.P. Moeller⁵⁾, R.A. Olstad⁵⁾, H. Ikezi⁵⁾, R. Callis^5)
 
1) Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan
²⁾ Graduate School of Science, The University of Tokyo, Tokyo, Japan
³⁾ National Institute for Fusion Science, Toki, Japan
⁴⁾ Ibaraki University, Hitachi, Japan
⁵⁾ General Atomics, San Diego, U.S.A.

Abstract.  A fast wave travelling wave antenna is being developed for LHD. This antenna is equivalent to two combline antennas stacked vertically, but has only one input and one output. It consists of 10 nearly identical modules, each consisting of half-wavelength resonant structure approximately 1 meter long, grounded at the midplane. The frequency of operation is chosen to be in the neighborhood of 75 MHz, with a bandwidth of about 10 MHz, and the excited wavenumber is 14 inverse meters when the phase difference between adjacent current straps is 90 degrees. Electron Landau damping of the fast wave will be used to heat electrons and to drive current. Several mockup antennas were built to optimize the electrical properties of the antenna. The RF magnetic field in front of the Faraday shield was measured using a single-turn loop probe, and excitation of a travelling wave was confirmed. A low-power (1 kW) test with plasma load performed on the TST-2 spherical tokamak indicated very high loading resistance, suggesting the need to install the antenna sufficiently far away from the plasma in order to ensure wave excitation with high directivity.

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