An analysis of an ICRF antenna with controllable toroidal wavenumber in LHD

In order to excite fast wave in the ion cyclotron range of frequencies (ICRF) using multiple antennas with the phase difference in the Large Helical Device (LHD), a controllable wavenumber antenna that consists of two single-strap antennas is designed, and the electrical characteristic features of the antenna are estimated by using a three-dimensional electromagnetic commercial code and the simplified antenna model. Controlling the radio-frequency (RF) phase difference between these two single-straps, reverse-phase excitation can be achieved in order to reduce the impurity production. According to this estimate, the RF current profile on the strap surface is strongly concentrated on the both horizontal strap edges, and the electrical strap length at frequency of 85 MHz is longer than the quarter wavelength of 85 MHz (λ85MHz/4). Excitable wavenumber spectra are different in various RF phases and frequencies. At the low frequencies (< 60 MHz) with in-phase, effective wavenumbers between k = 0 m-1 and k = 10 m-1 can be excited. During the reverse-phase excitation, large (k = 6, 15 m-1) wavenumber spectra with low (k = 0 m-1) wavenumber kept small are obtained. (author)