Results of lower hybrid wave experiments using a dielectric loaded waveguide array antenna on TST-2

Lower hybrid current drive experiments were performed on the TST-2 spherical tokamak (R = 0.38 m, a = 0.25 m, B-t = 0.3 T, I-p = 0.1 MA). A waveguide array antenna consisting of four dielectric (alumina, epsilon(t) = 10.0) loaded waveguides was used. The coupling characteristics were investigated over a wide range of input power (0.1 W 40 kW). The reflection coefficient of this antenna increased when the input power exceeded approximately 1 kW. This result was compared with a numerical simulation based on the finite element method (FEM). The ponderomotive effect was calculated for the wave field calculated by COMSOL [1]. This calculation also showed variation of the reflection coefficient with the input power. Non-inductive plasma current start-up to 10 kA was demonstrated using 40 kW of lower hybrid wave (LHW) power. The current drive figure of merit (eta(CD) = I(p)n(e)R/P-RF) of this antenna was higher than that obtained using the combline antenna, which is designed to excite a travelling fast wave. The best current drive efficiency was obtained in the case in which the n(parallel to) (=ck(parallel to)/omega) spectrum of the excited LHW was peaked around 9 and the toroidal field was higher than in previous experiments