Plasma start-up experiment and modelling on the TST-2 spherical tokamak

Plasma start-up experiments using the lower hybrid wave (LHW) were performed on the TST-2 spherical tokamak. Two capacitively-coupled combline antennas (outboard-launch and top-launch) are presently being used. Bottom-launch can be simulated using the top-launch antenna with reversed toroidal field. The unexpectedly good result of the simulated bottom-launch case was investigated by a combined ray-tracing and Fokker-Planck analysis. In the case of simulated bottom-launch the parallel index of refraction (n||) downshifts initially, but because of the plateau formed by the upshifted n|| LHW after reflecting from the cut-off layer at the bottom of the plasma, strong absorption of the low n|| LHW occurs during the first pass through the plasma, generating a distribution function which extends to very high energies.