Non-inductive plasma initiation and plasma current ramp-up on the TST-2 spherical tokamak

Plasma current (I-p) start-up in a spherical tokamak (ST) by waves in the lower-hybrid (LH) frequency range was investigated on TST-2. Alowcurrent (similar to 1 kA) ST configuration can be formed bywaves over a broad frequency range (21 MHz-8.2 GHz in TST-2), but further Ip ramp-up (to similar to 10 kA) is most efficient with waves in the LH frequency range. I-p ramp-up to 15 kA was achieved with 60kW of net RF power P-RF in the fast wave (FW) polarization at 200MHz excited by the inductively coupled combline antenna. X-ray measurements showed that the photon flux and temperature are higher in the direction opposite to I-p, consistent with acceleration of electrons by a uni-directional RF wave. There is evidence that the LH wave is excited nonlinearly by the FW, based on the frequency spectra measured by magnetic probes. Similar efficiencies of I-p ramp-up were obtained with the inductive combline antenna and the dielectric-loaded waveguide array ('grill') antenna, and tendencies for the current drive efficiency to increase with plasma current and toroidal field were observed. During operation of the grill antenna, wavevector components were measured by an array of magnetic probes. Results were qualitatively consistent with expectations based on dispersion relations for the FW and the LH wave. A capacitively coupled combline antenna has been developed to improve coupling to the plasma and the wavenumber spectrum of the excited LH wave, and will be tested in 2013.