Investigations of sawtooth behaviour in ramp-up plasma assisted by lower hybrid current drive in the HT-7 tokamak

Current ramp-up experiments assisted by lower hybrid current drive (LHCD) have been performed with a newly developed multi-junction grill in the HT-7 tokamak. A maximum of driven current (similar to 107 kA) by LHCD has been obtained with an incident lower hybrid wave (LHW) power of P-LHW similar to 210kW. Analysis of sawtooth periods with different LHW power spectra (N-parallel to(peak) = 2.55, 2.95 and 3.55) shows that the sawtooth period is mainly affected by the power deposition and the driven current profile due to the different spectra, indicating the effect of parallel refractive index on the current profile. Results indicate that LHCD is an effective means to modify current profile. The characteristic time needed for reaching a steady period may be mainly dominated by the combination of the time needed to reach a steady driven current and the mutual inductance between the plasma current and primary Ohmic current.