Current profile control and improvement of current drive efficiency by combined lower hybrid waves in TRIAM-1M

Current profile control experiments were carried out in TRIAM-1M by the combination of two lower hybrid waves (LHWs) at 8.2 and 2.45 GHz with different spectra of parallel refractive index N||. In the combined lower hybrid current drive (LHCD) discharge, the current drive efficiency was improved by a factor of > 1.5 compared with 8.2 GHz LHCD only and 2.45 GHz LHCD only. Measurements of the internal inductance and the radial profile of hard X ray emission indicated that the current profile can be controlled by changing the additional power of the 2.45 GHz LHW to the 8.2 GHz LHCD plasma. The improvement of the current drive efficiency and the modification of the current profile are considered to be attributable to the enhancement of the absorption of lower N|| waves by higher N|| waves in the two kinds of LHWs. Moreover, the controlled current profile can be successfully sustained for about two orders of magnitude longer than the current diffusion time.