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 com bination of two lower hybrid waves (LHWs) at 8.2 and 2.45 GHz with differen t spectra of parallel refractive index N-parallel to. In the combined lower hybrid current drive (LHCD) discharge, the current drive efficiency was im proved by a factor of >1.5 compared with 8.2 GHz LHCD only and 2.45 GHz LHC D only. Measurements of the internal inductance and the radial profile of h ard 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 plasm a. 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-parallel to waves by higher N-parallel to waves in the two kinds of LHWs. Moreover, the controlled current profile can be s uccessfully sustained for about two orders of magnitude longer than the cur rent diffusion time.