Effects of the spectral broadening of lower hybrid waves on current drive efficiency in the ASDEX tokamak

Full current drive operation was achieved on ASDEX for different Ohmic target plasmas by means of 2.45 GHz lower hybrid waves. Variations of the current drive efficiency of up to 50% have been observed which are correlated with the spectral broadening of the pump wave, as detected by an electromagnetic probe. Parametric decay instabilities at the plasma edge or scattering of the lower hybrid waves can be responsible for these effects. The experiment can be explained within the approximations of the available current drive theory with a simple model assuming that the same process which leads to the frequency broadening causes a modification of the launched nparallel spectrum. (author). Letter-to-the-editor. 16 refs, 3 figs