Simple model to explain the temperature dependence of the lower hybrid current drive efficiency

Dissipated power in the spectral gap is taken into account in deriving analytically the lower hybrid current drive efficiency. The efficiency is determined by a minimum down-shifted phase velocity and the quasi-linear velocity diffusion coefficient at the spectral gap in addition to the original wave spectrum. To explain present experimental results in both JT-60 and ASDEX, must be close to the Landau damping limit and must be the same order of magnitude as the collisional friction at . With the suggested values of these two parameters from experimental results, the driven current is mainly determined by the launched wave spectrum, while most of the wave power is dissipated at the spectral gap. This characteristic can explain both the temperature and the dependence of the current drive efficiency.