Physical mechanism for the temperature dependence of lower hybrid current drive efficiency

The volume-averaged electron temperature (T e ) dependence of lower hybrid current drive efficiency is investigated by a newly-developed numerical simulation code in the framework of a ray-tracing code and a one-dimensional relativistic Fokker-Planck calculation. As a result of numerical simulations, the current drive efficiency increases with (T e ), and the dependence can be explained by the large shift of the parallel refractive index in the low-temperature region ((T e )<or=10 keV) and by the relativistic effect in the high-temperature region (10<(T e )<or=20 keV). However, the current drive efficiency is not improved in the region higher than 20 keV.