A simple model for lower‐hybrid current drive in tokamaks

The effects of scattering by density fluctuations on the single pass lower‐hybrid propagation in tokamaks for current drive are analyzed in a cylindrical plasma model with sheared magnetic field. Even for the launched N∥ spectrum peaked around 2, the turbulent scattering and propagation in the sheared field significantly modifies the N∥ spectrum so that the waves can effectively couple to slower electrons. At low densities, a small amount of the incident energy experiences a large N∥ upshift and creates a high‐energy electron tail which interacts with the main part of the wave energy. At higher densities, the bulk of the wave energy is shifted in N∥ before damping resulting in lower current drive efficiencies.