Lower hybrid current drive efficiency in tokamaks and wave scattering by density fluctuations at the plasma edge

The turbulence in the scrape-off layer (SOL) plasma of FTU is characterized in order to assess its effect on the current drive efficiency of the lower hybrid (LH) waves. Amplitude, frequency and perpendicular wave vector of the fluctuations are measured for a variety of the main plasma conditions in front of the LH antenna together with the temperature and density in the SOL and used as inputs for the linear scattering theory of the LH waves developed many years ago. This theoretical model can account for both the frequency spectral broadening of the LH pump and the variations of the driven current, inferred by the perpendicular fast electron bremsstrahlung signals. The fraction of the LH power that is then deduced to be effective for current drive appears to be well related to the calculated optical thickness tau of the SOL. It drops as low as 40% as tau increases, consistent with the model prediction. Possible ways to control the SOL optical depth are investigated and a clear relation of the fluctuation level with the collisionality is found.