The structure and the dynamics of the fast electron tail generated in Tore Supra during lower hybrid current drive experiments are investigated by means of a microwave transmission diagnostic at frequencies close to the electron cyclotron frequency, installed on a vertical chord. Measurements have been performed both in steady state and during the relaxation phase following the end of the lower hybrid power pulse. The measured transmission spectra are compared with simulations performed by means of a 3D Fokker-Planck code and are found to be in excellent agreement with them, both in the general structure and in the level, as well as in the dynamical behaviour. It is shown that this diagnostic yields the extension of the lower hybrid wave spectrum actually sustaining the non-inductive current, as well as substantial indications on its radial location. These results support the use of vertical transmission measurements as the most specific diagnostic method for lower hybrid current drive experiments.
Cross-effect on electron cyclotron and lower hybrid current drive in tokamak plasmas