Electron cyclotron current drive and suprathermal electron dynamics in the TCV Tokamak

Electron cyclotron current drive (ECCD) is an important prospective tool for tailoring the current profile in next-step devices. To fill the remaining gaps between ECCD theory and experiment, especially in the efficiency and localisation of current drive, a better understanding of the physics of suprathermal electrons appears necessary. On TCV, the fast electron population is diagnosed by a multichordal, spectrometric hard X-ray camera and by a high field side ECE radiometer. The main modeling tool is the quasi-linear Fokker-Planck code CQL3D, which is equipped with a radial particle transport model. Systematic studies of fast electron dynamics have been performed in TCV by square-wave modulation of the electron cyclotron power, followed by coherent averaging, in an attempt to identify the roles of collisional relaxation and radial diffusion in the dynamics of the suprathermal population. The temporal evolution on turn-on and turn-off is found to involve multiple and disparate time scales. The role of radial diffusion has been evidenced by CQL3D modeling, which is able to reproduce the experimental current drive efficiency only if a modest diffusivity is included. (author)