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 quasilinear Fokker-Planck code CQL3D, which is equipped with a radial particle transport model. Systematic studies of fast electron dynamics have been performed in TCV with modulated or pulsed 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. A consistent picture is beginning to emerge from experiment and modeling, pointing to the crucial role of the radial diffusion of suprathermal electrons in the physics of ECCD.