Current drive experiments at the electron cyclotron frequency

The experimental investigation of non-inductive current drive by electromagnetic waves in the electron cyclotron range of frequencies and the comparison with theoretical predictions attracts increasing interest in both, tokamak as well as stellarator research. In spite of the low current drive efficiency (compared to Lower Hybrid Current Drive) Electron Cyclotron Current Drive (ECCD) is a candidate for MHD-mode control and current profil shaping in tokamaks and stellarators due to the high localization of the driven currents and the capability to drive currents in the plasma centre in large machines. ECCD is an appropriate tool for the control of the pressure effects on the profile of the rotational transform, particularly the bootstrap current in stellarators. This is a crucial condition to maintain good confinement properties in low shear configurations such as W VII-AS. Basic experimental investigations were performed at the W VII-AS stellarator, where the small EC-driven currents are not masked by large inductively driven currents as in tokamaks. The theoretical treatment of ECCD in stellarators would require a Fokker Planck solution in full phase space taking into account the complex magnetic field configuration (trapped particles, loss cone effects) which is untractable. In a first approach, we have compared our experimental data to a simple analytical current drive model (linearized in slab geometry) which is incorporated in a ray-tracing code. In a second step, we have analysed the sensitivity of the model with respect to simplified assumptions on trapped particles and quasi-linear effects. (orig.)