Fundamental Investigation of Electron Bernstein Wave Heating and Current Drive at the WEGA Stellarator

Overdense plasma heating with electron Bernstein waves (EBWs) is of large importance for fusion, since ultra high density operation is a promising fusion scenario for stellarators and heliotrons. The EBW physics is also important for spherical tokamaks (ST). Here the plasma core is inaccessible for standard ECRH with electromagnetic waves. The EBW driven current could help to stabilize MHD-modes and to achieve steady state operation at high beta. The typical EC-frequency range in STs is 10-30 GHz. This paper reports about EBW heating and current drive experiments at frequencies of 2.45 GHz. WEGA is a classical five period l=2 stellarator with a major radius of 0.72 m and an aspect ratio of 7. For the 2.45 GHz the mode conversion could be investigated in detail by hf-probes. The results were consistent with full wave simulations. The short EBW wavelength of < 1mm enabled the calculation of their propagation by a 3Dray-tracing code. In addition the code predicted an EBW driven current. Both the resonant power deposition and the EBW driven current density profile was measured by power modulation technique with movable probes in the plasma. An agreement between the code results and experimental data was found.