Electron cyclotron resonance heating and current drive in the W7-X stellarator

Electron cyclotron resonance heating and electron cyclotron current drive (ECCD) in the W7-X stellarator, which is presently under construction, are investigated by means of a ray-tracing code. Two heating schemes are considered: launching of extraordinary and ordinary waves from the low magnetic field side at the second harmonic of the electron cyclotron frequency. Full power absorption is typically obtained for the extraordinary mode at the second harmonic for the predicted plasma parameters of W7-X. In this case, current drive calculations are also performed to determine the optimum launching conditions. A current drive efficiency, eta(ECCD), of the order of 20 A kW(-1) is obtained for the scenarios of low-field-side injection under consideration (for a density n(e) similar or equal to 2-4 x 10(19) m(-3), and a temperature T-e similar or equal to 3-4 keV), corresponding to a normalized efficiency gamma(ECCD) similar or equal to 0.02 x 10(20) A W-1 m(-2). ECCD can be used to tailor the radial profile of the rotational transform (in order, for example, to compensate the residual bootstrap current, and to avoid low-order rational surfaces inside the plasma). A variational calculation of ECCD, extended to take trapped-particle effects into account, and its straightforward implementation in the ray-tracing code are briefly presented. The results for the driven current are compared with estimates of the residual bootstrap current in the optimized magnetic configuration of W7-X. Significant absorption is found over a wide density range also in the case of ordinary mode heating at the second harmonic, allowing an extension of the operational parameter window to higher densities.