ELECTRON-CYCLOTRON-RESONANCE HEATING AND CURRENT DRIVE IN THE W7-X STELLARATOR

Electron cyclotron resonance heating and electron cyclotron current dr ive (ECCD) in the W7-X stellarator, which is presently under construct ion, are investigated by means of a ray-tracing code. Two heating sche mes are considered: launching of extraordinary and ordinary waves from the low magnetic held side at the second harmonic of the electron cyc lotron frequency. Full power absorption is typically obtained for the extraordinary mode at the second harmonic for the predicted plasma par ameters of W7-X. In this case, current drive calculations are also per formed 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 dens ity 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 efficienc y 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 o rder, for example, to compensate the residual bootstrap current, and t o avoid low-order rational surfaces inside the plasma). A variational calculation of ECCD, extended to take trapped-particle effects into ac count, 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 mag netic configuration of W7-X. Significant absorption is found over a wi de density range also in the case of ordinary mode heating at the seco nd harmonic, allowing an extension of the operational parameter window to higher densities.