The optimum ion confinement mode in W7-A stellarator induced by fast orbit losses of the nearby perpendicular NBI

The general discussion of improved confinement properties in both tokamaks and stellarators has concentrated on mainly two effects: the reduction of the plasma fluctuation level and the role of sufficiently large radial electric fields. Especially for the tokamak L-H transition, the generation of radial electric fields by ion orbit losses at the plasma edge affects the fluctuation level. A consistent analysis of radial electric field and its impact on fluctuations, however, is not available so far and the basic effect for confinement improvement is still an open question. In this context, a new analysis of the optimum ion confinement regime found in the W7-A stellarator, the predecessor of W7-AS, can contribute to the issue of improved confinement. In the W7-A stellarator (R=2m, A ≤ 9 cm) with nearly perpendicular neutral beam injection (P ≤ 1 MW), optimum ion confinement with ion temperature of up to 1 keV could be obtained in high density (∼1014 cm-3) NBI discharges. For this density regime, the highest Ti values found in stellarators or torsatrons so far were measured. In these NBI discharges in W7-A, where Ti significantly exceeded Te very high Er values (up to -500V/cm) were inferred from the analysis of Doppler shifted impurity lines. The analysis of the ion energy balance was in strong conflict to the neoclassical predictions disregarding radial electric fields. In this paper, we first analyse pure ECRH discharges (W7-A and W7-AS) at moderate densities. Then, we discuss for an NBI discharge in W7-A with optimum ion confinement the effects of the fast beam ions on the radial electric field and the confinement of thermal ions. For the neoclassical transport analysis, a DKES code database is used where strong radial electric fields are included also in the plateau collisionality regime. From the ambipolarity condition of all particle fluxes the radial electric fields are estimated and compared with the measured ones. (author) 6 refs., 2 figs