The neoclassical "electron root'' feature in the Wendelstein-7-AS stellarator

The neoclassical prediction of the "electron root,'' i.e., a strongly posit ive radial electric field, E-r (being the solution of the ambipolarity cond ition of the particle fluxes), is analyzed for low-density discharges in We ndelstein-7-AS [G. Grieger, W. Lotz, P. Merkel, et al., Phys. Fluids B 4, 2 081 (1992)]. In these electron cyclotron resonance heated (ECRH) discharges with highly localized central power deposition, peaked T-e profiles [with T-e(0) up to 6 keV and with T-i much less than T-e] and strongly positive E -r in the central region are measured. It is shown that this "electron root '' feature at W7-AS is driven by ripple-trapped suprathermal electrons gene rated by the ECRH. The fraction of ripple-trapped particles in the ECRH lau nching plane, which can be varied at W7-AS, is found to be the most importa nt. After switching off the heating the "electron root'' feature disappears nearly immediately, i.e., two different time scales for the electron tempe rature decay in the central region are observed. Monte Carlo simulations in five-dimensional phase space are presented, clearly indicating that the ad ditional "convective'' electron fluxes driven by the ECRH are of the same o rder as the ambipolar neoclassical prediction for the "ion root'' at much l ower E-r. For the predicted "electron root, '' the ion fluxes calculated ba sed on the traditional neoclassical ordering are much too small; shortcomin gs of the usual approach are indentified and a new ordering scheme is propo sed. (C) 2000 American Institute of Physics. [S1070-664X(99)04012-4].