The neoclassical “Electron Root” feature in the Wendelstein-7-AS stellarator

The neoclassical prediction of the “electron root,” i.e., a strongly positive radial electric field, $E_r$ (being the solution of the ambipolarity condition of the particle fluxes), is analyzed for low-density discharges in Wendelstein-7-AS [G. Grieger, W. Lotz, P. Merkel, et al., Phys. Fluids B 4, 2081 (1992)]. In these electron cyclotron resonance heated (ECRH) discharges with highly localized central power deposition, peaked $T_e$ profiles [with $T_e\text{(0)}$ up to 6 keV and with $T_i{\mathrm{\ll T}}_e]$ 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 generated by the ECRH. The fraction of ripple-trapped particles in the ECRH launching plane, which can be varied at W7-AS, is found to be the most important. After switching off the heating the “electron root” feature disappears nearly immediately, i.e., two different time scales for the electron temperature decay in the central region are observed. Monte Carlo simulations in five-dimensional phase space are presented, clearly indicating that the additional “convective” electron fluxes driven by the ECRH are of the same order as the ambipolar neoclassical prediction for the “ion root” at much lower $E_r.$ For the predicted “electron root,” the ion fluxes calculated based on the traditional neoclassical ordering are much too small; shortcomings of the usual approach are indentified and a new ordering scheme is proposed.