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

Abstract

The neoclassical prediction of the “electron root,” i.e., a strongly positive radial electric field, Er (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 Te profiles [with Te(0) up to 6 keV and with Ti≪Te] and strongly positive Er 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 Er. 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.

Reduction of Ion Thermal Diffusivity Associated with the Transition of the Radial Electric Field in Neutral-Beam-Heated Plasmas in the Large Helical Device

“…However, there has been no experimental study to test these issues on the neoclassical ion transport in stellarator plasmas. This is because the transition of the radial electric field from small negative (ion root) to large positive (electron root) was observed only in plasmas with the assistance of electron cyclotron heating (ECH), where electron heating is dominant [7][8][9][10][11]. The ion temperature is much lower than the electron temperature because ions are heated only by the energy exchange between ions and electrons.…”

“…In previous experiments [7][8][9][10][11], the transition from ion root to electron root was observed only in plasmas with ECH, therefore it has been also open to question as to whether the nonthermal electrons driven by ECH are required to achieve the transition. The transition of radial electric field from ion root to electron root clearly demonstrates that nonthermal electrons are not necessary for the transition, because there are no nonthermal electrons in NBI heated plasma.…”

ECRH scenarios with selective heating of trapped/passing electrons in the W7-X Stellarator

“…Usually, similar argumentation can be applied also for trapped electrons and this is the basic assumption for the transport theory. However, there is a number of experiments (see for example [7][8][9][10][11]) as well as theoretical investigations [8-10, 12, 13] which indicate significant kinetic effects for the transport in the case when a substantial fraction of the RF power is absorbed by trapped electrons, P trap abs > P pass abs . This can be especially important for stellarators, where the trapped electrons, being locked within the toroidal ripples, do not spread the absorbed power toroidally over the magnetic surface, and in the poloidal direction only do so by their precessional motion.…”

“…The magnetic equilibriums with finite-β effects accounted were precalculated by the VMEC code [17] as well as the transport coefficients calculated by the DKES code [18]. The present understanding and all theoretical assumptions that adopted for modeling are based on the substantial experience from W7-AS and other experiments [8,9,11,19].…”

Development of Momentum Conserving Monte Carlo Simulation Code for ECCD Study in Helical Plasmas

“…We have developed GNET code [1], which can solve a linearized drift kinetic equation in the five dimensional phase-space (two velocity and three real spaces). We have studied the radial transport of supra-thermal electrons by the ECH [1,2] and the electron cyclotron current drive (ECCD) [3] in helical plasmas.…”