Major results from the stellarator Wendelstein 7-AS

Abstract

Wendelstein 7-AS was the first modular stellarator device to test some basic elements of stellarator optimization: a reduced Shafranov shift and improved stability properties resulted in β-values up to 3.4% (at 0.9 T). This operational limit was determined by power balance and impurity radiation without noticeable degradation of stability or a violent collapse. The partial reduction of neoclassical transport could be verified in agreement with calculations indicating the feasibility of the concept of drift optimization. A full neoclassical optimization, in particular a minimization of the bootstrap current was beyond the scope of this project. A variety of non-ohmic heating and current drive scenarios by ICRH, NBI and in particular, ECRH were tested and compared

Theory of tokamak disruptions

“…The Wendelstein 7-X stellarator was designed to make the magnetic configuration as independent as possible of the plasma parameters, 45 so the system should be robust against a catastrophic loss of magnetic surfaces. This robustness was seen 46 in Wendelstein 7-AS and is seen in the large helical device (LHD). 47 In tokamak-like, quasi-axisymmetric systems, 41 42 with dB=B of 10% to 20%.…”

Neoclassical transport simulations for stellarators

“…Since no physics-based turbulent transport model is available for stellarators, the simplified form χ e,i ∝ 1/n e (as was indicated in W7-AS [1]) is used inside of the density gradient region (and further suppressed in the flat density region). Although experimentally indicated [35], a power dependence of χ e,i is not taken into consideration.…”

“…Although a detailed analysis of the experimental energy balance and its comparison with the neoclassical predictions is lacking for most devices, the W7-AS results clearly indicate that neoclassical confinement in the lmfp-regime limits the achievable temperatures; see e.g. the review [1]. At low temperatures, however, neoclassical transport is less important due to the fairly strong temperature dependence of the radial transport coefficients.…”

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].…”