W7-AS: One step of the Wendelstein stellarator line

Abstract

͑W7-AS͒. W7-AS ͓G. Grieger et al., Phys. Fluids B 4, 2081 ͑1992͔͒ has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code ͓Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 ͑2002͔͒. It has extended the operational range to high density ͑4 ϫ 10 20 m −3 at 2.5 T͒ and high ͗␤͘ ͑3.4% at 0.9 T͒; it has demonstrated successfully the application of electron cyclotron resonance heating ͑ECRH͒ beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X ͓J. Nührenberg et al., Trans. Fusion Technol. 27, 71 ͑1995͔͒, aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a "non-tokamak" and has extended H-mode operation toward high density by the discovery of the high-density H-mode ͑HDH͒, characterized by H-mode energy and L-mode-level impurity confinement. In the HDH-mode quasisteady state operation is possible close to operational limits without noticeable degradation in the plasma properties. High-␤ phases up to t pulse / E = 65 have been achieved, which can already be taken as an indication of the intrinsic stellarator capability of steady-state operation. Confinement issues will be discussed with emphasis on the similarities to tokamak confinement ͑general transport properties, H-mode transition physics͒ but also with respect to distinct differences ͑no confinement degradation toward operational boundaries, positive density scaling, lack of profile resilience, no distinct isotope effect, H-mode operational window͒. W7-AS turned out to be an important step in the development of the Wendelstein stellarator line towards an independent fusion power plant concept.

Scaling and transport analyses based on an international edge turbulence database

“…As of March 2014, experimental data from the two tokamaks ASDEX Upgrade (AUG) 18 and MAST, 19 as well as the five stellarators HSX, 20 TJ-K, 21 Uragan-3M (U3M), 22 Wendelstein 7-AS (W7AS) 23 and WEGA 24 has been collected in the ETDB. There are two available datasets from W7AS and WEGA, taken at different magnetic field strengths.…”

Significance of MHD Effects in Stellarator Confinement

“…Wendelstein W7-AS [17,18] changing the center of the current in the helical coils. In addition, a set of external saddle coils allows to drive an n/m = 1/1 magnetic island at the plasma edge, which can be utilized for local island divertor (LID) operation and island studies [3,7].…”

Wendelstein 7-X, Overview and Status of Construction

“…the toroidal and poloidal field components were produced by a single set of non-planar coils. W7-AS clearly demonstrated the viability of the advanced stellarator [1]. II.…”