Experiments close to the beta-limit in W7-AS

Abstract

A major objective of the experimental program in the last phase of the W7-AS stellarator was to explore and demonstrate the high-β performance of advanced stellarators. MHD-quiescent discharges at low impurity radiation levels with volume averaged β-values of up to β = 3.4% have been achieved. A very important prerequisite was the attainment of the high density H-Mode (HDH) regime. This was made possible by the installation of extensive graphite plasma facing components designed for island divertor operation. The co-directed neutral beam injection provided increased absorbed heating power of up to 3.2 MW in high-β plasmas with B 1.25 T. The anticipated improved features concerning equilibrium and stability at high plasma β could be verified experimentally by the comparison of x-ray data with free boundary equilibrium calculations. The maximum β found in configurations with a rotational transform around ι = 0.5 is determined by the available heating power. No evidence of a stability limit has been found in the accessible configuration space, and the discharges are remarkably quiescent at maximum β, most likely due the increase of the magnetic well depth. An increase in low m/n MHD activity is typically observed during the transition towards high β. The beneficial stability properties of net-current-free configurations could be demonstrated by comparison with configurations where a significant inductive current drive was involved. Current driven instabilities such as tearing modes and soft disruptions can prevent access to β-values as high as in the currentless case. The experimental results indicate that optimized stellarators such as W7-X can be considered as a viable option for an attractive stellarator fusion reactor.

Systems Studies and Optimization of the ARIES-CS Power Plant

“…There is no good model for the beta limit in stellarators; values of b& ϭ 4.8% have been obtained in LHD and it appears that the b limit is more determined by equilibrium quality than by instability in stellarators. 19 The equilibrium b& values for NCSX-type plasmas is significantly higher than 5%~Ref. 7!.…”

Non-Axisymmetric Shaping of Tokamaks Preserving Quasi-Axisymmetry

“…The effective current I ef f is the current that would give the poloidal field, including the field produced by the helical component. Much larger values of β N were seen in W7AS experiments [8].…”

“…β N ≡ β/(I ef f /aB) ≈ 4, when estimated using the formula given in [8]. The effective current I ef f is the current that would give the poloidal field, including the field produced by the helical component.…”

“…The experiments have found stable high volume-averaged beta, β ≡ 2µ 0 p/B 2 plasmas that last hundreds of energy confinement times [6], high density (≥ 10 21 /m 3 ) regimes exceeding the Greenwald density limit of tokamaks [7], and a scaling of confinement comparable to that of H-mode tokamaks [8]. These achievements coupled with the observed robust stability [1] without disruptive plasma terminations show that threedimensional shaping of fusion plasmas addresses important issues that must be addressed before fusion power systems can be a reality.…”

Electron Cyclotron Heating for W7-X: Physics and Technology

“…The exploration of this limit is one of the most important scientific objectives of W7-X. The high-density regime was investigated at W7-AS [19] and a density limit was derived [20], which depends on the absorbed power density and magnetic field as ( )…”