A. Elsner

The impact of boundary plasma conditions on the plasma performance of the Wendelstein 7-AS stellarator

In the modular advanced stellarator W7-AS, the plasma performance and the main characteristics of the plasma-wall interaction are strongly affected by the three-dimensional edge topology. Both limiter-and separatrix-dominated configurations are possible. TiC and bulk-boronized limiter materials have been used. The impurity behaviour and the accessible plasma parameter ranges are compared for different limiter and wall conditions. With limiters, optimum plasma performance in currentless ECRF-or NBI-heated discharges was achieved with bulk-boronized graphite limiter material and boronized walls. Solid target sputter boronization, however, was found to be ineffective in comparison with boronization by He/B 2 H 6 glow discharges. For separatrix-dominated discharges, conditioning by wall coating has short-term effects only. Enhanced, localized plasma outflow to the wall due to islands at the boundary quickly erodes the layers. The possibility to develop a divertor concept is discussed. Basic properties of the plasma edge edge as derived from Langmuir probes and limiter calorimetry are described. Modeling is complicated by three-dimensionality. In a first approach, a 1D edge transport model on the basis of distinct flux bundles is applied.

Radiation measurements and modeling of the density limit on the W7-AS stellarator

Density limit discharges in the W7-AS stellarator with a strong density ramp were compared to a series of discharges with constant line integrated density approaching the maximum value achieved in the density ramp. The physics of the density limit in stellarators was demonstrated to be consistent with the predictions of the two-point model, indicating that this model successfully describes the density limit process in both stellarators and tokamaks. The discharges with a strong density ramp were found to have broader density profiles than those discharges with constant line integrated density. The latter discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. Modeling of the radiation profile, to simultaneously match the measured bolometer and soft X-ray radial profiles of radiated power, implies that impurity density profiles were peaked and continuously increased during the discharge. The increase in radiated power decreased the net deposited power to the plasma and the diamagnetic energy fell. The aim of producing steady-state discharges at the highest possible density is aided by the reduction of impurity sources by helium glow discharge cleaning.

Plasma response on impurity injection in W7-AS

Abstract In order to study impurity transport and radiation behavior nitrogen has been injected into the scrape-off plasma of limiter-dominated discharges by gas puffing or into natural magnetic islands at the plasma edge of separatrix-dominated discharges by a reciprocating erosion probe. Strong radiative plasma edge cooling with a reduction of the power flux to the limiter could be obtained. The accompanied degradation of the energy confinement and the observed decrease of the central electron temperature can partly be explained by the reduction of the effective heating power. At high plasma densities and sufficiently strong impurity injection phenomena well known from tokamaks, like plasma shrinking, plasma detachment from the limiters and MARFEs were transiently observed.