P. Pech

Results from a double-sided langmuir probe in T-10, and an extended model of a probe in a streaming magnetized plasma

Abstract By means of a JANUS-type Langmuir probe operated in the scrape-off layer (SOL) in T-10 the temporal evolution and radial profiles of plasma density, electron temperature, and toroidal asymmetry of saturation currents were measured in dependence on discharge parameters. Of particular importance turned out to be the B T field direction and the radial probe position relative to the active limiter structure (rail and aperture limiter). The interpretation of the results is based on the local SOL structure in terms of connection lengths to the active limiter and special flow effects in T-10, and makes use of a theoretical probe model by Hutchinson. As to the latter model, a refinement concerning its basic principles is presented independently.

Analysis of the stainless steel limiter and of shields of the wasaprobe after operation in T-10

Abstract The stainless steel limiter and shields of the surface analysis probe used in T-10 were analyzed by SEM, EDS, SAM, RBS, and SIMS with respect to modifications of the surface structure and the chemical composition. Results on deposition of surface layers and microparticles and on erosion by arcing are presented.

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.

Deposition probe investigations of the sol of T-10

Abstract Several hundred systematic deposition probe measurements were carried out in the SOL of T-10 to investigate the time development of the Fe-impurity flux at various radial positions. During these investigations the discharge conditions as well as the limiter configuration were varied over a wide range. Some qualitative guiding principles are given for deposition probe measurements and their interpretation. On the basis of the radial dependence of the iron flux during both the plateau and the end phase of the tokamak discharge, the influence of erosion processes as well as of local sources and enhanced transport on the iron deposition is discussed. The experiments show an immediate relation between the direction of the toroidal magnetic field and the evolution of the impurity flux during the plateau phase, no influence of various limiter arrangements could be detected.

Characteristics of electric probes in the tokamak SOL: influences of the magnetic field

Abstract The paper describes, discusses and interprets observations made with electric probes in the tokamak SOL to check simple assumptions about the transport of probe current through the plasma, following the theoretical approach of Guenther [7]. Within the framework of the supposed model, it can be concluded that in practice a probe tip expected to operate as a classical single Langmuir probe acts as a double probe. As a consequence, the values for the temperature are generally lower than those obtained by the single-probe formula.

On the distribution of metals deposited onto the limiter and the liner of tokamaks after long-term operation

Abstract Three inspections of the inner parts of the discharge vessels of T-10 and TM1-MH after long-term operation revealed that metals originating from the various construction materials are distributed inhomogeneously over the first wall of these tokamaks. This partially allows one to identify local metal sources and to indicate anisotropies of the transport. Different materials from inner structures, even if they were only used in earlier experiments, are observed at all limiter surfaces and as components of the debris consisting of macroparticles of different size, shape and elemental composition. There are metallic deposits of the form of structured films or of solidified droplets.

Surface changes on probes inserted into the limiter shadow of the T-10 tokamak

Abstract Time-resolved measurements of impurity fluxes which are oriented parallel and antiparallel to the toroidal and poloidal magnetic field have been performed in the limiter shadow of the T-10 tokamak. A qualitative model is proposed which explains the main features of the experimental results.

Transport Study on the Boundary Plasma of the Wendelstein 7-AS Stellarator

For a limiter defined SOL, as given by small values of the rotational transform, the edge topology of W7-AS is characterized by large flux bundles distributed regularly over the poloidal angle. These bundles are poloidally decoupled to a good approximation, as indicated by 2D resolved Langmuir probe data. Therefore, for ι≤0.4, a 1D radial plasma model is a reasonable first step to provide a qualitative description of the transport in the W7-AS boundary layer. The toroidal variation of the flux bundle geometry is taken into account by averaging the transport equations along the corresponding field lines. The effects of the neutral gas sources and the sensitivity of the diffusion coefficient to the unknown Ti profile are discussed. A ι scan of the diffusion coefficient shows a minimum at ι≅0.35, where smooth magnetic surfaces exist throughout the SOL. This suggests that topological effects related to perturbations at the “natural” 5/m resonances may be responsible for the observed ι dependence of the diffusion coefficient.

Studies of impurity recycling by the collector probe technique

Abstract In order to study recycling effects of the nonintrinsic impurity Li discharges with and without LiD-pellet injection were investigated. The observed maximum impurity level of Li in the SOL plasma of discharges without injection reaches less than 10% of that observed in discharges with injection. The measurements offer the possibility to distinguish between influxes from the wall and those which reach the collector probe via the core plasma. The time evolution, orientation and radial dependence of the impurity fluxes are characteristic features of their origin. The consideration of all these features facilitates a better understanding of collector probe measurements in the SOL-plasma.

Studies on plasma-surface interaction in the stellarator W7-AS

Abstract The analysis of long term collector samples has given information on wall erosion by ion sputtering in W7-AS. There is strong indication that this erosion occurred during high - t -discharges which are separatrix-dominated by 5/n island chains. Langmuir and calorimeter probe measurements have demonstrated that in this case particle and heat fluxes leave the plasma locally with high densities. Erosion of limiter surfaces measured post-mortem can be satisfactorily correlated with results from measurements with flush mounted limiter probes. Measurements with a moveable collector probe have shown that the plasma impurity contamination by limiter material is higher after wall boronization in limiter-dominated discharges. This is explained by less contamination of the limiter surface after removal of the boronization layer from the limiter surface. On the basis of a simple 1D-impurity transport model a comparison of Langmuir and collector probe measurements with spectroscopic measurements in the core plasma was made, supporting this hypothesis.