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An experimental investigation of particle parallel flows has been carried out at Wendelstein 7-X (W7-X), one of the most advanced stellarators in the world. The studies are restricted to the outermost plasma region, the scrape-off layer (SOL), which is shaped to tackle the exhaust problem in vision of future fusion reactors based on plasma magnetic confinement. The aim of the measurements is to set the basis for a physics analysis of the SOL dynamics by obtaining direct information on convective heat transport, together with the assessment of the predominant flow directions of the main plasma ions and of fusion-products or wall-released impurities. In this way, a better comprehension of the interplay between the transport parallel and perpendicular to the SOL field lines can be achieved, contributing to the understanding of the effectiveness of the island divertor configuration.
The chosen instrument for the experimental studies is the Coherence Imaging Spectroscopy (CIS) diagnostic, a camera-based interferometer capable of measuring 2D Doppler particle flows associated with a selected visible line from the plasma. The diagnostic is distinguished by its high time resolution and spatial coverage, allowing the visualisation and measurements of flow velocities for a full module of W7-X simultaneously. A CIS diagnostic has been fully designed for W7-X with an improved level of accuracy achieved thanks to the implementation of a new calibration source, a continuous-wave-emission tunable laser. The laser allowed a full characterization of the diagnostic and a frequent precise calibration, making the CIS system reliable for parallel flow investigations during the operational campaign OP1.2. The validity and importance of the CIS measurements have been further confirmed with dedicated simulation of the SOL plasma parameters by the EMC3-EIRENE code, and by comparisons with other edge diagnostics. The CIS results show the effects related to dynamical changes in the SOL due to impurity gas puffs or the development of a plasma current. Moreover, CIS can be used as a powerful tool to test the limits of the current theoretical models, for example in the case of forward and reversed field experiments.
Impurity ions pose a potentially serious threat to fusion plasma performance by affecting the confinement in various, usually deleterious, ways. Due to the creation of helium ash during fusion reactions and the interaction of the plasma with the wall components, which makes it possible for heavy ions to penetrate into the core plasma, impurities can intrinsically not be avoided. Therefore, it is essential to study their behaviour in the fusion plasma in detail. Within the framework of this thesis, different problems arising in connection with impurities have been investigated. 1. Collisional damping of zonal flows in tokamkas: The effect of impurities on the collisional damping of zonal flows is investigated. Since the Coulomb collision frequency increases with increasing ion charge, heavy, highly charged impurities play an important role in this process. The effect of such impurities on the linear response of the plasma to an external potential perturbation, as caused by zonal flows, is calculated with analytical methods and compared with numerical simulations, resulting in good agreement. 2. Impurity transport driven by microturbulence in tokamaks: Fine scale turbulence driven by microinstabilities is a source of particle and heat transport in a fusion reactor. A semi-analytical model is presented describing the resulting impurity fluxes and the stability boundary of the underlying mode. The results are compared with numerical simulations. Both the impurity flux and the stability boundary are found to depend strongly on the plasma parameters such as the impurity density and the temperature gradient. 3. Pfirsch-Schlüter transport in stellarators: Due to geometry effects, collisional transport plays a much more prominent role in stellarators than in tokamaks. Analytical expressions for the particle and heat fluxes in an impure, collisional plasma are derived from first principles. Contrary to the tokamak case, where collisional transport is exclusively caused directly by friction, in stellarators an additional source of transport exists, namely pressure anisotropy. Since this term is, contrary to the contribution from friction, non-ambipolar, it plays an important role regarding the ambipolar electric field. Furthermore, the behaviour of heavy impurities in the presence of strong radial temperature and density gradients is studied, which lead to a redistribution of the impurities on the flux surfaces. As a consequence, the radial impurity flux is decreased considerably compared with a plasma in which the impurities are evenly distributed on the flux surfaces.