Interpretation of D_alpha Imaging Diagnostics Data on the ASDEX Upgrade Tokamak

Harhausen, Jens (2009): Interpretation of D_alpha Imaging Diagnostics Data on the ASDEX Upgrade Tokamak. Dissertation, LMU München: Fakultät für Physik

Abstract

The Tokamak configuration is a promising concept for magnetic confinement fusion. Cross-field transport in the plasma core leads to a plasma flux across the separatrix into the scrape-off layer, where it is guided along field lines towards the divertor targets. A return flux of neutral particles after plasma-wall interaction is directed towards the plasma chamber. Each discharge scenario is accompanied by a characteristic recycling pattern. The dominant mechanisms of neutralplasma interaction are ionisation and atom-ion charge exchange. The impact of neutrals on the particle-, momentum- and energy-balance of the plasma is relevant for the understanding of the properties of the edge plasma and the state of the divertor plasma. Neutrals may cause energyand momentum-detachment, a state of reduced power and particle fluxes, at the targets, which is a prerequisite for acceptable wall loads under reactor conditions. The distribution of neutral particles in the plasma chamber can be determined by the analysis of line emission. Parameters of neutrals have been available so far only from localised measurements and it is therefore desired to extend the observation area. At the ASDEX Upgrade Tokamak, two cameras are installed to record the Deuterium Balmer-alpha (D_alpha) emission with high spatial resolution and dynamic range in the divertor and midplane regions. Two methods for data deconvolution are presented. A simple profile-fit is used to resolve the radial profile of emission at the low and high field sides for low and medium density discharges. This emission profile is translated to neutral parameters by comparison to the results obtained from kinetic modelling of neutral penetration (KN1D). An algorithm for tomographic reconstruction is applied to image data of the divertor region. In general, radiance data recorded is blurred due to the impact of diffuse reflection from surfaces of the plasma facing components in the all-Tungsten machine. Therefore, the tomographic algorithm has been extended by a model for reflection based on a solid angle resolved measurement. The sensitivity of the procedures is proven by the accurate analysis of different edge plasma configurations. Poloidally resolved neutral flux densities at the plasma edge and corresponding core fuelling rates are presented for the high field side. Underlying estimates of plasma parameters indicate a drop of static plasma pressure along the magnetic field towards the inner target. Changes of the poloidal flux density profile during a radial shift of the plasma column, indicate a correlation of plasma-wall gap and scrape-off layer parameters at the high field side. From the deconvolution of divertor view data separate emission patterns have been resolved. Besides the character of emission at the strike zones which can give a hint on the level of detachment, the occurrence of radiation above the inner target indicates that the distribution of plasma parameters is probably more complex than expected from simple radial decay lengths. The experimental emission profiles and inferred neutral parameters display an important boundary condition for complex 2D edge modelling codes like SOLPS. The comparison of experimental and code results question the applicability of the standard recipe (concerning code settings) for arbitrary plasma scenarios. An interface to theory is required to reasonably exploit the experimental data on neutral penetration. The essential result of this thesis is a reference frame for the quantitative analysis of video diagnostics data recorded on a Tokamak plasma, including the impact of reflecting plasma facing components.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: plasma diagnostics, tomography, ASDEX Upgrade, neutral penetration, plasma boundary
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 7. Mai 2009
• 1. Bericht­erstatter:in: Zohm, Hartmut