PhD Code: 2016-DC-19:
- Host institute 1: FP1-Ghent University (Home University) - FP9-Max-Planck-Institut für Plasmaphysik Garching und Greifswald (Home Institution)
- Host institute 2: AM07-Aix- Marseille Université (Host University)
- F6. Diagnostics, plasma control and data analysis
- Prof. Kristel Crombé (promotor) - Dr. Rodolphe D''Inca (mentor)
- Prof. Fabrice Doveil (co-promotor) - Dr. Laurence Kovacic (mentor)
Background: Launching electromagnetic waves in the Ion Cyclotron Range of Frequency (ICRF) domain in plasmas is a promising auxiliary heating technique, in order to obtain the high temperatures needed for nuclear fusion reactions. However, to ensure good ICRF wave accessibility to the plasma core, the ICRF antenna must be placed as close as possible to the plasma. Experimentally it is observed that active ICRF antennas modify key plasma parameters (electron temperature, plasma density, plasma potential…), as well as generate enhanced plasma-wall interactions, caused by the creation of so-called RF sheaths. Examples of such interactions include increased heat loads and hot spots on limiters and enhanced sputtering of plasma facing components. Theoretical descriptions and numerical simulations of RF sheaths are active research topics, but quantitative experimental verifications are missing since they require simple geometries, specific diagnostics and sufficient experimental time. The work for the thesis will therefore be performed at a novel dedicated test facility, IShTAR, Ion cyclotron Sheath Test Arrangement, located at the Max-Planck-Institut für Plasmaphysik in Garching (Germany). It aims at studying antenna near-fields and RF sheath effects in the presence of a plasma and magnetic field in the framework of a European collaboration. IShTAR operates at tokamak edge-like conditions for density and temperature. It has been equipped with a helicon plasma source. The characterisation of the plasma is on-going. A single strap ICRF antenna is under construction; the design was simplified as far as possible to facilitate the modelling. In addition, IShTAR is intended to serve as a test environment for the development of antenna and sheath diagnostics.
Objective: The aim of the work is to develop an optical diagnostic to measure RF electric fields in front of an antenna in IShTAR. The electric field is a key parameter for the modelling of RF sheaths; it influences moreover the modifications of the density profile in front of the antenna, the acceleration of the ions and the hot spots on plasma facing components. So far it has never been measured directly in existing fusion devices for ICRF antennas. The method that will be used is based on the EFILE (Electric Field Induced Lyman-a Emission) technique developed by the Plasma Turbulence team at Aix-Marseille University: when a beam of 2s (metastable state) hydrogen atoms is exposed to an electric field, Stark mixing between 2s and 2p levels induces the Lyman-alpha transition to the ground state which intensity is directly related to the amplitude of the field. The EFILE (Electric Field Induced Lyman-alpha Emission) technique is a new, direct and non-intrusive way to measure a constant or oscillating electric field amplitude in vacuum and in a plasma. It will be the first time that this diagnostic will be used in a magnetised plasma to measure RF electric fields. The existing experimental set-up in Marseille consists of two parts, the source of the metastable test-beam and the detection device. They will be rebuilt and adapted to IShTAR to measure the fields in front of an ICRF antenna. The diagnostic will first be tested and calibrated in vacuum for different antenna powers. Then the experiment will be done in the presence of a plasma. Results will be compared with Langmuir probe data and numerical modelling.
Time line and mobility scheme (research need to be performed for at least six month in two different countries): First 6 - 9 months:stay at AMU to get familiar with the diagnostic and perform measurements on the laboratory set-up. Design of system for IShTAR. Rest of the period: stay to IPP Garching for construction, installation, calibration of the diagnostic, and to perform measurements at the ICRF antennas in IShTAR.
Original document: 2016-DC-19