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Calibration of a Lyman-α radiation diagnostic in presence of RF field and measurements in a plasma-wall sheath

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Thesis advisor : Alexandre Escarguel
Email and address : alexandre.escarguel, 04 91 28 82 04, centre St Jérôme, service 362
Co-advisor : Kovacic Laurence laurence.kovacic, 06 42 54 87 97, centre St Jérôme, service 362

Subject description :
EFILE is a diagnostic which allows to directly measure an electric field in vacuum or in a plasma. 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-α transition to the ground state which intensity is directly related to the amplitude of the electric field. Its absolute value is determined with a proper calibration. It has already been applied to the study of plasma-wall sheath in the case of static field*.
The aim of this work is to study a radiofrequency (RF) electric field in vacuum and in a plasma, especially in the sheath between the plasma core and the surface of the polarized plate which serves as antenna. Theoretical descriptions and numerical simulations of RF sheaths are active research topics in tokamak physics, showing that an antenna modifies key plasma parameters (electron temperature, plasma density, plasma potential...), as well as generates 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. Quantitative experimental verifications are missing since they require simple geometries, specific diagnostics and sufficient experimental time. The existing diagnostic EFILE will thus be used in a simple cylindrical configuration first, where the plasma is not magnetized and where the field is generated between two parallel plates. This will allow to study the physical problem of sheath and also calibrate the diagnostic. Experimental results will be compared to theoretical models or numerical calculations (developed through a collaboration with Fresnel Institute of AMU). The perspective of this work is to install this diagnostic in MISTRAL (magnetized plasma device in the Turbulence Plasma team) where results will complete those of other diagnostics (Langmuir probes, Laser Induced Fluorescence) in order to study turbulence and transport. Then, EFILE should be installed in IShTAR (test bench operating at tokamak edge-like conditions for density and temperature), of a RF sheath in front of an ICRF antenna thanks to a collaboration with Institute of Plasma Physics in Garching (Germany).
Bibliography :
*L. Chérigier-Kovacic, P. Ström, A. Lejeune and F. Doveil, Review of Scientific Instruments 86, 063504 (2015) ; doi : 10.1063/1.4922856