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22–27 Oct 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
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Experimental Evidence of Lower Hybrid Wave Scattering in Alcator C-Mod due to Scrape Off Layer Density Fluctuations

26 Oct 2018, 15:40
20m
Mahatma Mandir Conference Centre

Mahatma Mandir Conference Centre

Gandhinagar (nearest Airport: Ahmedabad), India
Oral EXW - Magnetic Confinement Experiments: Wave–plasma interactions; current drive; heating; energetic particles EX/8, PPC/2 - TH/6 Heating, Current Drive & Steady State

Speaker

Dr Gregory Wallace (MIT Plasma Science and Fusion Center)

Description

We present new experimental measurements of the Lower Hybrid (LH) wave electric field vector, $E_{LH}$, obtained in Alcator C-Mod and provide a direct comparison with 3D full-wave COMSOL simulations using the cold plasma dielectric tensor and reflectometry measured density profiles. Two key results are reported: 1) The direction of $E_{LH}$ was found to have a substantial poloidal component and is in strong disagreement with the nearly radial full-wave simulation result. 2) Adding Scrape Off Layer (SOL) density fluctuations to the density profile implemented in the full-wave simulations can be used to explain the $E_{LH}$ direction discrepancy.

Polarized passive optical emission spectroscopy was implemented to determine $E_{LH}$. This technique entails measuring two orthogonally polarized $D_\beta$ spectral line profiles. The spectra are simultaneously fit to the Schrodinger equation containing both magnetic and time periodic electric field operators. The three components of $E_{LH}$ are the only fit variables. The experimental $E_{LH}$ results were compared to axisymmetry 3D full-wave COMSOL simulations via a synthetic diagnostic. Comparing the experimental and simulation results, good agreement was found with regard to the magnitude of $E_{LH}$ both as a function of measurement location and LH power. However, it was found experimentally that $E_{LH}$ contained a poloidal component having a magnitude on the order or greater than that of the radial component. The poloidal component was found to be a strong function of poloidal angle, increasing towards the midplane, and a weak function of toroidal angle, remaining nearly constant. This result strongly disagrees with the nearly radial $E_{LH}$ predicted by the full-wave simulations. SOL density fluctuations based on an experimentally verified 3D BOUT turbulence simulation of a similar Alcator C-Mod discharge were added to the density profile. We found that diffraction and scattering from a realistic turbulence model generates a substantial poloidal component in $E_{LH}$, significantly closing the gap between the experimental and simulation results. This result indicates that SOL turbulence can have a detrimental effect on LHCD performance if the wavelength is on the order of the turbulence characteristic scale length.

Country or International Organization United States of America
Paper Number EX/8-2

Primary author

Dr Elijah Martin (Oak Ridge National Laboratory)

Co-authors

Dr Andris Dimits (Lawrence Livermore National Lab) Dr Cornwall Lau (Oak Ridge National Laboratory) Dr David Green (Oak Ridge National Laboratory) Dr Gregory Wallace (MIT Plasma Science and Fusion Center) Dr John Wright (MIT - PSFC) Dr Joseph Ilon (Lawrence Livermore National Lab) Dr Maxim Umansky (Lawrence Livermore National Lab) Dr Paul Bonoli (Massachusetts Institute of Technology) Dr Robert Mumgaard (MIT Plasma Science and Fusion Center) Dr Syun'ichi SHIRAIWA (PSFC, MIT)

Presentation materials