Speaker
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 |
