Effects of LH power on SOL density profiles and LH coupling on Alcator C-Mod

Abstract

A swept-frequency X-mode reflectometer has been used to measure the scrape-off-layer (SOL) density profiles with and without lower hybrid (LH) power at three poloidal locations adjacent to the LH launcher for various plasma parameters in order to understand the coupling of LH waves on Alcator C-Mod. LH power has been observed to create significant poloidal SOL density profile asymmetries that are correlated with visible video camera images of emissivity patterns in front of the LH launcher. The observed density profile asymmetries depend on LH power, , magnetic geometry and magnetic field direction. A 2D diffusive–convective model has been used to show that these density profile modifications are consistent with a LH vortex, where LH power drives E × B drifts that then modify the SOL density profile. In particular, the simulations show that the density profile can possibly create a net poloidally averaged density depletion in front of the waveguide rows. A LH slab coupling model is then used to show that the simulated reflection coefficients strongly depend on the poloidal density profile asymmetries. The simulated LH power reflection coefficients agree with the experimental reflection coefficients only after the observed density depletion is included in the model.