Coupling study of fast wave near the lower hybrid frequency range in VEST

Abstract

The fast wave branch in lower hybrid resonance frequency range, especially higher than 2ωlh, has been proposed for the central or off-axis electron heating and current drive in higher density plasmas than the slow wave scheme. With a higher cutoff density for launching, efficient coupling between the antenna and plasma would be a priority issue for feasibility. The fast wave coupling characteristics with the wave frequency, gap size, and electron density profile of the Versatile Experiment Spherical Torus (VEST) device are investigated using a commercial full wave FEM solver, COMSOL. Maximum coupling between combline antenna and plasma is expected to be at ∼500 MHz with n∥∼4.5. The coupled power ranges from 90% to 60% in the gap size between 0.5 cm and 1.5 cm. The relative power fraction of the fast wave is larger than 80% at these conditions. The propagation and coupling power of the fast wave is crucially dependent on the plasma density window by launching and confluence densities. Initial experimental result with low power shows that measured coupling efficiency starts to increase as electron density in front of antenna attains the level of cutoff density for the fast wave propagation. It varies from 30% to 90% with the edge density evolution, which is consistent with the coupling simulation using the measured edge density profile. Coupling simulation verified in this study will make it possible to predict and analyze the coupling characteristics of future lower hybrid fast wave experiments.