Development of a microwave polarimeter for the measurement of the lower-hybrid driven current profile on the TST-2 spherical tokamak

Non-inductive plasma startup is considered to be essential for a spherical tokamak fusion reactor. On the TST-2 spherical tokamak at the University of Tokyo, plasma current ramp-up up to a quarter of the Ohmically driven plasma current has been achieved using only the lower-hybrid current drive. Equilibrium analysis suggests a formation of a very broad, possibly hollow, current profile, but the analysis has a large uncertainty due to lack of internal magnetic field measurements. A microwave polarimeter is being developed to constrain the equilibrium reconstruction and determine the current profile experimentally. Due to the relatively low plasma current, the Cotton-Mouton effect is significant and needs to be taken into account in the fitting process. Current profile estimation of a fully non-inductive lower-hybrid driven discharge will be presented if available.