Thomson scattering measurements in low-density plasmas in the TST-2 spherical tokamak

Thomson scattering (TS) diagnostics have been widely used in fusion studies to measure profiles of electron temperature Te and electron density ne. In order to measure the low-density plasmas (ne ≤ 1018 m−3) in TST-2, which is sustained by lower hybrid wave power, the signal-to-noise ratio in TS measurement has been improved by various means. For instance, optimization of the detecting system, accumulation of TS data obtained from reproducible discharges, and application of a coaxial multi-pass scheme were carried out. As a result, the profiles have been measured successfully and a peaked ne profile and a hollow Te profile were obtained. Additionally, isotropy of Te near the plasma center was confirmed by coaxial double-pass TS measurement.