Simultaneous measurement of plasma pressure anisotropy with a double-pass Thomson scattering diagnostic system on the TST-2

We have developed a double-pass Thomson scattering diagnostic system for the TST-2 spherical tokamak device. By measuring the first- and second-pass scattering signals simultaneously, we obtained the two directional pressures and we measured the pressure anisotropy (i.e., the ratio of the pressures) with an error of 5% - 10% for moderate density (≃ 2 × 10¹⁹ m^-3) plasmas. We observed 30% and 100% anisotropy at the center and edge of the Ohmic-heated plasmas, respectively. We propose a three-temperature Maxwellian model, in which the fitting is better than in the shifted-Maxwellian model. The estimated plasma current density was close to the averaged current density. The results suggest that the contribution of thermal electrons to a plasma current is large in Ohmic-heated plasmas. (author)