Validations of calibration-free measurements of electron temperature using double-pass Thomson scattering diagnostics...

This paper evaluates the accuracy of electron temperature measurements and relative transmissivities of double-pass Thomson scattering diagnostics. The electron temperature (T_e) is obtained from the ratio of signals from a double-pass scattering system, then relative transmissivities are calculated from the measured T_e and intensity of the signals. How accurate the values are depends on the electron temperature (T_e) and scattering angle (θ), and therefore the accuracy of the values was evaluated experimentally using the Large Helical Device (LHD) and the Tokyo spherical tokamak-2 (TST-2). Analyzing the data from the TST-2 indicates that a high T_e and a large scattering angle (θ) yield accurate values. Indeed, the errors for scattering angle θ = 135° are approximately half of those for θ = 115°. The method of determining the T_e in a wide T_e range spanning over two orders of magnitude (0.01-1.5 keV) was validated using the experimental results of the LHD and TST-2. A simple method to provide relative transmissivities, which include inputs from collection optics, vacuum window, optical fibers, and polychromators, is also presented. The relative errors were less than approximately 10%. Numerical simulations also indicate that the T_e measurements are valid under harsh radiation conditions. This method to obtain T_e can be considered for the design of Thomson scattering systems where there is high-performance plasma that generates harsh radiation environments.