Investigation on Double-Pass Configurations for Thomson Scattering Measurements

Double-pass Thomson scattering is a simple and reliable scheme to measure two-directional (perpendicular and parallel) electron temperatures in plasmas. In this study, we configured a double-pass Thomson scattering configuration so that the laser beam passing through plasma is reflected by a mirror and passes through the plasma again to generate the second scattering light with a different scattering angle. To avoid direct re-entering of the beam to the laser, the reflected beam was tilted slightly. This study investigated the configuration in terms of the measurement performance and laser damage risk by the backward beam. Furthermore, this study clarified several requirements on the optical configuration and quantified the parameters' effects on the performance of the configuration. Through optimization procedures, three optimal configurations were figured out: (i) a simple configuration with two lenses and one mirror, but with a long distance from the laser to the plasma, (ii) another simple configuration that slightly breaks the requirement of sufficient deviation of the backward beam from the laser output, and (iii) a modified configuration with three lenses and one mirror.