Real-Time 2-D Optical Polarization Properties of the Fusion Reactor First Mirror Based on Active Polarized Beams

First mirrors (FMs) are essential plasma-facing components for optical measurements including spectroscopy and image systems to diagnose plasma characteristics in controlled fusion devices. Under the action of plasma, the FM surface morphology, structure and composition, and optical properties will be changed, which seriously affect the optical performance and service life of the FM. The existing FM detection methods are usually based on offline detection, which need to take FMs out from tokamak. With the development and requirements of ITER, fusion reactor running time will become longer and longer, even several years. The FM detection must be real-time and online. Based on the polarization characteristics of a metal object, the experiment system of active polarized beam detecting FM optical polarization properties was proposed. The polarization direction of incident beams was modulated at horizontal, -45°, and vertical direction, respectively. The real-time 2-D polarization information varieties, including the degree of polarization, polarization azimuth, and polarization ellipticity, reflecting from different samples with surface roughness Ra = 0.012, 0.025, and 0.05 $\mu \text{m}$ , respectively, were studied in detail. It was indicated that depolarization happened after reflection from the samples. The greater the Ra, the more changed the polarization parameters, and the worse of the uniformity of polarization parameters. It is found that the 2-D polarization properties can be used to detect a different roughness degree Ra, which provides a significant basis for further quantitative detection of FM damage.