Design of a multi-channel polarization-preserving optical system for the KSTAR motional Stark effect diagnostic

An optical system capable of preserving, or minimizing the change of, the polarization properties of incident light has been designed and fabricated for the motional Stark effect diagnostic system which measures internal magnetic field structures inside the tokamak for the Korea Superconducting Tokamak Advanced Research. A dual photoelastic modulator (PEM) with a linear polarizer is included in the optical train with four lenses, a mirror and a dichroic beam splitter. Particular cares have been taken for the polarization properties of the delivered light to be perturbed as little as possible under a strong magnetic field and high vacuum. The lenses are made of STIH-6, a material with low Verdet constant, which minimizes the Faraday rotation. The residual Faraday rotation that takes place in the non-STIH-6 optical elements such as the vacuum window and the optical apertures of the PEM is calibrated out from the in-situ measurements using an in-vessel reference polarizer while energizing the magnetic field coils. In the lens design, the object plane is rotated by 44.8° from the optical axis because of the tilted setup-port. The image surface has a finite curvature to reduce the aberration from the four lenses. The fiber dissector is designed based on the focal plane that aligns the focal points from 25 lines-of-sight, each of which constitutes a bundle of 19 fibers. The fibers run about 35 m from the front optics in the tokamak vacuum vessel to the detector in the diagnostic area remote from the tokamak hall. The footprint images at the intersections of the lines-of-sight on the neutral beam trajectory confirms the imaging quality is sufficient to the diagnostic requirement with the designed magnifications.