Calibration and performance evaluation of MSE Wavelength-Interpolation Background Subtraction on KSTAR

In 2018, fifteen additional sightlines of a background polychromator system were fabricated and installed on the KSTAR MSE diagnostic, which now can subtract partially-polarized background light via wavelength interpolation on all 25 sightlines. This paper will discuss several calibrations of this instrument: standard polarization-angle calibrations with a rotating linearly-polarized light source; beam-into-gas calibrations to optimize filter passband placement relative to the beam-generated MSE spectrum; and calibrations to quantify Faraday rotation in the optical system. The overall system performance including measurement accuracy and ability to accurately measure /compensate the partially polarized background light over a range of plasma conditions will be discussed, based on data acquired in the upcoming KSTAR 2018 experimental run campaign. Preliminary considerations for the use of the polychromator channels that measure background light to provide a measurement of the Zeff profile will be presented.