Relative calibration correction using fast LEDs to enhance the reliability of Thomson scattering diagnostic on KSTAR

Abstract A Thomson scattering diagnostic system on KSTAR has been operated to measure the electron temperature and density profiles, which are among the most essential parameters in the study of fusion plasmas. The system was optimized by employing a fast digitizer for full-pulse fitting and by performing absolute calibration with rotational Raman scattering. While performing the Raman calibration, a discrepancy between filters 1 and 2 at the edge polychromators was observed, suggesting limited reliability of the calibration. Moreover, the distribution of minimum chi-square values, which quantify the statistical goodness of fit of the electron parameter measurements, deviated noticeably from the theoretical expectation, particularly for certain polychromators. These discrepancies were attributed to a systematic error in the relative responsivity of polychromators, caused by differences in pulse duration between the pulsed laser and the halogen lamp, which was used for relative calibration. To address this issue, the relative responsivity was corrected using fast LEDs with a pulse waveform similar to that of the laser. As a result, the reliability of the electron temperature and density obtained after the correction was enhanced.