Systematic evaluation of the effect of multi-ion-source neutral beam injection on motional Stark effect diagnostic in KSTAR

Abstract A newly developed systematic approach to evaluate the effect of multi-ion-source neutral beam injection on the motional Stark effect (MSE) diagnostic in Korea Superconducting Tokamak Advanced Research (KSTAR) is introduced. Based on a set of model equations combined with measured MSE data, this approach attempts to decompose the spectral overlap mixed by the neutral beam injected from multiple ion sources. An optimization algorithm minimizes the error between the measured Stokes parameters and those reconstructed by the separated spectra and thereby, evaluates a time-varying systematic offset caused by multi-ion-source beam injection in MSE measurements, regardless of the energy combination among the ion sources. From its application to a group of KSTAR plasma discharges, this methodology, attempted for the first time in the MSE community, has demonstrated its capability to provide researchers with a guideline on the neutral beam operation along with high-quality MSE measurements.