Development and experimental results of a new dispersion interferometer system, Single Crystal Dispersion Interferometer with a single detector on KSTAR

Dispersion interferometers using an acousto-optic modulator (AOM), capable of suppressing phase shifts associated with mechanical vibrations, have been used widely in fusion research. We have developed a dispersion interferometer system using not only a single nonlinear crystal but also with a single detector so that the beam paths of the first and second harmonics are exactly same from the beam source to the detector. Thus, our dispersion interferometer removes a possible source of errors due to different beam paths of the harmonics around the AOM and detectors, if multiple detectors were used. The new dispersion interferometer system using an Nd:YAG laser with a fundamental wavelength 1064nm has been installed to measure line integrated densities of KSTAR discharges, especially during shattered pellet injection (SPI). Since SPI results in more than one order higher density and density gradient (order of 10²² m^-2) compared to conventional tokamak plasmas, using a short-wavelength laser is beneficial where our new design of the dispersion interferometer becomes valuable. We introduce the new dispersion interferometer system and its measurement results from KSTAR. The experimental results are compared with measurements from the KSTAR TCI (two-color interferometer) system, which is a routine measurement system for electron density in KSTAR but with a weakness of frequent fringe jumps right after SPI.