Fluctuation effects on the plasma density profiles measured by using a frequency modulated continuous wave reflectometer

Abstract

Scanning diagnostics requires that for the correct measurement the target should not move during the scan. However, even if a frequency modulated continuous wave reflectometer is operated with a frequency modulation (FM) time of 10 µs, the high-temperature fusion plasma fluctuates even for such a short duration. The fluctuation effect on the reflectometer measurements is investigated by simulating the reflectometer response for various plasma conditions. Because the uncertainty of the zero density position might give an additional distortion to the reconstructed profile, this effect is also investigated. The simulation results show that the density profile reconstruction is not very sensitive to the zero density position but very sensitive to the fluctuations. The FM rate must be 100 times faster than the fluctuation frequency in order to suppress the radial distortion below about 20% of the fluctuation radial amplitude. An algorithm is proposed to reduce the distortion more effectively even at much lower FM rates. The distortion is canceled out by averaging two profiles, one of which is measured as increasing the frequency and the other as decreasing the frequency. The algorithm is applied to the experimental data analysis to confirm the simulation results. Through this study, it is concluded that the FM time of the Korea Superconducting Tokamak Advanced Research reflectometer should be reduced to 2 µs to be able to measure the H-mode plasma density profiles.