Microwave imaging reflectometry for density fluctuation measurement on KSTAR

A dual-frequency microwave imaging reflectometry system was commissioned to measure both coherent and turbulent electron density fluctuations in KSTAR plasmas. Imaging of the density fluctuations is achieved with an array of 16 vertically aligned detectors and two X-mode probe beam frequencies (tunable over 78–92 GHz between plasma discharges). The system provides the capability of fluctuation measurements with poloidal wavenumbers (kθ) up to ~3 cm−1 at the maximum sampling rate of 2 MHz. Following extensive laboratory tests, the system was further tested with known coherent density fluctuations during the precursor oscillation of the m/n = 1/1 internal kink mode. The phase information of the reflected beam was compared with the precursor oscillation of the electron temperature measured by an electron cyclotron emission (ECE) radiometer. Density fluctuation levels (δne/ne) at two radial positions separated by the inversion radius (inside and outside) were comparable to temperature fluctuation levels (δTe/Te) from ECE signals. Subsequently, two correlation analysis methods were applied to turbulent fluctuation measurements in a neutral beam heated L-mode plasma to determine the mean poloidal rotation velocities of density fluctuations at two radial positions. The measured mean poloidal velocities were ~8.4 km s−1 at r/a ~ 0.6 and ~5 km s−1 at r/a ~ 0.7 in the clockwise direction, which differed by 1–2 km s−1 with the projected poloidal velocities from the toroidal rotation velocity measured by charge exchange recombination spectroscopy.