Microwave imaging reflectometry for density fluctuation measurement on KSTAR

Abstract

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.

Full-wave feasibility study of anti-radar diagnostic of magnetic field based on O-X mode conversion and oblique reflectometry imaging

“…Furthermore, the signal to noise ratio is higher, by simply adopting a sufficiently intense reflectometric source. The spatial and temporal resolution are comparable with other reflectometric imaging systems [12][13][14] .…”

“…Full-wave simulations exhibited the expected "hole" in reflectivity for that special direction of incidence. At the frequencies analyzed in our study (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), simulations show also that this reflectivity hole is complicated by reflections at the X-mode cutoff layer, which is itself a function of the the angle of incidence.…”

Intrinsic rotation reversal, non-local transport, and turbulence transition in KSTAR L-mode plasmas

“…On the other hand, the maximum value of core V shows little changes for the ECH power variation. MIR can also provide the information of poloidal rotation velocity v θ [23,27]. Fig.8 shows the cross-phase spectra of MIR in the core region at a low density ECH phase and a high density OH phase.…”

“…Moreover, the electron cyclotron emission imaging system (ECEI) [22] can provide the information of temperature fluctuation. Microwave imaging reflectometry (MIR) [23] measure the density fluctuation in KSTAR.…”

Role of Radio Frequency and Microwaves in Magnetic Fusion Plasma Research

“…This combination of an almost-perfect symmetric magnetic field and perturbation tool allows for the systematic testing of stability and confinement physics. The last unique feature of the KSTAR is its state-of-the-art 2D microwave imaging system-which employs microwave imaging reflectometry (MIR) and a 2D/3D electron cyclotron emission imaging (ECEI) system-which can visualize the MHD and turbulence physics at excellent spatial and temporal resolutions [13,14]. These diagnostic systems have demonstrated new physics not available in the past with 1D diagnostic systems.…”