Technical overview of the millimeter-wave imaging reflectometer on the DIII-D tokamak (invited)

Abstract

The two-dimensional mm-wave imaging reflectometer (MIR) on DIII-D is a multi-faceted device for diagnosing electron density fluctuations in fusion plasmas. Its multi-channel, multi-frequency capabilities and high sensitivity permit visualization and quantitative diagnosis of density perturbations, including correlation length, wavenumber, mode propagation velocity, and dispersion. The two-dimensional capabilities of MIR are made possible with 12 vertically separated sightlines and four-frequency operation (corresponding to four radial channels). The 48-channel DIII-D MIR system has a tunable source that can be stepped in 500 μs increments over a range of 56 to 74 GHz. An innovative optical design keeps both on-axis and off-axis channels focused at the cutoff surface, permitting imaging over an extended poloidal region. The integrity of the MIR optical design is confirmed by comparing Gaussian beam calculations to laboratory measurements of the transmitter beam pattern and receiver antenna patterns. Measurements are presented during the density ramp of a plasma discharge to demonstrate unfocused and focused MIR signals.

Evolution of E × B shear and coherent fluctuations prior to H-L transitions in DIII-D and control strategies for H-L transitions

“…During the ELM-free period starting shortly after NBI power is reduced to minimal diagnostic levels and before the start of back transition activity, a powerful coherent fluctuation is seen with consistent properties in all fast density measurements: BES, Doppler Back Scattering (DBS), 32 Millimeter-wave Imaging Reflectometer (MIR), 33 and the CO 2 interferometer. 34 Unfortunately, fast electron temperature measurements from the Electron Cyclotron Emission (ECE) 35 diagnostic are unavailable because typical densities in these experiments result in cut-off of the critical ECE chords.…”

Millimeter-wave system-on-chip advancement for fusion plasma diagnostics

“…All of them have already demonstrated encouraging results of visualizing density fluctuations thus offering more potential on instability observations. The DIII-D MIR instrument currently uses a 12-vertical-channel Schottky diode heterodyne receiver array with 4 simultaneously tunable transmitter signals, thus providing a 12x4 pixels image 2 .Both of the MIR systems share the same port and window on the tokamak including large aperture optics with the ECEI diagnostic systems, and therefore the combination is capable of probing the density and temperature fluctuations over a specific area inside the plasma.…”

Optics System Design of Microwave Imaging Reflectometry for the EAST Tokamak

“…The typical density fluctuation structure not only has radial wavenumbers, but also has significant poloidal wavenumbers, which means the onedimensional geometric assumption breaks down. In order to keep the simple data interpretation method valid, where the density fluctuation is proportional to phase variation, imaging optics with large aperture are employed to collect the reflected wavefront onto the detector array to reduce the interference pattern caused by the curved unsmooth cutoff layer [9,10] . The ideal transmitter and receiver optics should generate a curved wavefront or image plane to match the cutoff layer perfectly.…”