MIR on DIII-D

A microwave imaging reflectometer (MIR) instrument, capable of simultaneously measuring the poloidal and radial structure of density fluctuations, has been developed for the DIII-D tokamak and installed in May 2013. 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. 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.

Fig 1: Typical DIII-D MIR parameters, source: https://fusion.gat.com/pubs-ext/NuclFus/A27746abs.pdf, credit to C. M.Muscatello

Highlighted Innovations

The DIII-D MIR concept has undergone numerous technological and system-level upgrades both in optics and electronics since earlier microwave imaging systems, thereby permitting a higher level of robustness and flexibility:

Upgraded Optical design with mini-lens and dual-dipole antenna: The shape of the wavefront of the probing beam and the curvature of the cutoff layer strongly affect the integrity of the reflected signal. This is addressed with transmitting optical elements that are designed to control the shape of the probing beam. The innovative optical design keeps both on-axis and off-axis channels focused at the cutoff surface, permitting imaging over an extended poloidal region.[reference:Review of Sciencitific instruments 85, 11D702 (2014)]
Advances in microwave electronics make it possible to transmit and detect multiple frequencies simultaneously, permitting fluctuation measurements at multiple radial locations.Interesting physics occurs over the entire poloidal cross-section of the plasma, on disparate spatial scales. MIR is flexible in this respect, allowing a remote user to rapidly tune the individual probing frequencies for a variety of correlation studies.
Computer based control system: A master control module, which also fits into the backplane, is used to control all MIR module microcontrollers and the combined RF switches.This method allows up to 127 MIR modules to be controlled through a single master control module giving the system easy up scaling capabilities [reference: Review of Sciencitific instruments 85, 11D834 (2014)]
Synthetic diagnostic simulations: The feasibility of the DIII-D MIR optics design has been assessed through synthetic diagnostic simulations utilizing a full-wave code FWR2D coupled to an optical ray tracing platform that models realistic imaging components, help to corroborate our confidence in a successful implementation of MIR on DIII-D. [reference: Review of Scientific Instruments 83 (10), 10E338 (2012)]

Physics Results

The MIR system on DIII-D shares the same 270 degree midplane port with the ECEI system (Fig 2), allowing for simultaneous measurements for both temperature and density fluctuations of the same plasma volume. Numerous exciting physics results, like ELM (Fig 3) , EHO (Fig 4-6) related studies , Alfven Eigenmodes (Fig 7) etc., have been obtained from it.

Fig 2: Schematic of co-located ECEI/MIR system

Fig 4: Spectrogram of EHO from MIR data

Fig 3: Images of electron density and temperature fluctuations inside the DIII-D tokamak fusion plasma. The top two plots are spectra of representative channels from (a) MIR and (b) ECEI showing fluctuations in the interval between edge-localized mode activity. The bottom two plots are (c) density and (d) temperature image reconstructions of the 66-kHz fluctuation at 1860 ms and each panel is separated by 1.9 μs (source: IEEE Transactions on Plasma Science 42 (10), 2734-2735)

Fig 5: Frequency versus wavenumber spectrogram from MIR shows clearly the EHO and broadband MHD have opposite sign in phase velocity, partially implies they are drove from different mechanism.

Fig 6: Measured and synthetic MIR power spectra in poloidal wave numbers with frequencies (a) 57 GHz and (b) 58 GHz for an n = 1 EHO in DIII-D shot #157102, at 2420 ms

Fig 7: a) A single-channel spectrogram from MIR reveals a number of reversed-shear-induced Alfvén eigenmodes that sweep between 40 and 120 kHz as the safety factor, q, is varied during the Lmode phase of DIII-D discharge 161127. b) At 450 ms, both RSAEs and TAEs are observed by the diagnostic. Fitting a line through the spectrum of TAE modes provides an estimate of the Doppler shift and natural TAE frequency, i.e. the observed frequency in the absence of rotation. c) As qmin decreases, RSAEs sweep upward toward the TAE band. d) At the upper limit of their sweep, RSAEs couple to modes in the TAE gap.

Future Upgrade

Expanded to eight illuminating frequencies: In the near term, the 4-frequency up-converting probe source will be replaced with an 8-frequency source, and a longer term goal of 16 frequencies. This not only expands the number of pixels and coverage area but also provides the flexibility to arbitrarily position the center frequency and thus the radial position of the accessibility window
System-on-chip: More detail can be found here
Digital Beam Forming: In the long term, we will seek greater flexibility to shape and to focus the MIR beams. The large aperture lenses will eventually be phased out and replaced by electronic beam forming [reference: F. Hu et.al., Microwave Imaging Radar Reflectometer System Utilizing Digital Beam Forming]

Fig 5: Phased arrays for beam shaping and steering, and their application in fusion plasma diagnostics

Fig 6: Block diagram of the electronically controlled phase shifter transmitter system

System Related Publications

B. Tobias et.al., "Microwave imaging reflectometry on DIII-D",1st EPS conference on Plasma Diagnostics (ECPD2015)
Christopher M Muscatello et.al., "Technical overview of the millimeter-wave imaging reflectometer on the DIII-D tokamak (invited)", Review of Scientific Instruments 85, 11D702 (2014)
A. G. Spear et.al., "2D microwave imaging reflectometer electronic"s, Review of Scientific instruments" 85, 11D834 (2014)
X.Ren et.al., "Process to generate a synthetic diagnostic for microwave imaging reflectometry with the full-wave code FWR2D",Review of Scientific Instruments 85 (11), 11D863 (2014)
X.Ren et.al., "Evaluation of the operating space for density fluctuation measurements employing 2D imaging reflectometry", Review of Scientific Instruments 83 (10), 10E338 (2012)
Christopher M Muscatello et.al., "Microwave imaging reflectometry (MIR) for visualization of the 2-dimensional structure of density fluctuations on DIII-D"

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