The MAST Synthetic Aperture Microwave Imaging (SAMI) radiometer is an antenna array designed to image thermal microwave emission from MAST fusion plasmaaSAMl is now installed on MAST and preliminary data has been taken. This data clearly show the presence of electrostatic to electromagnetic mode conversion and the circumstances under which this mode conversion take place are strongly related to the pedestal density gradient and magnetic field. These quantities are valuable in understanding tokamak pedestal behaviour and are especially important to models of the Edge Localised Mode (ELM). This paper describes SAMIs design and construction, as well as fist signals showing the presence of mode conversion

Freethy, S.

Huang, B. K.

Shevchenko, V.

Vann, R. G.L.

White Rose Research Online

This is a repository copy of Recent progress with the MAST synthetic aperture imaging radiometer.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/175778/Version: Accepted VersionProceedings Paper:Freethy, S., Huang, B. K., Shevchenko, V. et al. (1 more author) (2011) Recent progress with the MAST synthetic aperture imaging radiometer. In: 38th EPS Conference on PlasmaPhysics 2011, EPS 2011 - Europhysics Conference Abstracts. 38th EPS Conference on Plasma Physics 2011, EPS 2011, 27 Jun - 01 Jul 2011 38th EPS Conference on Plasma Physics 2011, EPS 2011 - Europhysics Conference Abstracts . , FRA , pp. 736-740. eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Recent progress with the MAST synthetic apertureimaging radiometerS. FreethyYork Plasma Institute, Department of Physics, University of York, YO10 5DD, U.K.EURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon,Oxfordshire OX14 3DB, U.K.E-mail: simon.freethy@ccfe.ac.ukB. K. HuangCentre for Advanced Instrumentation, Department of Physics, Science Laboratories,Durham DH1 3LE U.K.CCFE, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UKE-mail: billy.huang@ccfe.ac.ukV. ShevchenkoEURATOM/CCFE Fusion Association, Culham Science Centre, Abingdon,Oxfordshire OX14 3DB, U.K.E-mail: vladimir.shevchenko@ccfe.ac.ukR. G. L. VannYork Plasma Institute, Department of Physics, University of York, YO10 5DD, U.K.E-mail: rglv500@york.ac.ukAbstract. The MAST Synthetic Aperture Microwave Imaging (SAMI) radiometeris an antenna array designed to image thermal microwave emission from MAST fusionplasmas.SAMI is now installed on MAST and preliminary data has been taken. Thisdata clearly show the presence of electrostatic to electromagnetic mode conversion andthe circumstances under which this mode conversion take place are strongly relatedto the pedestal density gradient and magnetic field. These quantities are valuable inunderstanding tokamak pedestal behaviour and are especially important to models ofthe Edge Localised Mode (ELM). This paper describes SAMIs design and construction,as well as fist signals showing the presence of mode conversion.38th EPS Conference on Plasma Physics (2011) P2.050Recent progress with the MAST synthetic aperture imaging radiometer 21. How SAMI worksSAMI employs a technique called aperture synthesis, which is widely used in microwaveto radio wave imaging. Some of the more famous examples include the Very LargeArray and the SMOS satellite [1]. The Vann Cittert Zernike theorem states that thecross correlation between two points illuminated by a far field source is approximatelya sample of the Fourier transform of the brightness distribution [2].xzffxzFigure 1. Correlation between pairs of antennas represented by the circles (right) formsamples of Fourier space at positions defined by the vector between the pair normalisedto the wavelength of the radiation being observed (left). There is π rotational symmetryin Fourier space.The position in Fourier space at which the sample corresponds is given by the vectorbetween the pair of antennas which are being correlated normalised to the wavelengthof the wave being observed, termed baselines, as illustrated by figure 1. Enough pairsof antennas can be used to reconstruct Fourier space which can then be inverted to findthe image. The number of pairs for n antennas grows as n(n − 1)/2 so the number ofeffective pixels using this technique grows rapidly with the number of antennas.2. Array shape optimisation and hardwareThe problem with measuring only a few places in Fourier space is that this is equivalentto multiplying the Fourier function by a sharply peaked gridding function. The netresult being oscillations in real space which are not part of the image. This effect canbe mitigated by placing the antennas in such a way that these unwanted oscillations areminimal. By minimising a beam efficiency function which calculates the ratio of powerin the central lobe with total power via a simulated annealing approach, reasonable38th EPS Conference on Plasma Physics (2011) P2.050Recent progress with the MAST synthetic aperture imaging radiometer 3approximations to the optimal layout of antennas were found. SAMI is capable of usingsub-microsecond broadband switches to allow switching between groups of antennas inorder to enhance its coverage of Fourier space. Figure 2, shows two groups of 8 antennasoptimised simultaneously to find complimentary groups of antennas to switch between.θφFigure 2. Top left is the synthetic aperture equivalent of an array beam pattern, topright are the optimal antenna positions for two groups of eight antennas and bottomis the coverage of Fourier space for the optimal groupsSAMI employs a full vector heterodyne down converter (see figure 2 left), convertingreal and imaginary components of the microwave signal for 8 antennas at any of 16frequencies to a digitsable frequency range. These signals are then digitised by anFPGA controlled digitiser providing 14 bit resolution at 250MSs−1. SAMI uses printedcircuit board antipodal Vivaldi antennas which are 2cm wide by 6cm long and can beseen in array form in the top right of figure 2. These antennas are operational from 5 to35 GHz and were characterised by measurements made at the university of Strathclyde.SAMI includes sub microsecond switches to switch between antenna groups.3. Initial resultsThe first results with MAST plasmas have been obtained and we have clear signaturesthat we are able to see the mode conversion between the electrostatic Electron Bernstein38th EPS Conference on Plasma Physics (2011) P2.050Recent progress with the MAST synthetic aperture imaging radiometer 4Channel 1Channel 8Power splitters16 to 1 switchfixed frequency DROsFPGA digitiserIFAsLNAHybridIQPCFigure 3. Left: A schematic of the SAMI radiometer. SAMI covers the region from10GHz to 35GHz. Right: An array of 37 Vivaldi antennas. The antennas are 2cm by6cm and are operational from 5GHz to 35GHz.Wave (EBW) and electromagnetic waves. Figure 3 shows power traces from one of ourantennas with each frequency plotted with an offset for clarity. Initially the signalsdevelop together in the low density plasma as Electron Cyclotron Emission (ECE).Then the plasma becomes over-dense strongly suppressing the ECE. Note that thisappears to happen later for higher frequencies due to the density growing in time. Thenthe lower frequency channels begin to rise due to the EBW emission mode convertinginto electromagnetic waves near the plasma edge. Note that the signals are strongestfor frequencies from 13 10 18GHz since this range corresponds to the EC resonance.Emission from the second harmonic in this example is weaker, due to the relatively lowplasma density being insufficient for mode conversion of these frequencies.SAMI digitises real and imaginary components in order to perform digital sidebandseparation, allowing both upper and lower sidebands to be kept. Not only is this far moreefficient for estimating the cross correlation phase, it effectively doubles the number offrequency channels. SAMI is able to achieve up to 30dB suppression of sidebands.4. conclusionSAMI is an aperture synthetic imaging radiometer designed to obtain the first images ofEBW mode conversion, which is sensitive to edge magnetic field and density gradient.The antenna array has been designed explicitly to reduce the sidelobes generated byunder-sampling of Fourier space. SAMI performs full vector heterodyne down conversionto allow for digital sideband suppression and image formation. It can switch betweenany of 16 frequencies in the range of 10-35GHz in sub microsecond time. First data38th EPS Conference on Plasma Physics (2011) P2.050Recent progress with the MAST synthetic aperture imaging radiometer 5Figure 4. Left: The power received by SAMI for the 1st 200ms of a MAST shot withdifferent frequency channels plotted with an offset for clarity. Right: The reflectometersweeps up in frequency providing an excellent means to test the sideband separation.has been taken showing strong signatures of mode conversion and very good sidebandsuppression capacity.[1] A Camps et al. Radiometric sensitivity computation in aperture synthesis interferometricradiometry. IEEE transactions on geoscience and remote sensing, 36(2), 1998.[2] A R Thompson, J M Moran, and G W Swenson Jr. Interferometry and synthesis in radio astronomy.Wiley - VCH, 2004.38th EPS Conference on Plasma Physics (2011) P2.050

Recent progress with the MAST synthetic aperture imaging radiometer

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Recent progress with the MAST synthetic aperture imaging radiometer

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