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2D Impurity Flow Imaging on MAST using Coherence Imaging

2D Impurity Flow Imaging on MAST using Coherence Imaging. Scott Silburn Centre for Advanced Instrumentation, Durham University, UK James Harrison (CCFE), Clive Micahel (ANU), Ray Sharples (Supervisor, Durham), John Howard (ANU), Hendrik Meyer (Supervisor, CCFE), Kieran Gibson (York)

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2D Impurity Flow Imaging on MAST using Coherence Imaging

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  1. 2D Impurity Flow Imaging on MAST using Coherence Imaging Scott Silburn Centre for Advanced Instrumentation, Durham University, UK James Harrison (CCFE), Clive Micahel (ANU), Ray Sharples (Supervisor, Durham), John Howard (ANU), Hendrik Meyer (Supervisor, CCFE), Kieran Gibson (York) FUSENET PhD Event, York, 24th June 2013

  2. Outline • Introduction • Coherence Imaging Technique • The MAST Coherence Imaging Diagnostic • Initial Data

  3. Introduction – Flow Measurements • Understanding plasma flow is important for the successful design and operation of magnetic fusion devices, e.g. it impacts impurity transport, exhaust & plasma confinement. High quality flow data is required to develop our understanding and could help benchmark modelling. • Doppler spectroscopy of ion line emission is commonly used to measure impurity flows, but provides a limited number of line-integrated spectra. Complex or spatially extended flow patterns are hard to interpret from this. • Coherence imaging is a technique which can be used for high spatial and time resolution Doppler shift imaging, developed at Australian National University and demonstrated on DIII-D, TEXTOR, KSTAR.

  4. Introduction - MAST • Mega Amp Spherical Tokamak - CCFE, Oxfordshire, UK • R = 0.85m, r = 0.65m • Plasma current typically 400 – 900 kA, pulse length up to 700ms • Up-down symmetric • Open vacuum vessel geometry with many ports offers excellent access for optical diagnostics

  5. Coherence Imaging Technique

  6. Coherence Imaging Technique • Doppler shift causes a tiny change in fringe frequency • This causes a phase shift between the shifted and un-shifted interferograms after many wave cycles • Doppler shift can be determined from the phase shift at a known value of N

  7. Coherence Imaging & MAST System Design Camera lenses Back-to-back Camera lens Delay plateN wave delay, Savart plateFringes Plasma image with interferogram superimposed From Plasma Narrow (~3nm) band pass filter Linear polarisers, axes at 45° to plates

  8. Coherence Imaging on MAST: Specs • Targeted ion species: • C2+ (465nm) • C+(514nm) • He+ (468nm) • Field of view 9° - 40°(up to 1.4m x 1.4m of plasma cross-section) • Detector: 1024x1024 pixels (but ~10px spatial resolution across fringes), up to 3kHz full frame. • Flow resolution ~1 – 4 km/s • Can view the lower divertor, or radially or tangentially at the midplane

  9. Initial Data • Data has been obtained with time resolution of 1 – 20ms in all 3 species • Various flow phenomena observed, including oppositely directed toroidal Carbon flows above & below the midplane (at the high field side) at early times during the shot. Quantitative analysis & interpretation is now required. Centre column

  10. Initial Data • Initial divertor data appears to show C III flows towards both divertor targets (Also seen in divertor coherence imaging on DIII-D). • The divertor data contains enough information to enable tomographic inversion of flow profiles….

  11. Initial Data – Tomographic Inversion • Assuming toroidal symmetry and flow mainly along B, obtaining intensity and flow cross-sectional profiles is a large least-squares problem. • This is solved using a simple Simultaneous Algebraic Reconstruction Technique implementation in MATLAB, which is under testing & development with divertor data. • Fits so far look reasonable (but more work required), time to process a raw image frame to inverted flow profile is ~ 3min on a laptop. C III, 28841 at 362ms (H-Mode) Flow data Best fit Line integrated flow (km/s)

  12. Conclusions • Wide angle flow imaging of multiple impurity species in both the Scrape-off-Layer & Divertor of MAST has been demonstrated using coherence imaging. • Flows observed include reversal about the midplane during startup in Carbon, and quantitative analysis and interpretation of the data is now required. • Initial divertor data shows C III flowing towards the divertor targets (as on DIII-D) • Tomographic inversion is being developed to obtain divertor flow profile cross-sections

  13. Additional Slides

  14. Coherence Imaging & MAST Remove horizontal fringes using FFT Raw data image Extract local fringe contrast using FFT divide out instrument function Extract local fringe phase using FFT, subtract instrument function.

  15. Choice of Fixed Delay • Due to the multiplet structure of the carbon lines, the contrast exhibits a beating pattern with delay. The delay must be chosen to optimise the fringe contrast and satisfy T << Tc for the tomography. Fringe Contrast Left: Calculations of fringe contrast as a function of waveplate thickness and plasma temperature for C III (top) and C II (bottom), based on measured MAST divertor spectra. Fringe Contrast • We will have multiple interchangable plate options to allow optimisation for different measurements.

  16. Position Registration & Available Views • Line-of-sight calibration done in two stages: • Calibration of camera properties (focal length, distortion etc) done with test pattern photos in the lab • Calibration of position and pointing on MAST done with flashlamp-illuminated vessel photos and feature matching, given the camera properties. Test pattern image Tangential midplane view Radial midplane view

  17. Position Registration & Available Views • Expected coil locations line up well with vessel photos • EFIT lines up reasonably with plasma images

  18. Calibration Setup Cd spectral lamp Small (~20cm) integrating sphere

  19. Calibration Setup • Monitoring of calibration drifts when in use: fibre feed of radial midplane light in to image.. • Designed to provide a zero flow reference in part of the image….but doesn’t work yet.

  20. Tomographic Inversion • With so many line-of-sight measurements, it’s possible to use the data to reconstruct poloidal profiles of plasma quantities, under some assumptions e.g. toroidal symmetry. • Ray trace the pixel lines-of-sight through the machine, and project on to reconstruction grid in the R-Z plane. Camera data Noise etc Plasma emissivity profile Calculated grid cell / pixel weightings

  21. Tomographic Inversion Line-integrated intensity Recovered R-Z emission profile

  22. what happened to Ti? • But it also means we can’t measure ion temperature.

  23. Tomographic Inversion • Test cases reconstructed with flow errors of ~1 – 5km/s • But these cases will be easier than real data! Room for improvement? Simulated line integrated image

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