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

• Introduction

• Coherence Imaging Technique

• The MAST Coherence Imaging Diagnostic

• Initial Data

Outline

• 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.

Introduction – Flow Measurements

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

Coherence Imaging Technique

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

Linear polarisers, axes at 45° to plates

Narrow (~3nm) band pass filter

Plasma image with interferogram superimposed

From Plasma

Delay plateN wave delay,

Savart plateFringes

Camera lensesBack-to-back Camera

lens

Coherence Imaging & MAST System Design

• 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

Coherence Imaging on MAST: Specs

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

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….

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.

Flow data Best fitC III, 28841 at 362ms (H-Mode)

Line

inte

grat

ed fl

ow (k

m/s

)

• 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

Conclusions

Additional Slides

Remove horizontal fringes using FFT

Extract local fringe phase using FFT, subtract instrument function.

Extract local fringe contrast using FFT divide out instrument function

Raw data image

Coherence Imaging & MAST

• 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.

Choice of Fixed Delay

Fringe ContrastFringe 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.

• We will have multiple interchangable plate options to allow optimisation for different measurements.

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.

Radial midplane view Tangential midplane viewTest pattern image

Position Registration & Available Views

• Expected coil locations line up well with vessel photos• EFIT lines up reasonably with plasma images

Calibration Setup

Cd spectral lamp

Small (~20cm) integrating sphere

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.

• 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

Plasma emissivity profile

Noise etc

Calculated grid cell / pixel weightings

Tomographic Inversion

Tomographic Inversion

Line-integrated intensity Recovered R-Z emission profile

what happened to Ti?

• But it also means we can’t measure ion temperature.

• Test cases reconstructed with flow errors of ~1 – 5km/s– But these cases will be easier than real data! Room for improvement?

Tomographic Inversion

Simulated line integrated image