John Howard J Chung, O Ford, R Wolf, J Svennson, R Konig 2D MSE imaging on the KSTAR tokamak and future prospects 1.

John Howard J Chung, O Ford, R Wolf, J Svennson, R Konig

2D MSE imaging on the KSTAR tokamak and future prospects

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Outline

• Measurement principles– Doppler imaging on DIII-D

• Optical system and calibration

• KSTAR measurements

• Modeling results (using full QM treatment)

An alternative approach to spectroscopy

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Simple polarization interferometer:

InterferogramS = I(1+ cos)

Polarizers

Waveplate at 45 degrees (delay LB)

Input

Spectral Lines

Fourier transform

Interferogram

To recover the fringe properties, measurements are required at multiple interferometric delays

a simple polarization interferometer gives contrast and phase at a single optical delay

Spatial heterodyne interferometerSavart plate introduces angular phase shear generates straight parallel fringes imprinted on image.

Demodulate for local fringe brightness, contrast and phase.

Phase shift tracks Doppler colour changes flow fields

DIII-D Divertor raw image

Tomographically inverted DIII-D divertor brightness and flow images

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Demodulated brightness (top) and phase (bottom) projections at representative times during the divertor detachment for DIII-D discharge #141170: (a) 500 ms, (b) 2000 ms and (c) 4000 ms.

With Diallo, Allen, Ellis, Porter, Meyer, Fenstermacher, Brooks, Boivin

Corresponding tomographic inversions of brightness (top) and phase (bottom)

Reasonable agreement with UEDGE modeling

Motional Stark effect polarimetry senses the internal magnetic field

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Top view KSTAR MSE viewing geometry

Motional Stark effect (MSE) polarimetry measures the polarization orientation of Stark-split D656 nm emission from an injected neutral heating beam. The splitting and polarization is produced by the induced E-field (E = v x B ) in the reference frame of the injected neutral atom. MSE can deliver information about the internal magnetic field inside a current-carrying plasma

Angle-varying Doppler shift every observation position requires its own colour filter.

Interferometric approach – periodic filter allows 2-D spatial imaging Bz(r,z)

Beam

View range

Edge

Centre

A typical Doppler shifted Stark effect spectrumEdge Centre

Modelled interferometric image of beamCourtesy, Oliver Ford, IPP

Imaging spectro-polarimetry for MSE

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Recall simple polarization interferometer:

Output signal S = I(1+ cos)

Polarizers

Waveplate (delay )

Input

If input is polarized already (angle ), remove the first polarizerResulting interferogram fringe contrast depends on polarization orientation:

S = I(1+ cos2cos)

Add a quarter wave plate. Fringe phase depends on polarization orientation:S = I[1+cos(2

The and components interfere constructively (no need to spectrally isolate)

Quarter waveplate

How do we image the multiplet?For one of the multiplet components (e.g. ), the interferometer output is:

S = I [1+ cos(2)]

For the orthogonal component (slightly different wavelength), it is

S = I [1- cos(2)]

For MSE triplet, after adding the interferograms, the effective signal contrast depends on the component contrast difference – . Choose optical delay to maximize the contrast difference –

Model of KSTAR isolated full energy Stark multiplet and associated nett contrast

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Good contrast (~80%) across full field of view (i.e. Stark splitting doesn’t change significantly). But significant phase variation due to large Doppler shift

Optical delay 1000 waves a-BBO plate thickness ~5 mm2nm bandpass filter tilted to track Doppler shift across FOV

KSTAR parameters: Bt = 2.0T on axis, Ip = 600 kAD beam, 85keV/amu, 1.0 degrees divergence

Centre

Edge

Centre

Edge

Final imaging MSE instrument

S = I0 [1 + cos(kxx cos(kyyInstrument produces orthogonal phase modulated spatial carriersDemodulate fringe pattern to obtain Doppler shift and polarization

A first quarter wave plate and shearing Savart plates results in a phase encoded double spatial-heterodyne system for and amplitude encoding for

Power spectrum of interference pattern

All information is encoded on distinct spatial heterodyne carriers:Polarimetric angles: (orientation and ellipticity)Interferometer contrast and phase: (splitting and Doppler shift)

Calibration image using Neon lamp at 660nm

Power spectrum of interference pattern

All information is encoded on distinct spatial heterodyne carriers:Polarimetric angles: (orientation and ellipticity)Interferometer contrast and phase: (splitting and Doppler shift)

Typical calibration data

(a) Central horizontal slices across a sequence of demodulated polarization angle images . The Doppler phase image is insensitive to the calibration polarizer angle.

(a) Deviation from linearity of the measured polarization angle at the centre of the calibration image versus polarizer angle. Cell size for averaging is ~1.5-2 carrier wavelengths (10-14 pixels). There is a small systematic variation. Random noise ~0.1 degrees (calibration image).

Optical system layout

From plasma

Telescope Cell Camera

Filter

Mirror

Typical MSE double heterodyne image

Pixelfly1300x1000

100ms exp

Frame rate10Hz Day 1

Beam direction

Radiation noisePlasma Boundary/ port opening

Orthogonal spatial carriers

Internal reflection and sparks(not an issue for imaging MSE)

Typical MSE double heterodyne image

Pixelfly1300x1000

100ms exp

Frame rate10Hz Day 2

Typical MSE double heterodyne image

Pixelfly1300x1000

100ms exp

Frame rate10Hz

Day 3

Typical MSE double heterodyne image

Conclusion: Need new camera Solution: CID camera + remote + shield

Pixelfly1300x1000

100ms exp

Frame rate10Hz

This is our beam-into-gascalibration image

Measured and modelled Doppler phase images are in good agreement

Line-of sight integration effects may account for the small discrepancies.

Viewing from above mid-plane accounts for tilt of phase contours

System tolerant of large beam energy changes (70-90 keV)

Measurement Model

Centre Edge

Measured and model “nett polarization” images

Simple circular plasma model - 2.0T, 600kA

A typical measured nett polarization image - 2.0T, 600kANote: A fixed constant shift of 16 degrees has been subtracted Window Faraday effect? thermal drift? misalignment?

Low brightness regions

Reflection artifact

Nett polarization angle = plasma MSE angle - Gas MSE reference angle

Typical KSTAR midplane radial profile evolution during RMP ELM suppression xpts

Edge

Axis

System should be self calibrating – edge polarization angle is determined by toroidal current and PF coils – other angles are referred to the edge.

(alleviates issues with thermal drifts, window Faraday rotation, in situ calibration problems etc.)

EdgeAxisCommon mode noise structures from beam-into-gas calibration have been partly removed

Ramp up

Tolerant of polarized background reflections

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Reflections from internal structures have little effect on inferred polarization angle.

True for broadband emission, polarized or unpolarized

QM modeling of system polarization response

• Apply QM model developed by Yuh, Scott, Hutchinson, Isler etal to estimate importance of Zeeman effect on MSE nett polarization (three directions with corresponding polarized components E, v and B)

• No line of sight integration effects• Statistical populations• Uniform brightness beam (no CRM modeling)• KSTAR viewing geometry• Simple circular flux surfaces with Shafranov shift• Spectro-polarimeter – sum over 36 Stark-Zeeman

component lines23

Modeled E,v and B components

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E ()B(5% of intensity)

V ()

Polarizationorientation

Ellipticity

Centre Edge

Nett ellipticity angles are comparable in magnitude to polarization tilt

Comparison with ideal Stark effect model

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Stark-Zeeman

Geometric model (no Zeeman)

Difference orientation angle variation across MSE image less than ~0.1o

Standard geometric models for interpretation are OK

Difference orientation angle

Plasma images show strong ellipticity

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Typical raw image of beam emission

Beam direction

Orthogonal carriers(mostly linear)

Elliptically polarized light

Ellipticity imagesBeam emission images show larger than expected ellipticity

Beam into gas Beam into plasma

80 keV 85 keVEllipticity unlike QM model. Window linear birefringence?

Dependence on beam energy indicates other than some B field dependent optical effect Zeeman-Stark coupling

Beam into gas

Attributes of MSE imaging approach

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Analyse full multiplet so no need for multiple discrete narrowband filters Simple inexpensive instrument ~103 more channels at <10% total cost No filter tuning issues or incidence angle sensitivities Tolerant of beam energy changes (10-20%), other beam energy components,

overlapping beams. Multiple heterodyne options (spatial/temporal), single channel or imaging

Insensitive to “broadband” polarized background contamination

Insensitive to non-statistical populations

Full Stokes polarimetry Possibility of self calibration based on unpolarized plasma radiation (Voslamber 1995) mirror/window degradation

Can be applied to spectrally complex elliptically polarized multiplets (Zeeman effect)

New opportunities. For example: 2D toroidal current imaging (in principle) and possibility of synchronous imaging of sawteeth, MHD, ELMs, Er etc.

Future directions/possibilities

• Replace crystal quartz window to eliminate Faraday rotation and anomalous ellipticity and install radiation hard camera for next KSTAR campaign.

• Fast system for real-time equilibrium• Time-multiplex system for high spatial resolution imaging

(RMP effects)• Use gated intensified camera to synchronously study

magnetic reconnection for comparison with ECE imaging.• MSE/Zeeman imaging at ASDEX and DIII-D (for pedestal

and ELM suppression studies)

Conclusion

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Modeling indicates that imaging MSE should be a reliable tool for obtaining 2d maps of the internal magnetic field in tokamaks.

IMSE significantly increases the information available to infer the current profile.