Fast Imaging of Visible Phenomena in NSTX

R. J. MaquedaNova Photonics

C. E. BushORNL

L. Roquemore, K. Williams, S. J. ZwebenPPPL

47th Annual APS-DPP MeetingOctober 24-28, 2005

Denver, Colorado

Poster RP1.00014

Movie clips are hyperlinked with this “camera” symbol.

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Abstract

Edge phenomena are important for global plasma confinement as well as

power and particle handling and distribution to plasma facing components. High

frame rate, 2-D imaging is a powerful tool to access the physics behind these

phenomena which include: edge turbulence and "blobs", ELMs, and MARFEs.

This diagnostic is also useful in general plasma equilibrium and dynamics

measurements, like those during Coaxial Helicity Injection discharges, and in

pellet injection experiments. A new Phantom 7 fast-framing digital camera has

been installed in NSTX which has been used at frame rates typically ranging

between 68000 frames/s and 120000 frames/s and full discharge coverage

(frames recorded for over 2 s). Examples will be presented showing the

usefulness of this diagnostic for physics studies in the areas mentioned above.

Work supported by DoE grant DE-FG02-04ER54520.

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OutlineTwo-dimensional imaging at fast frame rates (>10000 frames/s) has many applications in magnetically confined plasmas:

• Edge turbulence and “blobs”: Gas Puff Imaging (GPI) diagnostic.

• L-H transitions: Where does the transition start?

• Edge Localized Modes (ELMs): Heat pulse evolution and interaction with plasma facing components.

• Multifaceted Asymmetric Radiation From the Edge: Is a MARFE axisymmetric?

• Plasma positioning and equilibrium: Development of non-inductive current initiation by Coaxial Helicity Injection (CHI).

• Lithium pellet injection: Ablation dynamics and plume development.

A new fast framing camera capable of capturing 120,000 frames/s with full discharge coverage is being used in NSTX.

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NSTX’s Phantom 7 Camera*

• Frame rates of: ≤4,800 frames/s at 800 x 600 pixel resolution ≤68,000 frames/s at 128 x 128 pixel resolution≤120,000 frames/s at 64 x 64 pixel resolution

• Minimum frame exposures of 2 µs.

• Digitization: 12-bit.

• C-MOS detector with 30%-40% Q.E. and 22 µm x 22 µm pixels.

• Full discharge coverage with 2 GB of on-board memory.

• Fast download speeds through Ethernet connection (100 Mbit/s network).

• Control through LabView, synchronized to MDS+ shot cycle.

• Coherent fiber bundles used to transmit image to camera.

• Interference filters used to select visible bands of spectrum.

* Manufactured by Vision Research, Wayne, NJ.

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GPI Diagnostic

• Camera used to view visible emission from edge just above midplane.

• Gas puff is injected to increase image contrast and brightness. Gas puff does not perturb local (nor global) plasma.

• Emission filtered for D light from

gas puff: I none f(ne,Te)

• D emission only seen in range

~ 5 eV < Te < 50 eV

• View aligned along B field line to see 2-D structure B. Typical edge phenomena has a long parallel wavelength, filament structure.

• For more details: “Gas puff imaging of edge turbulence”, R.J. Maqueda et al., Rev. Sci. Instrum. 74(3), p. 2020, 2003.

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Summary of GPI ResultsOhmic H-mode

• Edge turbulence observed during Ohmic H-modes in NSTX is similar to that measured in neutral beam heated H-modes.

• Quiescent H-mode edge is present with the turbulence much reduced respect to the preceding L-mode phase.

• Only small amplitude poloidal modulations of the emission has been observed during H-modes.

• The fluctuation level decreases from a typical 10%-40% RMS level in L-mode to an also typical 5% RMS level in a quiescent H-mode.

• The poloidal autocorrelation lengths appear to be somewhat smaller than those previously reported in H-modes (S.J. Zweben et al., Nucl. Fusion 44, p. 134, 2004).

For details see: C. E. Bush, poster RP1.00028

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GPI: L-H Transition

Transition takes place at ~192.1 ms

L-mode

Separatrix

Antenna limiter

shadow

24 cm radial

24 cm poloidal

Spontaneous transition into quiescent H-

mode

“Blobs”

Ohmic H-mode

0.65 ms mosaicD2 puffD filter

~11 MB

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GPI: Time Evolution

Time (s)

Imagepixel(a.u.)

RMSfluctuation

level

DivertorD

(a.u.)

H-modeH-mode

drsep = -5 cm

drsep = 0 cm

drsep =+5 cm

Sho

t 115

513

L-modeburst

2 kHz“breathing”

mode

Reduced fluctuation level during

H-mode

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RMSfluctuation

level

Averageimage

brightness(a.u.)

Poloidalauto-corr.

length(cm)

Rmid-Rsep (m)

average 0.191 s - 0.192 s (L-mode)

average 0.1922 s - 0.1932 s (H-mode)

average 0.207 s - 0.208 s (H-mode)

average 0.209 s - 0.210 s (L-mode)

GPI: Radial Profiles

Sho

t 115

513

FWHM

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GPI: Active H-mode

H-mode edge with “blobs” ...micro-ELMs?

Separatrix Antenna limiter shadow 24 cm radial

24 cm poloidal

“Blobs”Active

Active

Quiet

Quiet

4.5 MW NBI

0.65 ms mosaicD2 puffD filter

~5.5 MB

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L-H TransitionWhere does the transition start?

• The image intensity is consistently reduced first at the midplane near the center stack.

• This is followed soon after (20-30 s) by the outer divertor strike point.

• The inner leg of the divertor region is delayed respect to the outer strike point by ~150 s, with a slower decay rate. This, perhaps, introduced by atomic physics of highly radiating MARFE-like region.

• NOTE: Data available for only LSN H-modes with high field side fuelling and fixed plasma parameters (800 kA, 4.5 kG, 4 MW NBI).

• Only shots with “clean transitions” (no dithers and low fluctuation levels) were selected.

• Time traces normalized to “1” before the transition and “0” after the transition

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D

CII (657.8 nm)

Time (s)

R.O.I.intensity

(a.u.)

L-H Transition: Time Sequence Image intensity drop sequence:

1) Center stack near midplane, with drop~100 s.

2) Outer divertor strike point, within 20-30 s.

3) Other locations later, with slower decays.

Similar sequence in D light.

Fish-eye view

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Edge Localized Modes (ELMs)• Type I, III, and V ELMs are routinely seen in NSTX’s H-mode shots.

(Type II ELMs have recently been observed too.)

• Fast camera imaging shows evolution characteristics of impurity emission layers in divertor region during the different types of ELMs.

• Type V ELMs show heat pulse propagation characteristics consistent with energy/particles ejection from the closed field line region near the lower strike point, low field side. (For more details see: R. Maingi, invited talk CI1b.003.)

Lower divertor tangential view

Phantom camera image

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Type I ELM

Sho

t 117

407

CII (657.8 nm)

Carbon sputtering

Energy/particle dump Recovery

• Energy dump into divertor region causes CII emission layer to move to smaller (and larger) major radii.

• EFIT reconstructions show the X-point moves upward and inward on the order of a few centimeters.

0-600 scale

0-1568 scale

0-4095scale

Unperturbed emission

~9.3 MB

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Type III ELM

Sho

t 117

432

Outer strike point

brightens

Recovery

• Energy dump affects inner leg detachment but emission layer persists close to separatrix.

• There is no measurable movement of X-point.

Inner legre-attaches

Modes on inner

separatrix

Relaxed to unperturbed

emission

Unperturbed emission

CII (657.8 nm)

800 kA4.2 MW NBIDouble null

Time (ms)

Divertor D (a.u.)

Sho

t 117

432

1 2 3

4 5 6

1 23 4 5 6

~4.4 MB

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Type V ELM

Sho

t 117

407

Secondary band on

outer strike point

...and reaches X-point region

• Heat pulse propagates on inner separatrix, delayed from outer strike point band.

• There is no measurable movement of X-point.

Heat pulse propagates

on inner separatrix

Propagation continues

Relaxed to unperturbed

emission

Unperturbed emission

CII (657.8 nm)

800 kA4.2 MW NBI

LSN

Time (ms)

Divertor D (a.u.)

Sho

t 117

407

1 2 3

4 5 6

1 2345 6

~4.8 MB

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Type V ELM

Time (s)

Imageintensity(counts)

Imageintensity(counts)

Outer divertor

Inner separatrix

From peak in cross-correlation function: In-out delay = 0.32 ms

Shot 117407

Sho

t 117

407

t = 255.477 ms

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Type V ELM: Heat Pulse Propagation

Time (ms)peak in cross-correlation function

Poloidal distance along

inner separatrix (cm)

1.1 Km/sSlow down reaching X-

point

Shot 117407

Sho

t 117

407

t = 255.587 ms

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MARFE evolutionAre MARFEs toroidally symmetric?

• MARFEs are seen on the divertor region and center stack of NSTX.

• Although the evolution of the MARFE is varied, some new characteristics have been observed.

• MARFEs born near the lower divertor move upward (against ion grad-B drift direction) as toroidally localized condensation, while rotating toroidally, following the magnetic field pitch.

• Upward movement stagnates and becomes a “more typical” toroidally symmetric ring.

• MARFE then moves downward towards lower divertor, while still rotating.

• Presence of highly radiating MARFE coincides with decrease in divertor recycling.

• MARFE: “Multifaceted Asymmetric Radiation From the Edge” [B. Lipschultz et al, Nucl. Fusion 24, p. 977, 1984].

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Center stack

Lower divertor

Toroidally localized, rotating

condensation

1.0 ms mosaicD filter

9 s exposurescontrast enhanced

MARFE evolution

Upper divertor

Stagnation

MARFE moves

downward

Ion grad-B drift

Fish-eye view

900 kA6.4 MW NBIDouble null

~3.8 MB

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Plasma Positioning and EquilibriumCoaxial Helicity Injection (CHI)

R. Raman (U. Washington)

• Solenoid-free plasma startup is important for the spherical torus concept. Coaxial Helicity Injection (CHI) is a promising method to achieve this goal.

• Fast-framing digital camera gives operators feedback on plasma positioning and equilibrium during CHI experiments.

• 60 kA of closed flux current generated using only 7 kJ of capacitor bank energy.

• In some discharges, the current channel shrinks to a small size and persists for more than 200 ms.

For details see: R. Raman, contributed oral GO3.00011

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Coaxial Helicity Injection (CHI)Discharge evolution

For details see: R. Raman, contributed oral GO3.00011Time (ms)

Plasma current

(kA)

Injector current

(kA)

Sho

t 118

342

Fish-eye viewNo filter

9 s exposures

Detached plasma

Decay

Fully grown plasma

Fast crowbar

Current persistenceBreakdown and growing plasma

~4.5 MB

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Lithium Pellet Injection

• Lithium pellet injection is used in NSTX to modify the conditions of plasma facing components, as well as, for diagnostic purposes.

• Lithium pellets with masses between 0.43 mg and 5 mg are injected just above the outer midplane at ~150 m/s.

• Fast-framing digital camera shows pellet penetration, ablation of pellet material and transport along field lines towards divertor regions.

• Ablated pellet material shows structure of underlying electron density (filamentary structure) and flux surfaces (if deep penetration).

For details on lithium pellet injection experiments see:H. Kugel, contributed oral GO3.00008

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Sho

t 117

909

LiII (548.5 nm)

Pellet injection~1.4 ms <-> ~20 cm penetration

0-255 scale

0-1023 scale

0-4095scale

Lithium Pellet InjectionNBI Heated H-mode

Fish-eye view

Ablation begins in SOL

Pellet material “diverted”

Pellet material burns-out

Filamentary structure

Pellet material deposited on divertor surfaces

600 kA - 4.4 MW NBI - double null

~4.5 MB

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Sho

t 117

094

LiII (548.5 nm)

0-600 scale

0-1568 scale

0-4095scale

Lithium Pellet InjectionOhmic Helium Plasma

Full penetrationFish-eye

view

Outer edgeCore flux surfaces Inside edge

500 kA - Ohmic – inner wall limited

Flux surfaceFlows?

Pellet ablation first seen at 240.47 ms

~7.7 MB

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Summary of Physics Results

• Ohmic H-modes in NSTX appear similar to neutral beam heated H-

modes. They are categorized among the “quiescent” group of H-modes.

• During the L-H transition (NBI shots), the recycling (and CII light) is first

reduced at the midplane near the center stack and soon followed (20-30

s) by the outer divertor strike point region.

• Type V ELMs show heat pulse propagation characteristics consistent with

energy/particle ejection from the closed field line region near the lower

strike point, low field side.

• MARFEs appear to originate as toroidally localized condensations that

later become “more typical” toroidally symmetric rings.

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Summary and Conclusions• Fast-framing visible cameras have multiple uses in magnetically confined

plasmas.

• Examples have been shown pertaining to edge turbulence, ELMs, L-H transitions, MARFEs, solenoid-free startup and pellet injection.

• Other possible uses include:- ELMs using GPI and fish-eye views.- “GPI” using pellets and/or supersonic gas injector.- Interaction between MARFEs and ELMs.

• With more than 210 Gbyte of Phantom camera data collected in the 2005 experimental campaign of NSTX the first challenge becomes automated analysis. (Note: Camera was used on only 1/3 of NSTX’s plasma shots.)

• Nearly every short portion of data contains interesting, valuable information!