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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!