APS 05 poster

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Fast Imaging of Visible Phenomena in NSTX
R. J. Maqueda Nova Photonics C. E. Bush ORNL L.
Roquemore, K. Williams, S. J. Zweben PPPL
47th Annual APS-DPP Meeting October 24-28,
2005 Denver, Colorado Poster RP1.00014
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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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Outline Two-dimensional imaging at fast frame
rates (gt10000 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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NSTXs 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 Da light fromgas puff
  I ? none f(ne,Te)
• Da emission only seen in range 5 eV lt
  Te lt 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
Separatrix
24 cm radial
11 MB
24 cm poloidal
Antenna limiter shadow
Blobs
L-mode
Spontaneous transition into quiescent H-mode
Ohmic H-mode
0.65 ms mosaic D2 puff Da filter
Transition takes place at 192.1 ms
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GPI Time Evolution
Divertor Da (a.u.)
2 kHz breathing mode
Image pixel (a.u.)
Reduced fluctuation level during H-mode
RMS fluctuation level
Shot 115513
Time (s)
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GPI Radial Profiles
Average image brightness (a.u.)
RMS fluctuation level
FWHM
Poloidal auto-corr. length (cm)
Shot 115513
Rmid-Rsep (m)
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GPI Active H-mode
24 cm radial
Separatrix
Antenna limiter shadow
5.5 MB
24 cm poloidal
Active
Blobs
Quiet
4.5 MW NBI
Active
Quiet
0.65 ms mosaic D2 puff Da filter
H-mode edge with blobs ...micro-ELMs?
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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 ms) by the
  outer divertor strike point.
• The inner leg of the divertor region is delayed
  respect to the outer strike point by 150 ms,
  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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L-H Transition Time Sequence
Image intensity drop sequence 1) Center stack
near midplane, with tdrop100 ms. 2) Outer
divertor strike point, within 20-30 ms. 3) Other
locations later, with slower decays. Similar
sequence in Da light.
Fish-eye view
CII (657.8 nm)
R.O.I. intensity (a.u.)
Da
Time (s)
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Edge Localized Modes (ELMs)

• Type I, III, and V ELMs are routinely seen in
  NSTXs 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
9.3 MB
Unperturbed emission
Energy/particle dump
Recovery
0-600 scale
0-1568 scale
Carbon sputtering
0-4095 scale
Shot 117407
CII (657.8 nm)

• 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.

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Type III ELM
4.4 MB
Outer strike point brightens
800 kA 4.2 MW NBI Double null
Inner leg re-attaches
Unperturbed emission
Divertor Da (a.u.)
3
1
2
4
5
6
1
2
3
4
5
6
Shot 117432
Shot 117432
CII (657.8 nm)
Recovery
Modes on inner separatrix
Relaxed to unperturbed emission
Time (ms)

• Energy dump affects inner leg detachment but
  emission layer persists close to separatrix.
• There is no measurable movement of X-point.

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Type V ELM
4.8 MB
Secondary band on outer strike point
Heat pulse propagates on inner separatrix
800 kA 4.2 MW NBI LSN
Unperturbed emission
Divertor Da (a.u.)
3
2
1
4
5
6
Shot 117407
Shot 117407
1
2
3
4
5
6
CII (657.8 nm)
...and reaches X-point region
Propagation continues
Relaxed to unperturbed emission
Time (ms)

• Heat pulse propagates on inner separatrix,
  delayed from outer strike point band.
• There is no measurable movement of X-point.

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Type V ELM
From peak in cross-correlation function In-out
delay 0.32 ms
Outer divertor
Image intensity (counts)
t 255.477 ms
Inner separatrix
Shot 117407
Image intensity (counts)
Shot 117407
Time (s)
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Type V ELM Heat Pulse Propagation
1.1 Km/s
Slow down reaching X-point
Poloidal distance along inner separatrix (cm)
t 255.587 ms
Shot 117407
Shot 117407
Time (ms) peak in cross-correlation function
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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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MARFE evolution
3.8 MB
Fish-eye view
Toroidally localized, rotating condensation
Upper divertor
Center stack
Lower divertor
Ion grad-B drift
Stagnation
900 kA 6.4 MW NBI Double null
MARFE moves downward
1.0 ms mosaic Da filter 9 ms exposures contrast
enhanced
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Plasma Positioning and EquilibriumCoaxial
Helicity Injection (CHI)

• 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
R. Raman (U. Washington)
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Coaxial Helicity Injection (CHI) Discharge
evolution
4.5 MB
Fish-eye view No filter 9 ms exposures
Detached plasma
Plasma current (kA)
Decay
Fully grown plasma
Breakdown and growing plasma
Current persistence
Injector current (kA)
Fast crowbar
Shot 118342
Time (ms)
For details see R. Raman, contributed oral
GO3.00011
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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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Lithium Pellet Injection NBI Heated H-mode
4.5 MB
Fish-eye view
Ablation begins in SOL
Pellet material diverted
Pellet material burns-out
Filamentary structure
Pellet material deposited on divertor surfaces
0-255 scale
0-1023 scale
0-4095 scale
Shot 117909
LiII (548.5 nm)
Pellet injection 1.4 ms lt-gt 20 cm penetration
600 kA - 4.4 MW NBI - double null
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Lithium Pellet Injection Ohmic Helium Plasma Full
penetration
7.7 MB
Fish-eye view
Core flux surfaces
Inside edge
Outer edge
0-600 scale
0-1568 scale
0-4095 scale
Shot 117094
LiII (548.5 nm)
Flux surface Flows?
Pellet ablation first seen at 240.47 ms
500 kA - Ohmic inner wall limited
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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 ms) 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
  NSTXs plasma shots.)
• Nearly every short portion of data contains
  interesting, valuable information!