Results and analysis of Gas Puff Imaging experiments in NSTX: turbulence, L-H transitions, ELMs and other phenomena

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Results and analysis of Gas Puff Imaging
experiments in NSTX turbulence, L-H transitions,
ELMs and other phenomena R.J. Maqueda Nova
Photonics S.J. Zweben, T. Munsat, T.
Biewer Princeton Plasma Physics Laboratory C.
Bush, R. Maingi Oak Ridge National Laboratory
Laboratory J. Boedo UC-San Diego N.A. Crocker, S.
Kubota, X.V. Nguyen, W.A. Peebles UC-Los
Angeles and the NSTX Team
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Abstract The 2-D structure of the plasma edge is
measured in the National Spherical Torus
Experiment (NSTX) by imaging its spectral line
emission with 4-10 ms time resolution. In the Gas
Puff Imaging (GPI) diagnostic a poloidally
elongated gas puff is used to localize the
emission region in the poloidal-radial plane. The
typical L-mode edge is turbulent with toroidally
elongated filaments moving poloidally and
radially. This is contrast with the H-mode edge
where a more quiescent emission is observed. By
using a PSI-5 ultra-fast framing camera developed
by Princeton Scientific Instruments, capable of
frame rates of up to 250000 frames/sec, the
transitions between L- and H-modes are recorded.
In addition, ELMs and MHD activity with high
poloidal modes numbers are also observed with
this diagnostic. Analysis of data obtained in
recent GPI experiments will be presented, as well
as comparison with other edge diagnostics. This
includes spatial mode number, time series and
velocity field analysis. Plans for future
research/experiments will also be presented.
Work supported by DoE grant DE-FG02-04ER54520.
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Motivation There are very many reasons why the
edge plasma is important...

• Core confinement depends on edge gradients and
  gradient driven instabilities.
• The transport at the edge and across the SOL
  depends on the level and characteristics of the
  local edge turbulence, either directly or through
  their effect on the edge profiles.
• The edge plasma interacts with the wall (divertor
  or limiters), liberating impurities.
• Impurity transport along SOL and across the edge
  determines impurity concentration in the core.
• Edge plasma interacts with injected auxiliary RF
  heating power.

Gas Puff Imaging (GPI) provides access to many of
the important phenomena at the edge turbulence,
L-H transitions, ELMs, RF coupling, etc.
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GPI Diagnostic

• Imaging camera used to view visible emission from
  edge.
• Gas puff is injected to increase image contrast
  and brightness. Gas puff does not perturb local
  (nor global) plasma.
• Emission filtered for Da (or HeI) light from gas
  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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GPI Diagnostic (cont.)

• Viewing area located just above midplane on outer
  edge of NSTX.
• 25 cm x 25 cm area imaged with 1-2 cm resolution.
  
• PSI-5 camera used to record 300 frames, 64 x 64
  pixels/frame, typically at 250,000 frames/s
  -gt 1.2 ms of discharge coverage
• Measurement complemented with 13 discrete chords
  having 200 kHz resolution and 128 ms of discharge
  coverage, each viewing a 2 cm spot of the
  emission cloud.

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Typical NSTX parameters Shot 108322 at 0.18s
Core B(0) 3.5 kG I 900 kA Te(0) 1 keV ne(0)
2.5 x 1013 cm-3 ltnegt 2 x 1013 cm-3
Outer edge (Rmid 1.46 m) ne 5 x 1012 cm-3 Te
13 eVb 10-3 Ln 2 cm rs 0.2 cmnei 6 x
106 s-1 Lc 5 m (connection length to
divertor)lei/Lc 0.05 q (BT/Bpol)(R/A)
2 LRBM 1 cm
High-speed imaging of edge turbulence in NSTX,
S.J. Zweben et al., Nucl. Fusion 44, p. 134, 2004.
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GPI Movie Clips
Gas injection
Gas injection
Movie clips available from
http//www.pppl.gov/szweben/NSTX04/NSTX_04.html
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Movie clips

• Shot 113830 L-mode
• Shot 113139 H-mode
• Shot 113420 H-mode
• Shot 113732 L-H transition
• Shot 113079 L-H transition
• Shot 113735 L-mode, just before L-H transition
• Shot 113062 H-L transition dither
• Shot 113074 H-L transition dither
• Shot 113409 Type II/III ELM
• Shot 113470 Type V ELM
• Shot 113012 MHD Breathing
• Shot 113371 MHD Bouncing
• Shot 113757 MHD Blinking

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Image analysisL-mode vs. H-mode comparison

• Turbulence is qualitatively similar in Ohmic and
  L-mode discharges.
• No differences have been (yet) observed in lower
  single null, upper single null, double null and
  limited discharges.

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Pre-LH transition dithers
Transient H-mode like periods can occur up to 10
ms before the main L-H transition
Filaments on 2 cm diameter spot
113737113741113743113744
Filamentfrequency(filaments/ms)
dithers
t-tLH (ms)
Although turbulence is much reduced in H-mode,
with quiet periods lasting up to 100 ms,
filaments (blobs) and waves can be occasionally
seen.
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L-H transition in other diagnostics
Fluctuations in 30 GHz reflectometer decrease
rapidly during L-H transition. For details see
Crocker et al. JP1.022
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Transition observations

• L-H transition dithers can precede the main
  transition.
• The main L-H transition appears like a continuous
  evolution from turbulent filaments to the
  characteristic quiescent state in H-mode. This
  evolution lasts lt 100 ms without any apparent new
  spatial features or flows. Filaments just appear
  to drain out.
• Similar suppression of turbulence observed in
  other diagnostics during L-H transition.
• The H-L back transition generally appears as a
  high-n poloidal perturbation that evolves into
  radially moving filaments.

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New analysis tool
Velocity fieldsTobin Munsat
Do shear or zonal flows develop at the time of
the L-H transition? ...work in progress.
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ELMs
Type I Mid DWMHD with very fast or no pre-cursor,
Pheat gtgt PL-H GPI images best described as a
momentary transition back to L-mode
characteristics. Type II/III Small DWMHD with low
frequency, long lived pre-cursor Increased
filament activity respect to quiescent H-mode
discharges. Type V Very small DWMHD with n1
pre-cursor Modest difference (if any) with
background H-mode turbulence (filament
frequency). ELMs in NSTX Maingi et al., CO3.005
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Compound ELMs
113737
DivertorDa (a.u.)
Auto- correlation
L-mode
Compound ELM
GPI fastchord(a.u.)
Powerspectrum
H-mode
No substantial differences observed between
L-mode turbulence and that during compound ELMs.
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Summary and open questions
Measurements
Open Questions

• Turbulence is similar in Ohmic and L-mode
  discharges. (Differences are observed during
  H-modes.)
• H-modes might be preceded by short quiescent
  phases.
• H-modes have short periods of L-mode type
  activity (filaments).
• H-L transitions (generally) appear first as
  high-n poloidal modes.

• Are there differences in Ohmic, L-mode, upper
  single null, lower single null, double null, and
  inner wall limited?
• Is the main transition, and any pre-transition
  dithers, associated with poloidal flows (shear or
  zonal)?.
• What triggers filament formation during H-modes?
• Is there a consistent instability pattern leading
  to the H-L transition?

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Summary and open questions
Measurements
Open Questions

• Fast divertor Da light appears to start
  decreasing before the midplane GPI images show
  H-mode characteristics.
• ELMs are seen with similar characteristics as
  L-mode blobs.
• Breathing, bouncing and blinking are seen
  associated with MHD activity?

• Where are the first signs of the L-H transition
  observed? Candidate X-point region.
• Are ELMs and L-mode filaments similar entities?
  Are the sporadic H-mode filaments minute ELMs?
• What causes breathing, bouncing and blinking?

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Next steps

• Use new fast framing camera, capable of recording
  gt 100000 frames/s and whole discharge coverage.
• Observe emission from X-point region during L-H
  and H-L transition..
• Use wide view of NSTX plasma to determine
  location of L-H transition and ELM birthplace.
• Study effects of low Z (lithium) and medium Z
  (neon), radiatively cooled edge, on the edge
  turbulence. Is there a relation with impurity
  enhanced modes?
• Search for poloidal flows (shear, zonal, etc.)
  during L-H transition, both at midplane and at
  the X-point region.