Institute of Interfacial Process Engineering and Plasma Technology
Gas-puff imaging of blob filaments at ASDEX
Upgrade
www.igvp.uni-stuttgart.de
TTF Workshop 2013
G. Fuchert1, D. Carralero2, T. Lunt2, P. Manz2, H. W. Müller2,
M. Ramisch1, V. Rohde2, U. Stroth2 and the ASDEX Upgrade Team
1Insitute of Interfacial Process Engineering and Plasma Technology IGVP,
University of Stuttgart, Stuttgart, Germany
2Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching,
Germany
Institute of Interfacial Process Engineering and Plasma Technology
Motivation
 „Blobs“ of increased pressure occur in SOL of many fusion plasmas
 Contribution to the turbulent SOL transport (radial propagation)
 Increased particle and energy flux to the walls
 Understanding essential to predict and control SOL transport
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vr
Blob filaments at MAST
[B. D. Dudson, PPCF 50, 2008]
Image of blob from GPI at AUG
Institute of Interfacial Process Engineering and Plasma Technology
Outline
 Experimental setup and blob detection
 Blob properties in L- and H-mode
 Generation rate, waiting-time distribution
 Cross-field size
 Blob velocity
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 Influence of MP coils on blobs
 Summary
Institute of Interfacial Process Engineering and Plasma Technology
Gas-puff imaging
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Increase neutral gas pressure locally
Deuterium puff with 6·1020 particles/sec
Observe Dα light emission I(ne, Te)
Camera: 120 kfps, FOV ≈ 5x5 cm near outer
midplane, resolution ≈ 1 mm ≈ 3 ρs
ρpol=1.05
ρpol=1.10
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G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Experiments
 After a purely ohmic heated phase, additional ECRH heating triggers
the L-H transition.
 Comparison of blob properties in both phases (omitting ELM phases)
ne
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POhmic
PECRH
Dα flux
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Institute of Interfacial Process Engineering and Plasma Technology
Blob detection
 Substract mean and normalize images.
 Determine standard deviation for every pixel.
 Detect connected structures with Imax> It
(e.g. It = 2σ) and I > Imax/2
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 Fit an ellipse to any detected structure to get
the location and size of the blob.
 Track stuctures over time to get their velocities.
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Institute of Interfacial Process Engineering and Plasma Technology
Mean images
Ohmic L-mode
ρpol=1.05
H-mode
ρpol=1.10
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Limiter
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Institute of Interfacial Process Engineering and Plasma Technology
Generation rate, waiting-time distribution
 Qualitatively comparable waiting-time distributions (WTD) in L- and H-mode
 ASDEX Upgrade: Similar PDFs for Isat in both modes [1]
 Exponential if blob generation is a stochastic (Poisson) process [2]
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 Generation rate in the range of a few kHz
 Blob generation by turbulence around the separatrix
[1] G. Antar et al., PPCF 50, 2008
[2] O. E. Garcia, PRL 108, 2012
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Institute of Interfacial Process Engineering and Plasma Technology
Comparable waiting-time distributions
Ohmic L-mode
H-mode
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 Measurements do not contradict exponential WTDs
 Detection rate of a few kHz (consistent with turbulence generated blobs)
 No significant differences in L and H-mode
G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Comparable waiting-time distributions
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 Measurements do not contradict exponential WTDs
 Detection rate of a few kHz (consistent with turbulence generated blobs)
 No significant differences in L and H-mode
G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Cross-field size
No clear expectation from theory, typically ~ 1 cm (a few ρs)
Ohmic L-mode
H-mode
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 Comparable cross-field sizes of L- and H-mode filaments of a few ρs
 Tendency for larger H-mode blobs
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Institute of Interfacial Process Engineering and Plasma Technology
Blob velocity
 Poloidal velocity mainly determined by ExB background.
 Poloidal polarization of blob filament due to e.g. curvature drifts [1].
 Resulting ExB-drifts point outwards.
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 Size dependence of velocity has different regimes, e.g.
– 2D models [2,3]:
– 3D simulation (tokamak geometry) and analytical estimation [4]:
[1] D. A. D’Ippolito et al., Phys. Plasmas 9, 2002
[2] J. R. Myra et al., Phys. Plasmas 12, 2005
[3] J. R. Myra et al., Phys. Plasmas 13, 2006
[4] D. A. Russel et al., PRL 93, 2004
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Institute of Interfacial Process Engineering and Plasma Technology
Poloidal Velocity
Ohmic L-mode
ρpol=1.05
H-mode
ρpol=1.10
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 Shear layer in the SOL in L-modes [B. Nold et al., to be published]
 Poloidal velocity profile changes significantly after L-H transition
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Institute of Interfacial Process Engineering and Plasma Technology
Radial velocity
Ohmic L-mode
ρpol=1.05
H-mode
ρpol=1.10
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Institute of Interfacial Process Engineering and Plasma Technology
Radial dependence of the velocity
ρpol=1.05
ρpol=1.10
Ohmic L-mode
H-mode
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 Comparable vr profiles in both regimes.
 Radial deceleration of blobs (as observed in other experiments, e.g. DIII-D)
[J. A. Boedo et al., PoP 8, 2001]
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Institute of Interfacial Process Engineering and Plasma Technology
Size dependence of radial velocity
Ohmic L-mode
H-mode
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 Large spread in the measured velocities
 Weak negative correlation for every analyzed shot
Additional scaling parameters needed (p, l‖?)
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Institute of Interfacial Process Engineering and Plasma Technology
Size dependence of radial velocity
Comparison of local mean values for different shots
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 Tendency for slightly reduced vr after L-H transition
 Differences only marginal (within the resolution)
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Institute of Interfacial Process Engineering and Plasma Technology
MP coils/ELM mitigation at AUG
Magnetic perturbation (MP) coils at ASDEX Upgrade
 In-vessel saddle coils are used to produce magnetic perturbations.
 ELM mitigation is observed when the coils are switched on.
[W. Suttrop et al., PRL 106, 2011]
 Recent studies aim to elucidate the influence on the edge turbulence.
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G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Influence of MP coils on blob properties
H-mode, MP coils off
H-mode, MP coils on
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 Blob detection rate increases in phases where MP coils are on.
 Either generation rate increases or blobs propagate further outwards.
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Institute of Interfacial Process Engineering and Plasma Technology
Influence of MP coils on blob propagation
H-mode, MP coils off
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ρpol=1.15
H-mode, MP coils on
ρpol=1.15
 |vr| comparable
 Increased blob penetration length (at least locally)
 3D structure?
G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Summary
Blob properties in outer SOL almost unchanged after the L-H transition
 Similar exponential WTDs and detection rates in the kHz range
 Cross-field blob size slightly increased in H-mode
 Comparable radial velocities in both phases
 Weak negative correlation between size and velocity
 Size not sufficient as scaling parameter (p, l‖?)
 Drastic changes in the poloidal velocity profile
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MP coils influence blob propagation properties
 Detection rate in the field of view increases
 Larger penetration length (at least locally)
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Institute of Interfacial Process Engineering and Plasma Technology
Thank you for your attention
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Institute of Interfacial Process Engineering and Plasma Technology
Statistical properties
Fluctuation levels
Ohmic L-mode
H-mode
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G. Fuchert, DPG-Frühjahrstagung 2013
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Institute of Interfacial Process Engineering and Plasma Technology
Statistical properties
Skewness
Ohmic L-mode
H-mode
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G. Fuchert, DPG-Frühjahrstagung 2013
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