Gas-puff imaging of blob filaments at ASDEX Upgrade
description
Transcript of Gas-puff imaging of blob filaments at ASDEX Upgrade
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Institute of Interfacial Process Engineering and Plasma Technology
Gas-puff imaging of blob filaments at ASDEX Upgrade
TTF Workshop 2013
G. Fuchert1, D. Carralero2, T. Lunt2, P. Manz2, H. W. Müller2,
M. Ramisch1, V. Rohde2, U. Stroth2 and the ASDEX Upgrade Team1Insitute of Interfacial Process Engineering and Plasma Technology IGVP,
University of Stuttgart, Stuttgart, Germany2Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany
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Institute of Interfacial Process Engineering and Plasma Technology
„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
Motivation
Blob filaments at MAST
[B. D. Dudson, PPCF 50, 2008]
Image of blob from GPI at AUG
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Outline
Experimental setup and blob detection
Blob properties in L- and H-mode Generation rate, waiting-time distribution Cross-field size Blob velocity
Influence of MP coils on blobs
Summary
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Gas-puff imaging
4G. Fuchert, TTF workshop 2013
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 outermidplane, resolution ≈ 1 mm ≈ 3 ρs
ρpol=1.05 ρpol=1.10
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Experiments
5G. Fuchert, TTF workshop 2013
After a purely ohmic heated phase, additional ECRH heating triggers the L-H transition.
Comparison of blob properties in both phases (omitting ELM phases)
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POhmicPECRH
Dα flux
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Blob detection
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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
Fit an ellipse to any detected structure to getthe location and size of the blob.
Track stuctures over time to get their velocities.
G. Fuchert, TTF workshop 2013
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Mean images
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Ohmic L-mode H-mode
G. Fuchert, TTF workshop 2013
Limiter
ρpol=1.05 ρpol=1.10
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Institute of Interfacial Process Engineering and Plasma Technology
Generation rate, waiting-time distribution
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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]
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
G. Fuchert, TTF workshop 2013
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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
Ohmic L-mode 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
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No clear expectation from theory, typically ~ 1 cm (a few ρs)
Comparable cross-field sizes of L- and H-mode filaments of a few ρs
Tendency for larger H-mode blobs
Ohmic L-mode H-mode
G. Fuchert, TTF workshop 2013
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Blob velocity
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Poloidal velocity mainly determined by ExB background.
Poloidal polarization of blob filament due to e.g. curvature drifts [1].
Resulting ExB-drifts point outwards.
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
G. Fuchert, TTF workshop 2013
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Poloidal Velocity
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Ohmic L-mode H-mode
Shear layer in the SOL in L-modes Poloidal velocity profile changes significantly after L-H transition
G. Fuchert, TTF workshop 2013
ρpol=1.05 ρpol=1.10
[B. Nold et al., to be published]
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Radial velocity
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Ohmic L-mode H-mode
G. Fuchert, TTF workshop 2013
ρpol=1.05 ρpol=1.10
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Comparable vr profiles in both regimes.
Radial deceleration of blobs (as observed in other experiments, e.g. DIII-D)
Radial dependence of the velocity
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Ohmic L-mode H-mode
[J. A. Boedo et al., PoP 8, 2001]
G. Fuchert, TTF workshop 2013
ρpol=1.05 ρpol=1.10
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Size dependence of radial velocity
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Large spread in the measured velocities Weak negative correlation for every analyzed shotAdditional scaling parameters needed (p, l‖?)
Ohmic L-mode H-mode
G. Fuchert, TTF workshop 2013
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Comparison of local mean values for different shots
Tendency for slightly reduced vr after L-H transition
Differences only marginal (within the resolution)
Size dependence of radial velocity
17G. Fuchert, TTF workshop 2013
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MP coils/ELM mitigation at AUG
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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.
Recent studies aim to elucidate the influence on the edge turbulence.
[W. Suttrop et al., PRL 106, 2011]
G. Fuchert, TTF workshop 2013
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Influence of MP coils on blob properties
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Blob detection rate increases in phases where MP coils are on. Either generation rate increases or blobs propagate further outwards.
H-mode, MP coils off H-mode, MP coils on
G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
|vr| comparable
Increased blob penetration length (at least locally) 3D structure?
Influence of MP coils on blob propagation
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H-mode, MP coils off H-mode, MP coils on
G. Fuchert, TTF workshop 2013
ρpol=1.15 ρpol=1.15
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Summary
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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
MP coils influence blob propagation properties Detection rate in the field of view increases Larger penetration length (at least locally)
G. Fuchert, TTF workshop 2013
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Institute of Interfacial Process Engineering and Plasma Technology
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Thank you for your attention
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Statistical properties
23G. Fuchert, DPG-Frühjahrstagung 2013
Ohmic L-mode H-mode
Fluctuation levels
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Statistical properties
24G. Fuchert, DPG-Frühjahrstagung 2013
Ohmic L-mode H-mode
Skewness