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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„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


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


Limiter


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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


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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


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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


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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


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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


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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



[B. Nold et al., to be published]

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Radial velocity

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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)

Radial dependence of the velocity

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Ohmic L-mode H-mode

[J. A. Boedo et al., PoP 8, 2001]



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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


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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


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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]


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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


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|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



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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)


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22

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

gas-puff imaging of blob filaments at asdex upgrade

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