Institute of Interfacial Process Engineering and Plasma Technology Gas-puff imaging of blob filaments at ASDEX Upgrade TTF Workshop 2013 G. Fuchert 1, D. Carralero 2, T. Lunt 2, P. Manz 2, H. W. Müller 2, M. Ramisch 1, V. Rohde 2, U. Stroth 2 and the ASDEX Upgrade Team 1 Insitute of Interfacial Process Engineering and Plasma Technology IGVP, University of Stuttgart, Stuttgart, Germany 2 Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany

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 vrvr

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  Influence of MP coils on blobs  Summary

Institute of Interfacial Process Engineering and Plasma Technology Gas-puff imaging 4G. Fuchert, TTF workshop 2013  Increase neutral gas pressure locally  Deuterium puff with 6·10 20 particles/sec  Observe D α light emission I(n e, T e )  Camera: 120 kfps, FOV ≈ 5x5 cm near outer midplane, resolution ≈ 1 mm ≈ 3 ρ s ρ pol =1.05 ρ pol =1.10

Institute of Interfacial Process Engineering and Plasma Technology 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) nene P Ohmic P ECRH D α flux

Institute of Interfacial Process Engineering and Plasma Technology Blob detection 6  Substract mean and normalize images.  Determine standard deviation for every pixel.  Detect connected structures with I max > I t (e.g. I t = 2σ) and I > I max /2  Fit an ellipse to any detected structure to get the location and size of the blob.  Track stuctures over time to get their velocities. G. Fuchert, TTF workshop 2013

Institute of Interfacial Process Engineering and Plasma Technology Mean images 7 Ohmic L-mode H-mode G. Fuchert, TTF workshop 2013 Limiter ρ pol =1.05ρ pol =1.10

Institute of Interfacial Process Engineering and Plasma Technology Generation rate, waiting-time distribution 8  Qualitatively comparable waiting-time distributions (WTD) in L- and H-mode  ASDEX Upgrade: Similar PDFs for I sat 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

Institute of Interfacial Process Engineering and Plasma Technology Comparable waiting-time distributions 9  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-modeH-mode G. Fuchert, TTF workshop 2013

Institute of Interfacial Process Engineering and Plasma Technology Comparable waiting-time distributions 10  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

Institute of Interfacial Process Engineering and Plasma Technology Cross-field size 11 No clear expectation from theory, typically ~ 1 cm (a few ρ s )  Comparable cross-field sizes of L- and H-mode filaments of a few ρ s  Tendency for larger H-mode blobs Ohmic L-mode H-mode G. Fuchert, TTF workshop 2013

Institute of Interfacial Process Engineering and Plasma Technology Blob velocity 12  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

Institute of Interfacial Process Engineering and Plasma Technology Poloidal Velocity 13 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 ρ pol =1.05 ρ pol =1.10 [B. Nold et al., to be published]

Institute of Interfacial Process Engineering and Plasma Technology Radial velocity 14 Ohmic L-modeH-mode G. Fuchert, TTF workshop 2013 ρ pol =1.05 ρ pol =1.10

Institute of Interfacial Process Engineering and Plasma Technology  Comparable v r profiles in both regimes.  Radial deceleration of blobs (as observed in other experiments, e.g. DIII-D) Radial dependence of the velocity 15 Ohmic L-modeH-mode [J. A. Boedo et al., PoP 8, 2001] G. Fuchert, TTF workshop 2013 ρ pol =1.05ρ pol =1.10

Institute of Interfacial Process Engineering and Plasma Technology Size dependence of radial velocity 16  Large spread in the measured velocities  Weak negative correlation for every analyzed shot  Additional scaling parameters needed (  p, l ‖ ?) Ohmic L-modeH-mode G. Fuchert, TTF workshop 2013

Institute of Interfacial Process Engineering and Plasma Technology Comparison of local mean values for different shots  Tendency for slightly reduced v r after L-H transition  Differences only marginal (within the resolution) Size dependence of radial velocity 17G. Fuchert, TTF workshop 2013

Institute of Interfacial Process Engineering and Plasma Technology MP coils/ELM mitigation at AUG 18 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

Institute of Interfacial Process Engineering and Plasma Technology Influence of MP coils on blob properties 19  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

Institute of Interfacial Process Engineering and Plasma Technology  |v r | comparable  Increased blob penetration length (at least locally)  3D structure? Influence of MP coils on blob propagation 20 H-mode, MP coils offH-mode, MP coils on G. Fuchert, TTF workshop 2013 ρ pol =1.15

Institute of Interfacial Process Engineering and Plasma Technology Summary 21 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

Institute of Interfacial Process Engineering and Plasma Technology 22 Thank you for your attention

Institute of Interfacial Process Engineering and Plasma Technology Statistical properties 23G. Fuchert, DPG-Frühjahrstagung 2013 Ohmic L-mode H-mode Fluctuation levels

Institute of Interfacial Process Engineering and Plasma Technology Statistical properties 24G. Fuchert, DPG-Frühjahrstagung 2013 Ohmic L-mode H-mode Skewness