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Gas-puff imaging of blob filaments at ASDEX Upgrade

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,

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Gas-puff imaging of blob filaments at ASDEX Upgrade

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  1. 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 Team 1Insitute of Interfacial Process Engineering and Plasma Technology IGVP, University of Stuttgart, Stuttgart, Germany 2Max-Planck-Institut für Plasmaphysik, EURATOM Association, Garching, Germany

  2. 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 vr Blob filaments at MAST [B. D. Dudson, PPCF 50, 2008] Image of blob from GPI at AUG

  3. 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

  4. Gas-puff imaging • 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=1.10 ρpol=1.05 G. Fuchert, TTF workshop 2013 4

  5. 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 PECRH POhmic Dα flux G. Fuchert, TTF workshop 2013 5

  6. 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 • 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 6

  7. Mean images Ohmic L-mode H-mode ρpol=1.05 ρpol=1.10 Limiter G. Fuchert, TTF workshop 2013 7

  8. 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] • 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 8

  9. Comparable waiting-time distributions Ohmic L-mode H-mode • 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 9

  10. Comparable waiting-time distributions • 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 10

  11. Cross-field size • 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 11

  12. 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. • 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 12

  13. Poloidal Velocity H-mode Ohmic L-mode ρpol=1.10 ρpol=1.05 • Shear layer in the SOL in L-modes • Poloidal velocity profile changes significantly after L-H transition [B. Nold et al., to be published] G. Fuchert, TTF workshop 2013 13

  14. Radial velocity Ohmic L-mode H-mode ρpol=1.10 ρpol=1.05 G. Fuchert, TTF workshop 2013 14

  15. Radial dependence of the velocity • Comparable vr profiles in both regimes. • Radial deceleration of blobs (as observed in other experiments, e.g. DIII-D) ρpol=1.05 ρpol=1.10 Ohmic L-mode H-mode [J. A. Boedo et al., PoP 8, 2001] G. Fuchert, TTF workshop 2013 15

  16. Size dependence of radial velocity Ohmic L-mode H-mode • Large spread in the measured velocities • Weak negative correlation for every analyzed shot • Additional scaling parameters needed (p, l‖?) G. Fuchert, TTF workshop 2013 16

  17. Size dependence of radial velocity Comparison of local mean values for different shots • Tendency for slightly reduced vr after L-H transition • Differences only marginal (within the resolution) G. Fuchert, TTF workshop 2013 17

  18. 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. • Recent studies aim to elucidate the influence on the edge turbulence. [W. Suttrop et al., PRL 106, 2011] G. Fuchert, TTF workshop 2013 18

  19. Influence of MP coils on blob properties H-mode, MP coils on H-mode, MP coils off • Blob detection rate increases in phases where MP coils are on. • Either generation rate increases or blobs propagate further outwards. G. Fuchert, TTF workshop 2013 19

  20. Influence of MP coils on blob propagation H-mode, MP coils off H-mode, MP coils on • |vr| comparable • Increased blob penetration length (at least locally) • 3D structure? ρpol=1.15 ρpol=1.15 G. Fuchert, TTF workshop 2013 20

  21. 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 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 21

  22. Thank you for your attention 22

  23. Statistical properties Fluctuation levels Ohmic L-mode H-mode G. Fuchert, DPG-Frühjahrstagung 2013 23

  24. Statistical properties Skewness Ohmic L-mode H-mode G. Fuchert, DPG-Frühjahrstagung 2013 24

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