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Observation of Turbulence in Wendelstein 7-AS

Observation of Turbulence in Wendelstein 7-AS. M. Endler and M. Hirsch Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-17491 Greifswald, Germany. 1. General considerations 2. Confinement region 3. Scrape-off layer (SOL). 1. General Considerations.

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Observation of Turbulence in Wendelstein 7-AS

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  1. Observation of Turbulence in Wendelstein 7-AS M. Endler and M. Hirsch Max-Planck-Institut für Plasmaphysik, EURATOM Association, D-17491 Greifswald, Germany 1. General considerations 2. Confinement region 3. Scrape-off layer (SOL)

  2. 1. General Considerations Reason for interest in plasma turbulence: Turbulent transport Radial heat transport in W7-AS – comparison between neoclassical theory and observation: ion heat transport electron heat transport experiment theory theory with additional effects From: M. Kick et al., IAEA 1996 (Montreal), vol. II, 27

  3. fusion plasma n, T core edge n0, T0 n1, T1 Turbulence and Transport pot of boiling water T0 T1 > T0 T1

  4. Twofold Motivation for Observing Plasma Turbulence • Directly measuring the turbulent transport ( synchronised observation of ≥ 2 quantities required) • Comparison with turbulence models, simulations, theory (aim: understanding parameters controlling turbulence; influencing turbulence)

  5. Langmuir probes (multi-tip) Measurement with limited resolution: spatial lowpass Mirnov probes, SX ECE BES reflectometry Measurement in k space: spatial band pass Doppler reflectometry microwave scattering CO2 laser scattering Which structure sizes can be observed? krs = 1 for 300 eV driving dissipation instabilities kinetic energy 6 cm–1 1 cm k [cm ] –1 0.1 1 10 100 a=17cm [cm] 60 6 0.6 0.06 l

  6. Raw data and statistical analysis Example: Langmuir probe data from the W7-AS SOL (Isat) probability distribution function raw data (auto)correlation function

  7. 2. Turbulence in the W7-AS confinement region • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL

  8. Te fluctuations in the plasma core by ECE – ~ Challenge: < 1 % Te/Te fluctuations are masked by thermal fluctuations of the radiation field by observing at slightly different frequencies = shifted volume (first demonstration on TEXT) ^ • decorrelate thermal fluctuations without decorrelating Te fluctuations by observing the same volume from two positions under sufficiently large angle (first demonstration on W7-AS)

  9. Decorrelation of thermal fluctuations Demonstration of the principle using an artificial source for “temperature fluctuations” but true thermal fluctuations lines of sight of observation below/above decorrelation angle From: S. Sattler and H.-J. Hartfuß, PPCF 35 (1993) 1285, figs. 9&10

  10. Different features in Te fluctuations normalized cross-correlation broadband fluctuations (bandwidth ~ 100 kHz) low-frequency fluctuations (< 5 kHz) quasicoherent modes From: S. Sattler et al., PRL 72 (1994) 653, fig. 2

  11. Broadband fluctuations disappear for Te = 0 In a region with Te = 0, only the low-frequency feature remains From: H.-J. Hartfuß et al., PPCF 38 (1996) A227, figs. 9&10

  12. ~ ~ Correlation between n and Te From: M. Häse et al., RSI 70 (1999) 1014, fig. 5

  13. 2. Turbulence in the W7-AS confinement region • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL

  14. Doppler reflectometry – using turbulence as a tracer for poloidal rotation Doppler reflectometry: use of (–1)st diffraction order of reflected signal From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 1 “ordinary” reflectometry: use of 0th diffraction order of reflected signal antenna microwave signal corrugated and fluctuating reflecting layer

  15. Comparison of poloidal velocity from Doppler reflectometry and from spectroscopic data poloidal velocity of fluctuations ≈ poloidal velocity of impurities ≈ vEB From: M. Hirsch et al., PPCF 43 (2001) 1614, fig. 7

  16. Time resolution of Doppler reflectometry 4 µs resolution reveals strong and fast changes in poloidal velocity and scattered power at HL backtransition From: M. Hirsch et al., PPCF 48 (2006) S155, fig. 6

  17. 2. Turbulence in the W7-AS confinement region • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL

  18. ELM-like transient transport events Correlation analysis: From: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P-5.023, fig. 1 • profile flattening in ECE Te signals • in region of strong pressure gradient • causing cold pulses propagating inward on diffusive time scale • simultaneously bursts in broadband Mirnov activity and small-scale density fluctuations From: M. Hirsch et al., 25th EPS (Prague, 1998) 2322, fig. 1a

  19. Transient magnetic activity – poloidal mode structure • Magnetic activity: • poloidal mode number related to edge rotational transform • bursts of ~ 100 µs Arrangement of Mirnov coils in poloidal cross section: From: S. Zoletnnik et al., PPCF 44 (2002) 1581, fig. 24 See: M. Anton et al., J. Plasma Fusion Res. SERIES 1 (1998) 259

  20. Correlation between magnetic and density fluctuations ^ = radial correlation of Mirnov signal with various BES channels along the Li beam See: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 5 Complement poloidal resolution of Mirnov coils with radial resolution of Li beam

  21. Tentative model for transient transport events • poloidally localised event (associated with broadband turbulence) causes radial transport of hot, dense plasma • flattening of pressure (temperature, density) gradient • initial poloidal gradient causes MHD oscillations with m = 1/i, until gradients on flux surface are balanced (after a few ion transit times ~ 100 µs) After: S. Zoletnik et al., 32nd EPS (Tarragona, 2005) P-5.023

  22. 2. Turbulence in the W7-AS confinement region • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL

  23. Turbulence and transport in the confinement region ~ ~ • Sometimes, yes: ne, Te amplitude is correlated with heat diffusivity for density variation (at fixed heating power) ~ ~ • Sometimes, not in the expected way: ne, Te amplitude is anti-correlated with heat diffusivity for heating power variation (similar: for i variation) • Turbulent transport cannot be measured directly in the confinement region • Is fluctuation amplitude related to transport? • We may still be lacking important diagnostic information (phase between quantities? small scales, e.g., ETG turbulence?)

  24. 2. Turbulence in the W7-AS confinement region • Topics: • Te fluctuations • Doppler reflectometry • Transient events • Turbulence and transport • Transition edge/SOL

  25. Density fluctuations inside and outside the last closed magnetic surface (LCMS) (from fast Li beam diagnostic) • no significant radial correlation across the LCMS • different character of density fluctuations in edge and SOL See: S. Zoletnik et al., PoP 6 (1999) 4239, fig. 4

  26. Transport in the scrape-off layer Definition of last closed magnetic surface (LCMS): B radial transport ^B transport ||B to the target plates confinement region scrape-off layer (SOL) by a magnetic separatrix by a limiter

  27. 3. Turbulence in the W7-AS scrape-off layer • Topics: • Spatial structure of turbulence • Phase between fluctuating quantities • Transport • Topics: • Spatial structure of turbulence • Phase between fluctuating quantities • Transport

  28. Langmuir probe heads: 1 cm Diagnostics – Langmuir probes Positions of Langmuir probes in W7-AS:

  29. photomultipliers (H /D ) 2 2 Diagnostics – Ha fluctuation diagnostic vacuum vessel emissivity: µ ne n0 f(Te) window only weak temperature dependence lens filters plasma glass fiber 16 ´ gas valve

  30. Raw data from the Ha fluctuation diagnostic (density fluctuations) poloidal position 9 cm time 500 µs Individual “fluctuation events” are propagating in poloidal direction lifetime: several 10 µs poloidal correlation length: 1–5 cm poloidal velocity: O(100)–O(1000) m/s

  31. Frequency spectrum f [kHz] 0 200 400 600 800 Auto power density spectrum same, double logarithmic 10-3 10-4 10-5 10-6 10-7 arb. units 10 100 f [kHz] (floating potential data) From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 5

  32. Poloidal-temporal correlation function (floating potential data) 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 3 2 1 0 -1 -2 -3 grey scale dq [cm] -20 -10 0 10 20 t [µs]

  33. Correlation/coherency || B Ffl data from SOL, 6.3 m probe tip separation || B, torus outboard side cross correlation cross coherency From: J. Bleuel et al., NJoP 4 (2002) 38, figs. 20&22

  34. Correlation || B in W7-AS – comparison of poloidal-temporal correlation functions Correlation function between single probe tip and the tips of the poloidal array displaced by 6 m || B (at radial position of maximum correlation) Correlation function between the tips of the poloidal array From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 21

  35. Radial-poloidal correlation function – obtained from the angular array 5 different time lags: poloidal separation d [cm] radial separation dr [cm] (floating potential data) From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 11

  36. Comparison with spatial structure from model calculations 3D simulation of ITG/drift wave turbulence, Te fluctuations at fixed time: From: B. Scott, Phys. Plasmas 7 (2000) 1845–1856

  37. 3. Turbulence in the W7-AS scrape-off layer • Topics: • Spatial structure of turbulence • Phase between fluctuating quantities • Transport

  38. Energy (for each species): Transport due to “electrostatic” turbulence Particles:

  39. Correlation and phase between different fluctuating quantities Correlation between floating potential and ion saturation current: Typically, a phase of p/2...p/3 between these quantities is observed, maximising transport, if Ffl fluctuations are considered equivalent to Fpl fluctuations From: J. Bleuel et al., NJoP 4 (2002) 38, fig. 8

  40. Phases between n and F in interchange instability  B r p0  j|| Ffl Isat E vExB Ffl j ≠ 0 due to curvature Target plate p 

  41. Phase between n, Te and Fpl fluctuations n - Te n - Fpl Te - Fpl Isat - Ffl From: M. Schubert, PhD thesis, Greifswald (2005), figs. 5.24&25 (accessible through http://edoc.mpg.de/)

  42. Modelling of SOL turbulence • The observed phases are consistent with a drift-interchange type of turbulence • The impact of the target plate boundary conditions has not yet been fully explored • The changes of the phases in radial direction are not yet understood in detail

  43. 3. Turbulence in the W7-AS scrape-off layer • Topics: • Spatial structure of turbulence • Phase between fluctuating quantities • Transport

  44. Fluctuation-induced radial energy transport Observed: (6.6 ± 1.5) kW/m2 Expected from global energy balance: 24 kW/m2 (assuming homogeneous transport across LCMS, taking into account local flux expansion) From: M. Schubert, PhD thesis, Greifswald (2005), fig. 5.33 (accessible through http://edoc.mpg.de/)

  45. Summary • Confinement region: • Progress to be expected from improvement of diagnostic capabilities • SOL: • detailed knowledge of spatial structure of turbulence • improving knowledge about relations between different quantities • capability to observe directly the turblence-induced transport • qualitative agreement with transport to be expected from global confinement

  46. Tentative outlook • Confinement region: • high temporal & spatial resolution required  problem of intensity – could progress in lasers help? • combine several methods to obtain information on different quantities, or complementary information on one quantity • SOL: • improve advanced methods (fast sweeping of electrostatic probes?) and perform parameter studies • continue detailed comparison with theory and modelling • “turbulence engineering” by suitable shaping of targets or by active methods?

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