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S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013

Geodesic acoustic modes: simultaneous observation of density, magnetic-field, and flow components in the TCV tokamak. S. Coda, C.A. de Meijere , Z. Huang, L. Vermare 1 , T. Vernay , V. Vuille , S. Brunner, J. Dominski , P. Hennequin 1 , A. Kr ä mer-Flecken 2 , G. Merlo, L. Porte.

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S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013

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  1. Geodesic acoustic modes:simultaneous observation of density,magnetic-field, and flow componentsin the TCV tokamak S. Coda, C.A. de Meijere, Z. Huang, L. Vermare1, T. Vernay, V. Vuille, S. Brunner, J. Dominski,P. Hennequin 1, A. Krämer-Flecken2, G. Merlo, L. Porte 1LPP, CNRS-EcolePolytechnique, Palaiseau, France 2ForschungszentrumJülich, Germany S. Coda, U.S.-E.U. Joint Transport Task Force Workshop, Santa Rosa, CA, 9-12 April 2013

  2. Outline • Geodesic acoustic modes • Multi-diagnostic measurements of GAMs in TCV • Modeling of GAMs in TCV • Summary and outlook

  3. Zonal flows • Electric potential perturbations: symmetric over flux surface (m=n=0), low-frequency (w0) • Nonlinearly generated by broadband drift-wave turbulence • Associated poloidal, sheared (kr≠0) EB flows break apart turbulent eddies and effectively regulate turbulence  self-organization

  4. Geodesic acoustic modes • Finite-frequency (wcs/R) zonal-flow component • n=0, m=1 standing-wave density fluctuation • n=0, m=2 standing-wave magnetic component (recent prediction, Wahlberg 2009) •  recent proposal to excite GAM with external magnetic perturbation (Hallatschek 2012)

  5. Geodesic acoustic modes • Flow and density components observed on several devices(Doppler backscattering, reflectometry, beam emission spectroscopy, heavy ion beam probe)

  6. Outline • Geodesic acoustic modes • Multi-diagnostic measurements of GAMs in TCV • Modeling of GAMs in TCV • Summary and outlook

  7. GAMs in TCV • Unique, correlated multi-diagnostic observation • First sighting of magnetic-field component for turbulence-driven GAM • Axisymmetry unambiguously determined • Density: tangential phase contrast imaging • Magnetic field: Mirnov coils • Flow: Doppler backscattering • Radiative temperature: correlation ECE

  8. GAMs in TCV • Initial study: L-mode, limited plasma with 1 MW central ECRH  magnetic analysis then extended to broad range of past shots (including Ohmic)

  9. TCV R = 0.88 m, a = 0.25 m Ip < 1 MA, BT < 1.54 T k < 2.8, -0.6 < d < 0.9 ×4 ×2 4.5 MW ECRH power, 7 steerable launchers

  10. Tangential phase contrast imaging (TPCI) • Established technique for measuring line-integrated density fluctuations • Tangential geometry + spatial filtering adds spatial resolution

  11. Tangential phase contrast imaging (TPCI) Ultimate specs:0.9 cm-1 < k < 60 cm-1 (0.2 < krs < 90) spatial resolution down to 1% of minor radius multi-MHz bandwidth

  12. Tangential phase contrast imaging (TPCI) Current specs:1 cm-1 < k < 9 cm-1 line-integratedmeasurement only 1.5 MHz bandwidth

  13. TPCI provides GAM’sspatial distribution and radial wavelength • k is radial  TPCI signal comes from tangency point • scan r by moving plasma vertically

  14. TPCI provides GAM’sspatial distribution and radial wavelength

  15. 22-40 kHz peaks near edge kr 1.7-2.1 cm-1 (mainly outward)krs 0.4-0.5

  16. Magnetic component of the GAM

  17. Theory: magnetic component of the GAM Bq (r,q,t)  q2b sin(2q) sin(krr-wt) • short radial wavelength: faint signal outside plasma • nodes on LFS and HFS, so toroidal mode number should be measured away from equatorial plane

  18. Magnetic component of GAM has n=0

  19. At GAM node location,residual signal dominated by n=1 toroidal mode number n

  20. Magnetic component of GAM has m=2

  21. Magnetic component of GAM has m=2 antinodes and LFS phasing consistent with sin(2q)

  22. Magnetic component of GAM has m=2 HFS phasing indicates presence of m>2 components (effect of shape?)

  23. GAM scales with sound speed

  24. Doppler backscattering on TCV • Flow measurements performed with a 50-75 GHz tunable, heterodyne system on loan from LPP and Tore Supra • Collaboration with LPP (L. Vermare and P. Hennequin)1 • Monostatic antenna = replica of ECRH launcher, can be oriented in real time 1L. Vermare et al, Nucl. Fusion 52, 063008 (2012)

  25. Doppler backscattering on TCV

  26. Oscillating EB GAM poloidal flowis clearly seen in the edge region GAM flow  0.7 km/s rms (background flow  2 km/s)

  27. GAM seen also by correlation ECE Six-channel tunable X2 system, LFS detection

  28. Strong correlation between TPCI & ECE

  29. GAM on C-ECE vs TPCI: a few puzzles • plasma is invariably optically thin (t<0.5): ECE measurement is unknown mix of ne and Te fluctuations • kr (TPCI)  1.7-2.1 cm-1, kr (C-ECE)  0.9 cm-1 • predominantly outward-propagating on TPCI, propagation direction depends on location on C-ECE

  30. Global vs local GAM • All diagnostics on TCV see a single-frequency mode irrespective of location • Other devices have reported a single-frequency mode, several discrete modes, or a continuum over r • This variation in behavior is not well understood

  31. Outline • Geodesic acoustic modes • Multi-diagnostic measurements of GAMs in TCV • Modeling of GAMs in TCV • Summary and outlook

  32. Gyrokinetic modeling • ORB5: global particle-in-cell ∂f code • Collisionless, electrostatic simulation using TCV experimental equilibrium and kinetic profiles: turbulence is dominated by trapped electron modes • Model breaks down for r > 0.85, so simulation restricted to inner region (fluctuation level artificially scaled down in edge)

  33. Good, semi-quantitative agreement between experiment and modeling

  34. Good, semi-quantitative agreement between experiment and modeling Multiple discrete modes below a critical density gradient, single mode above (as in experiment)

  35. Good, semi-quantitative agreement between experiment and modeling kr 2.3 cm-1 f  33 kHz coherent over several wavelengths peak amplitude 3 km/s rms peaks at outermost properly simulated radius (r=0.85)

  36. Outline • Geodesic acoustic modes • Multi-diagnostic measurements of GAMs in TCV • Modeling of GAMs in TCV • Summary and outlook

  37. Summary • Initial study on TCV has revealed GAM in density, magnetic-field, and flow fields (plus ECE radiative temperature) • First multi-probe analysis of magnetic component has clearly confirmed axisymmetry • Frequency, radial wave number, poloidal and toroidal mode numbers, radial profile, direction of propagation have all been measured • Good agreement with gyrokinetic modeling

  38. Outlook • Much more to come from the experiment: parametric studies (dependence on q profile, shape, collisionality, etc.), exploration of damping mechanism, etc. • Better diagnostics will be used: fully commissioned TPCI, C-ECE using movable antenna • Much more to come from modeling: synthetic diagnostics for TPCI and C-ECE, parametric studies, etc. • Further challenges to theory: e.g. m>2 magnetic GAM components (finite-b, toroidicity effects)

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