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Turbulent transport in collisionless plasmas: eddy mixing or wave-particle decorrelation? Z. Lin

Turbulent transport in collisionless plasmas: eddy mixing or wave-particle decorrelation? Z. Lin Y. Nishimura, I. Holod, W. L. Zhang, Y. Xiao, L. Chen University of California, Irvine, California 92697, USA P. H. Diamond University of California, San Diego, California 92093, USA

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Turbulent transport in collisionless plasmas: eddy mixing or wave-particle decorrelation? Z. Lin

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  1. Turbulent transport in collisionless plasmas: eddy mixing or wave-particle decorrelation? Z. Lin Y. Nishimura, I. Holod, W. L. Zhang, Y. Xiao, L. Chen University of California, Irvine, California 92697, USA P. H. Diamond University of California, San Diego, California 92093, USA T. S. Hahm, S. Ethier, G. Rewoldt PPPL, Princeton University, Princeton, New Jersey 08543, USA F. Zonca Associazione EURATOM-ENEA sulla Fusione, Frascati, Italy S. Klasky Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA Supported by US DOE SciDAC GPS Center

  2. Turbulence Structure & Transport in Tokamak • ITG: isotropic eddies • ETG: radial streamers • Fluid picture: eddy mixing • Kinetic process: wave-particle decorelation

  3. Motivation: electron heat transport in tokamak • To understand physical mechanism of electron heat transport in tokamak driven by driftwave turbulence • Eddy mixing or wave-particle decorrelation? • Resonant vs. non-resonant transport? • Accuracy of mixing length estimate? • Choice of time scales in transport models? • Relation between instability drive, nonlinear saturation and turbulent transport? • Gyrokinetic particle simulation of microturbulence • Systematic measurement of nonlinear spatial & temporal scales • Quantitative test of quasilinear theory in tokamak geometry

  4. Transport: eddy mixing or wave-particle decorrelation? • Case studies of electron heat transport mechanism in tokamak • Comparative studies of CTEM, ITG, & ETG • GTC simulations: while saturation can be understood in context of fluid processes, kinetic processes related to instability drive often responsible for transport

  5. GTC global gyrokinetic particle simulation • GTC [Lin et al, Science1998] global field-aligned mesh: reduces computation by a/r~100 • Twisted across flux surfaces by magnetic shear • # of spatial grids N~(a/r)2 • Respect physical periodicity • Radial variations of equilibrium quantities • Gyrokinetic particle-in-cell approach • Efficient sampling of 5D phase space • Massively parallel computing • Resources made available by US SciDAC • GTC selected for early applications of 250TF ORNL computer • Object-oriented GTC for collaborative code development and for integrating kinetic electron, electromagnetic, multiple ion species, and MHD equilibrium

  6. GTC nonlinear convergence in ETG simulation • Convergence from 400 to 2000 particles per cell • Weak Cyclone parameters: R/LT=5.3, s=0.78, q=1.4, a/re=500, g/wr~1/4 • ORNL Cray XT3, 6400 PE, 4x1010 particles • Noise driven flux is 4000 times smaller than ETG driven flux • Noise spectrum in ETG simulation measured. Noise driven flux calculated & measured [Holod and Lin, PoP2007] • Initial saturation: nonlinear toroidal coupling[Lin, Chen, Zonca, PoP2005; PPCF2005] ce (vere2/LT) time (LT/ve)

  7. Turbulence Evolution • Initial expansion of fluctuation envelop • Eddies flow along streamers in steady state? • Breaking and reconnection of streamers • Scale separation important • Enabled by ORNL XT3 • Advanced visualization and statistical analysis needed!

  8. ETG radial streamers; Length scales • Streamers generated via nonlinear toroidal coupling • Streamer generation growth rate > linear growth rate • Mixing length arguments: long streamer drive large transport? • Electrons excursion distance < streamer length [Lin, Chen, Zonca, PoP2005], [Joiner, Applegate, Cowley, Dorland, Roach, PPCF2006] • Streamer length > 102 distance of mode rational surfaces • Phase space island overlap due to parallel motion; diffusive processes Time= 400 LT/ve Time= 1400 LT/ve

  9. Transport driven by local fluctuation intensity • Effective wave-particle decorrelation time twp=4ce/3dvr2 ~ 4.2LT/ve • twp << 1/g ~ 33: linear time scale not important to transport • Wave-particle correlation length dvrtwp << streamer length • Electron radial excursion diffusive:streamer length does not determine transport directly • From linear to nonlinear, ce/dvr2decreases by a factor of ~5 • Nonlinear loss of wave-particle correlation ce ce/dvr2 r/re time (LT/ve)

  10. Relevant time scales in ETG turbulence

  11. Parallel wave-particle decorrelation time ~twp • Parallel decorrelation time due to parallel spectral width ~ 5.3 • Radial turbulence scattering time due to radial width of m-harmonics, together with radial diffusion and parallel motion ~ 8.0 time (LT/ve)

  12. Fluid eddy turnover time >> twp • Calculate radial-toroidal two-point correlation function • Calculate radial correlation function along the ridge • Streamer correlation length Lr ~54re>> electron excursion distance • Eddy turnover time ~ 42 • Resonance broadening ~ 437 • Eddy trapping not important Dz Radial separation (re)

  13. Streamer auto-correlation time tauto>> twp Calculate two-time, two-point (t-z) correlation function Streamers move with a toroidal velocity ~ linear phase velocity Calculate Lagrangian time correlation function in wave frame Auto-correlation time tauto ~ 346 >>twp Kinetic time scales shorter than fluid time scales Dz/2p Time separation (LT/ve)

  14. Kinetic & fluid time scales in ETG turbulence tauto >> 1/g >>twp ~ 1/Dk||ve Wave-particle decorrelation of parallel resonance d(w-k||v||) dominates Quasilinear calculation of ce agrees well with simulation Saturation: wave-wave coupling determines fluctuation intensity Transport: wave-particle decorrelation determines transport level

  15. “Preaching to the choir” • Collaboration: TREND in large scale simulation • US SciDAC:Scientific Discovery through Advanced Computing • Turbulence: GPS & PMP • CEMM • RF • Energetic particle • US FSP: Fusion Simulation Project • CPES: edge +MHD + atomic+… • SWIM: MHD + RF • FACETS: core + edge + wall • EU ITM • Japan BPSI

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