System Size Effects on Gyrokinetic Turbulence

Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne, PPB, 1015 Lausanne, Switzerland.
Physical Review Letters (Impact Factor: 7.51). 12/2012; 105:155001. DOI: 10.1103/PHYSREVLETT.105.155001
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The scaling of turbulence-driven heat transport with system size in magnetically confined plasmas is reexamined using first-principles based numerical simulations. Two very different numerical methods are applied to this problem, in order to resolve a long-standing quantitative disagreement, which may have arisen due to inconsistencies in the geometrical approximation. System size effects are further explored by modifying the width of the strong gradient region at fixed system size. The finite width of the strong gradient region in gyroradius units, rather than the finite overall system size, is found to induce the diffusivity reduction seen in global gyrokinetic simulations.

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Available from: Ben Fynney Mcmillan, Sep 30, 2015
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    • "Such an approach enables us to work with a sufficiently simple but nevertheless complete physics model for studying the scaling of turbulent transport spanning the range from present generation experiments to the large ITER-scale plasmas [15] [22]. Specifically, this approach includes all of the important physics captured in numerous global PIC simulation studies of plasma size scaling over the years extending from the work by Z. Lin, et al. [15], up to the more recent work by B. F. McMillan, et al. [22] on system size effects on turbulent transport. These techniques reduce the complexity of developing the algorithmic advances required to take advantage of rapidly evolving architectural platforms. "
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    ABSTRACT: Reliable predictive simulation capability addressing confinement properties in magnetically confined fusion plasmas is critically-important for ITER, a 20 billion dollar international burning plasma device under construction in France. The complex study of kinetic turbulence, which can severely limit the energy confinement and impact the economic viability of fusion systems, requires simulations at extreme scale for such an unprecedented device size. Our newly optimized, global, ab initio particle-in-cell code solving the nonlinear equations underlying gyrokinetic theory achieves excellent performance with respect to \textquotedblleft time to solution at the full capacity of the IBM Blue Gene/Q on 786,432 cores of Mira at ALCF and recently of the 1,572,864 cores of Sequoia at LLNL. Recent multithreading and domain decomposition optimizations in the new GTC-P code represent critically important software advances for modern, low memory per core systems by enabling routine simulations at unprecedented size (130 million grid points ITER-scale) and resolution (65 billion particles).
    Proc. ACM/IEEE Conf. on Supercomputing (SC), Denver, CO; 11/2013
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    • "The local approximation is expected to hold when the tokamak minor radius a becomes much larger than the gyroradius ρ i . While finite machine size effects usually stabilize modes like the ITG instability due to profile shearing,[28] [29] [30] it is a priori not clear how microtearing modes are affected. In the present work, we relax these constraints in the context of linear and nonlinear gyrokinetic simulations, employing realistic ASDEX Upgrade (AUG) parameters. "
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    Physics of Plasmas 05/2012; 19(5). DOI:10.1063/1.3694663 · 2.14 Impact Factor
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    ABSTRACT: Using the Eulerian code GENE [1], gyrokinetic simulations of microturbulence were carried out under conditions relevant to electron-Internal Transport Barriers (eITB) in the TCV tokmak [2], generated under conditions of low or negative shear. For typical density and temperature gradients measured in such barriers, the corresponding simulated fluctuation spectra appears to simultaneously contain longer wavelength Trapped Electron Modes (TEM, for typically k_perp*rho_i < 0.5, k_perp being the characteristic perpendicular wavenumber and rho_i the ion Larmor radius) and shorter wavelength Ion Temperature Gradient modes (ITG, k_perp*rho_i > 0.5). The contributions to the electron particle flux from these two types of modes are respectively outward/inward and may cancel each other out for experimentally realistic gradients. This mechanism may partly explain the feasibility of eITBs. The non-linear simulation results confirm the predictions of a previously developed quasi-linear model [3], namely that the stationary condition of zero particle flux is obtained through the competitive contributions of ITG and TEM. Parameter scans of gyrokinetic microturbulence simulations were carried out with an attempt to identify combinations of density and electron/ion temperature gradients which not only cancel out particle fluxes but minimize electron heat fluxes as well.
    Plasma Physics and Controlled Fusion 12/2011; 53:054011. DOI:10.1088/0741-3335/53/5/054011 · 2.19 Impact Factor
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