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
Source: OAI


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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    • "With only one-dimensional domain decomposition in the toroidal dimension, this algorithm has each process keep a copy of the full poloidal grid, and the number of grid points in the poloidal grid increases 4x for a plasma device of 2x in minor radius. In order to effectively address the open questions in fusion plasma physics, such as the scaling of the energy confinement time with system size [14] [22] [29] [37], a key additional level of domain decomposition in the radial dimension was introduced. "
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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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    • "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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