(TH/P2-3) Gyrokinetic Simulations of ETG and ITG Turbulence

A.M. Dimits1), W.M. Nevins1), D.E. Shumaker1), G.W. Hammett2), T. Dannert3), F. Jenko3), W. Dorland4), J.N. Leboeuf5), T.L. Rhodes5), J. Candy6), C. Estrada-Mila7)
 
1) Lawrence Livermore National Laboratory, Livermore, United States of America
2) Princeton Plasma Physics Laboratory, USA
3) Max Planck Institut fur Plasmaphysik, Germany
4) University of Maryland, USA
5) University of California, Los Angeles, USA
6) General Atomics, San Diego, USA
7) University of California, San Diego, USA

Abstract.  We have carried out an investigation [1], using simulations and analytical theory, of the discrepancy between gyrokinetic continuum-code [2] and particle-in-cell (PIC) simulations [3] of electron-temperature-gradient (ETG) turbulence. Ref. [2] indicated a sufficiently large value of the electron thermal conductivity to account for anomalous electron thermal transport in tokamaks, while [3] gave significantly lower values. We have reproduced the key features of the results reported in [3] using the flux-tube gyrokinetic PIC code, PG3EQ [4], thereby eliminating global effects and as the cause of the discrepancy [1]. We show [1] that the late-time low-transport state in both these PG3EQ simulations and those reported in [3] is a result of discrete particle noise. Since discrete particle noise is a numerical artifact, the low value reported in [3(b)], along with conclusions about anomalous transport drawn from it, are unjustified. A detailed theory of the spectrum of noise fluctuations in a gyrokinetic particle simulation has been developed which greatly facilitates this demonstration [1]. This theory has no free parameters, and gives excellent agreement both with the fluctuation spectrum and the transport levels observed at late times in gyrokinetic particle simulations when the noise dominates [1]. In our attempts to benchmark PIC [4] and continuum [2] codes for ETG turbulence at the plasma parameters used in [3], both produce very large intermittent transport. We have therefore undertaken benchmarks at an alternate reference point, magnetic shear s=0.1 instead of s=0.796, and have found that PIC and continuum codes reproduce the same transport levels. Scans about this new reference point have been used to investigate the parameter dependence of ETG transport and to elucidate previously proposed saturation mechanisms. New results on the balances of zonal-flow driving and damping terms in late time quasi-steady ITG turbulence and on real-geometry gyrokinetic simulations of shaped DIII-D discharges will also be reported.
References:
[1] W.M. Nevins, G.W. Hammett, et al., Phys. Plasmas 12, 122305 (2005).
[2] F. Jenko, et al., Phys. Plasmas 7, 1905 (2000).
[3] (a) Z. Lin, 2004 IAEA Mtg.; (b) Z. Lin, et al, Phys. Plasmas 12, 056125 (2005).
[4] A.M. Dimits, et al, Phys. Rev. Letters 77, 71 (1996).
* Work performed at U.C. LLNL for US DOE under Contract No. W7405-ENG-48.

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