Recent Gyrokinetic Simulations with GYRO: Bohm
Transport in DIII-D, the Local Limit of Global
Simulations, and Transport Across a Minimum-qSurface∗
J. Candy
General Atomics, P.O. Box 85608
San Diego, California 92186-5608 USAAbstract
In this presentation we summarize results from three major transport studies carriedout with the global Eulerian gyrokinetic code, GYRO.1 First, and most importantly, we
discuss comprehensive simulations2 which match, within experimental error bounds, theturbulent transport levels observed in DIII-D L-mode discharges 101381 and 101391. The
experimentally-observed Bohm-like scaling of ion transport in these discharges was recovered, and the sensitive dependence of both electron and ion transport on ion temperature
gradient was also explored. These were full-physics simulations which included trapped andpassing electrons with pitch angle collisions, finite-β fluctuations, real DIII-D flux-surface
shape, linear and nonlinear E × B rotation, parallel flow shear, and radial variation of all
equilibrium quantities.
Next, we revisit the issue of gyroBohm scaling (and the breaking thereof) in the context
of a certain adiabatic-electron test case favoured by the GTC code.3 For this case, wehave shown that transport levels from local and global GYRO simulations are perfectly
matched in the limit of small ρ∗ = ρi /a. We also show that this limiting value is identicalto the local value obtained by Dimits.4 The confluence of local and global results, which weobserve repeatedly and robustly, has been previously noted5 in a more general context. For
this particular case, we show that the transition point in ρ∗ , where gyroBohm gives way toworse-than-gyroBohm scaling, is strongly dependent on the temperature-gradient profile. In
particular, use of the original “humped” GTC profile leads to the requirement ρ∗ . 1/375
for gyroBohm scaling, whereas the use of a more physical temperature-gradient profile inthe same scenario gives rise to gyroBohm scaling and recovers the local limit at the muchlarger value ρ∗ . 1/150.
Finally, we examine a conjecture regarding ion transport barrier formation in the vicinityof a minimum-q surface.6 The validity of the conjecture relies, ostensibly, on the absence of
ITG modes in the minimum-q region. Indeed, fluid transport simulations7 have shown thatwhen so-called nonresonant modes are ignored, a minimum-q region can give rise to a barrier.
However, a series of linear and nonlinear, as well as local and global, GYRO simulations
show no evidence of mode suppression or of transport reduction in the minimum-q region.
Instead, we find that transport is smoothly increasing across a minimum-q surface, and that
this region is populated by nonresonant ITG modes characteristic of the low-shear regime.
We also observe that as ρ∗ → 0, transport computed via a global simulation agrees withthat from local simulations, even at the point where s = 0.
*Work supported by the U.S. Department of Energy under Grant Nos. DE-FG03-95ER54309
and the SciDAC Plasma Microturbulence Project.
1

J. Candy and R.E. Waltz, J. Comput. Phys. 186 (2003) 545.
J. Candy and R.E. Waltz, Phys. Rev. Lett. 91 (2003) 045001.
3
Z. Lin, et al., Phys. Rev. Lett. 88 (2002) 195004.
4
A.M. Dimits, et al., Phys. Plasmas 7 (2000) 969.
5R.E. Waltz, J. Candy and M.N. Rosenbluth, Phys. Plasmas 9 (2002) 1938
6Y. Kishimoto, et al., Nucl. Fusion 40 (2000) 667
7X. Garbet, et al., Phys. Plasmas 8 (2001) 2793

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