Speaker
Description
See the full abstract here:
http://ocs.ciemat.es/EPS2019ABS/pdf/P1.1064.pdf
It is highly desirable to find a common regime of validity for gyrokinetic and Braginskii fluid codes, which would give us a handle to extend the regime of validity of either to the very edge and H-mode transition conditions. Up to now the highly nonlinear conditions there are more naturally the domain of fluid codes, while the short scale lengths and collisionless effects are the expertise of gyrokinetics.
In this vein, the gyrokinetic code CGYRO [1] implementing the Sugama model-collisionoperator [2] and the non-local Braginskii two-fluid code NLET [3] have both been applied to the highly collisional, resistive ballooning turbulence scenarios relevant to the edge of a tokamak, which approach the fluid limit. These comparisons yielded a good match for the collisional regime for dominant density perturbations. However, significant temperature fluctuations (due to temperature gradients) systematically gave differing transport and turbulence intensity. (Surprisingly, completely collisionless scenarios in the high gradient ITG regime also agree.)
The reason for the remaining mismatch in the highly-collisional fluid regime are inaccuracies in the gyrokinetic model collision operator. That collision operator uses an ad-hoc field operator with the purpose of restoring energy and momentum balance while maintaining Onsager symmetry, Galilean and temperature shift invariance.
The transport coefficients produced by the model collision operator in CGYRO have been compared to the predicted Braginskii values [4, 5]. Terms which depend only on the test particle component of the operator, such as the frictional heat flux, perpendicular resistivity, are exactly right, while like-particle collision dependent terms, such as the parallel resistivity or the perpendicular heat flux are overestimated.
A way to improvement, are corrections to the collision operator taking into account higher moments of the "field" collision operator, or a (costly) transition to the full Landau operator, which is complicated in gyrokinetics by the necessary pull-back operation.
[1] J. Candy, E.A. Belli, R.V. Bravenec, J. Comput. Phys. 324, 73 (2016)
[2] H. Sugama, T.-H. Watanabe, M. Nunami, Phys. Plasmas 16 (2009) 112503.
[3] K. Hallatschek, A. Zeiler, Phys. Plasmas 7, (2000) 2554
[4] S. I. Braginskii, in Reviews of Plasma Physics, ed. M. A. Leontovich (New York, 1965), Vol. I, 205
[5] Hinton, Fred L. Handbook of Plasma Physics 1, 147 (1983).
