Speaker
Klaus Hallatschek
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P4.1106.pdf
Highly collisional two-fluid and gyrokinetic simulations of tokamak edge
turbulence and the transition between kinetic and fluid regime
K. Hallatschek1 , J. Candy2 , E.A. Belli2
1 Max-Planck-Institute for Plasma Physics, Boltzmannstr. 2, 85748 Garching, Germany
2 General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
To arrive at a common basis, the gyrokinetic code CGYRO [1] and the non-local two-fluid code
NLET [2] have both been applied to identical parameters sets ranging from highly collisional,
resitive ballooning turbulence scenarios - which approach the fluid limit - relevant to the edge of
a tokamak, up to the ITG turbulence at higher temperatures in the core-edge transitional regime.
Earlier attempts with a less sophisticated collision operator in the gyrokinetic code were un-
successful to come close to the expected transport values in the edge regime. Surprisingly, it has
been much easier to match fluid and gyrokinetic results in the lower collisionality ITG regime in
the core-edge transitional region. Even in the collisionless core the transport difference between
fluid and gyrokinetic results for non-marginal instabilities are . 30% [3].
A non-trivial, novel result is that the linear growth rate and nonlinear transport agree be-
tween the codes in the fluid limit of high collision numbers (νe ∼ 500 − 2000ci /R), not least,
because the kinetic code treats the collisions fully implicit and employs the Sugama collision
operator with momentum and energy conservation, Galileian invariance and numerically exact
self-adjointness property.
For example, runs with resistive ballooning turbulence in the edge regime using the CGYRO
and the NLET code were performed at the parameters R/Ln = 10, R/LT = 0, ε = r/R = 0,
q = 3.2, Ti = Te , νee = 586ci /R. For the fully developed turbulence the amplitude, pattern, time-
and length-scales of the turbulence agree within the statistical errors, indicating that the proper
fluid limit has been obtained. E.g., the gyro-Bohm particle diffusivities in the mentioned case are
χCGY RO = 772, χNLET = 796 (in units of ρi2 ci /R). The cross ion and electron heat diffusivities
i,Q i,Q e,Q e,Q
are slightly less then 3/2 times that, χCGY RO = 1047, χNLET = 1131, χCGY RO = 1034, χNLET =
1120.
Regarding the structure of the turbulence the typical fluid-like Kelvin-Helmholtz plumes ap-
pear in the simulations, which is very different from the case of collisionless turbulence, with
its strongly dispersive behaviour and rather diffuse and random perturbations
[1] J. Candy, E.A. Belli, R.V. Bravenec, J. Comput. Phys. 324, 73 (2016)
[2] K. Hallatschek, A. Zeiler, Phys. Plasmas 7, (2000) 2554
[3] K. Hallatschek, Phys. Rev. Lett. 93, 065001 (2004)
