Speaker
Dr
Klaus Hallatschek
(Max-Planck-Institute for Plasma Physics)
Description
Using the gyrokinetic code CGYRO [1] employing the
sophisticated collision operator of Sugama [2] - essentially the
gyro averaged lowest order Hirshman-Sigmar operator -- the transition between
the fully kinetic and the fully fluid regime has been mapped for GAMs and
zonal flows. The collision operator is sufficiently accurate to reproduce the
two-fluid damping of the GAMs and residual zonal flows in the limit of large
collision number.
For GAMs for edge typical safety factors, surprisingly the damping by
electrons is always important, even in collisionless cases without trapped
electrons. The damping can be understood by an argument similar to Fermi's
golden rule. Moreover with increasing collision numbers a maximum of the damping
rate occurs at $$\nu_{ii}\sim\omega_{GAM}.$$ The maximum damping is relatively
small (much smaller than the one of the zonal flows) so that the worst quality
factor of the GAM resonance is still of the order ~100, which would
allow, e.g., its external excitation. Finally at very high collision numbers
the damping rate decreases again, while the frequency approaches the fluid
value [3,4].
For zonal flows, while at small collision numbers the damping is of the order
of an ion collision time, at sufficiently high collision numbers the
collisional damping decreases again, while the residual approaches the fluid
value, which is higher than the collisionless value.
References
[1] J. Candy, E.A. Belli, R.V. Bravenec, J. Comput. Phys. **324** 73 (2016)
[2] H. Sugama et al., Phys. Plasmas **16**, 112503 (2009)
[3] K. Hallatschek, A. Zeiler, Phys. Plasmas **7**, (2000) 2554
[4] K. Hallatschek, Plasma Phys. Control. Fusion **49**, B137-B148 (2007)
Primary author
Dr
Klaus Hallatschek
(Max-Planck-Institute for Plasma Physics)
