268 research outputs found
Modulation instability of drift waves
The instability of drift waves against zonal flows and streamers is discussed. Unlike in previous treatments, we do not make the assumption that their frequency is resonant with driftwave packets. In this more general treatment we find at least two unstable roots even in the simple case of a monochromatic pump drift wave, and potentially an infinite multitude of roots for a more complicated drift wave spectrum. One of them is the well known modulational instability in resonance with the drift wave packets; the other is a new instability corresponding to the inelastic refraction of drift waves at the streamer. It is nontrivial which of the many roots is the most unstable one
Radial propagation of geodesic acoustic modes
The GAM group velocity is estimated from the ratio of the radial free energy
flux to the total free energy applying gyrokinetic and two-fluid theory. This
method is much more robust than approaches that calculate the group velocity
directly and can be generalized to include additional physics, e.g. magnetic
geometry. The results are verified with the gyrokinetic code GYRO [J. Candy and
R. E. Waltz, J. Comp. Phys. 186, pp. 545-581 (2003)], the two-fluid code NLET
[K. Hallatschek and A. Zeiler, Physics of Plasmas 7, pp. 2554-2564 (2000)], and
analytical calculations. GAM propagation must be kept in mind when discussing
the windows of GAM activity observed experimentally and the match between
linear theory and experimental GAM frequencies.Comment: 9 pages, 6 figures. This article has been published in Physics of
Plasmas 5, 072503 (2009). It can be found at
http://link.aip.org/link/?PHPAEN/16/072503/
- …