Linear property of energetic geodesic acoustic modes and the impact of pressure anisotropy on tokamak plasma
Energetic geodesic acoustic modes (EGAMs) are n=0 bursting and chirping modes observed in a variety of toroidal magnetic confinement devices. They are usually associated with fast ion loss and a degradency of the energy confinement, which should be carefully minimized. Two regimes of unstable EGAMs exist: the wave-particle interaction driven EGAMs , and the reactive EGAMs with strong analogy to the two-stream instabilities, which was discovered through a linear multi-fluid theory. The transition between these two regimes, the application to explain some of the experimental results and an extension of the fluid model to resolve the radial mode structure are also discussed.
The radial structure of the EGAMs in both regime have not been well studied in the literature, main due to the difficulty to treat the finite drift orbit width (FOW) effects of the fast ions when the radial mode width is comparable to the drift orbit width. Based on the Lagrangian of the wave and fast particles, we’ve developed a linear kinetic code, the EGAM linear radial structure code (EGAMERS) which is capable of resolving full drift orbits in large aspect ratio geometry and their effects on EGAM mode structure. We have applied the code to study the ICRH trapped fast ion driven EGAMs.
External heating methods in tokamaks can induce strong pressure anisotropy. I will also briefly discuss inclusion of pressure anisotropy into the existing MHD equilibrium code HELENA and stability code MISHKA, as well as its physics effects on tokamak plasma equilibrium, stability and wave-particle interaction.
 Fu, G. Phys. Rev. Lett. 101, 185002 (2008).