Geodesic acoustic mode oscillation in the low frequency range

Abstract

In order to understand the various appearances of geodesic acoustic modes ͑GAM͒ in experiments, the following specific problems are theoretically addressed: ͑1͒ The asymmetry of the potential field of GAMs, which is enhanced by the coupling with ion acoustic modes. It may affect GAMs in plasmas with electron temperatures higher than those of the ions. ͑2͒ The possible existence of GAMs in the lower frequency range: This is discussed in connection with the uniqueness of the kinetic response of the plasma to an external field associated with the geodesic curvature of the magnetic lines of force. ͑3͒ The extension of the theory to cover both tokamaks and helical systems: Differences between the helical-type and the tokamak-type GAMs are discussed in terms of their differences in connection length. In a device of mixed helicity, helical natured GAMs are predicted to appear depending on the intensity of the corresponding geodesic curvature and electron temperature.

Energetic-Ion-Driven Global Instabilities Observed in the Large Helical Device and Their Effects on Energetic Ion Confinement

“…5 (b), a peculiar mode having n = 0 is also observed in the signals of the magnetic probes and interferometer/reflectometer, together with the RSAE [37,40]. The observed frequency agrees well with the GAM frequency evaluated at the zero-shear layer using the expression for a helical plasma [42,43]. This n = 0 mode is thought to be the energetic-ion-driven GAM.…”

Turbulence-driven zonal flows in helical systems with radial electric fields

“…From the viewpoint of improving plasma confinement, it is an important issue to investigate effects of magnetic configuration on zonal flows generated by turbulence. Theoretical works on collisionless time evolution of zonal flows in tokamaks [3][4][5][6] and in helical systems [7][8][9][10][11][12] such as heliotrons and stellarators elucidated how the zonal-flow response to a given turbulence source depends on the toroidal magnetic geometry by which particle orbits are determined. It was predicted in our previous works [7,8] that the zonal-flow response can be increased in helical systems by reducing radial drift velocities of helical-ripple-trapped particles.…”

Kinetic ion effect on geodesic acoustic Alfvén modes in tokamaks

“…Due to that property, the mode may be named as geodesic ion induced Alfvén mode ͑GIAM͒. In the difference from AW, intersection of ion sound wave 5 with GAM, which intersection points stay at the large distance ␦r / r of the order of q / sM, cannot form localized standing waves due to small dissipation. It was shown 4 that the GA mode diminishes their frequency inward to the plasma core where collisional frequency is reduced due to the high electron temperature and the mode might disappear due to Landau damping.…”

“…In this case, low frequency pressure perturbations are reestablished due to collisions. The M , N = 0 GA mode oscillations involve the pressure variations along magnetic field lines from high field side to low field side of tokamaks producing poloidal sideband harmonics M = Ϯ 1, Ϯ2, and Ϯ3 due to toroidicity, elipticity, or triangularity parameters of magnetic surfaces, 5 where possible coupling of the GA modes with ion sound oscillations had been discussed in frame of a drift kinetic equation. The GA modes may cross Alfvén wave ͑AW͒ continuum 6,7…”