Exciting and propagating characteristics of two coexisting kinetic geodesic acoustic modes in the edge of plasma

Abstract

Geodesic acoustic modes (GAMs) [1], which are the highfrequency branch of zonal flows, are azimuthally symmetric modes unique to toroidal plasma. GAMs are characterized by potential fluctuations with m = n = 0 (m/n is the polodial/ toroidal mode number). Recently, GAMs have received considerable attention in magnetic fusion plasma research due to the important role in controlling the transport level of plasmas through nonlinear interaction with drift-wave turbulence [2, 3]. Those conclusions have been further enhanced by the observation of a competition between the turbulence level and GAM flow shearing in the limit-cycle phase on ASDEX-U [4]. By now, it has been pointed out that the time-dependent

A gyro-fluid model of global geodesic acoustic modes supported by finite ion Larmor radius

“…A similar edge-localized single GGAM has also been observed in the L-mode plasmas on D-IIID tokamak [9]. Double GGAMs have been discovered by Kong et al in edge plasmas created by Ohmic heating on HT-7 [10] as well as on HL-2A [11], and both the lower and higher constant frequencies of GGAMs intersect with the theoretical curve of GAM continuum in the edge region. On ASDEX-Upgrade multiple (more than three) global GAMs (e.g., four in the discharge #18813) were observed in Ohmic, NBI and ECRH heated L-mode plasmas [12], with their amplitudes being narrowed down to the interval 0.97 < r < 1.0 (r is the normalized minor radius).…”

“…On the other hand, we note that this type of edge-localized GGAMs have been widely observed in L-mode plasmas heated in various ways [8][9][10][11][12].…”

“…Under condition (14) and making use of equations (A.2)-(A.5), equations ( 7)-( 9) and (11) are further simplified into the following ones,…”

“…On the one hand, naively one might attempt to fine-tune some typical T(x) and q(x) functions to create clean frequency gaps, that are situated outside the domains occupied by any of the positive forbidden frequency curve Ω 2 fi (x), so that the feasible eigen-frequencies of GGAMs could be accommodated in these gaps. On the other hand, it is tempting to look for multiple eigen-modes whose eigen-frequencies intersect with the higher branch of the local GAM frequency curve Ω 2 G+ (x), a phenomenon that has been observed in many toroidal plasmas [8][9][10][11][12][13][14][15]. From these two considerations, it is better to qualitatively position the possible forbidden frequency curves…”

Kinetic theory of geodesic acoustic modes in toroidal plasmas: a brief review

“…The solution in equation ( 61) provides direct information for the interpretation of experimental observations [21,168] and/or nonlinear simulations [120]. The parametrically excited GAM is characterized by a nonlinear radial wavenumber…”