Multi-field characteristics and eigenmode spatial structure of geodesic acoustic modes in DIII-D L-mode plasmas

Abstract

The geodesic acoustic mode (GAM), a coherent form of the zonal flow, plays a critical role in turbulence regulation and cross-magnetic-field transport. In the DIII-D tokamak, unique information on multi-field characteristics and radial structure of eigenmode GAMs has been measured. Two simultaneous and distinct, radially overlapping eigenmode GAMs (i.e., constant frequency vs. radius) have been observed in the poloidal E×B flow in L-mode plasmas. As the plasma transitions from an L-mode to an Ohmic regime, one of these eigenmode GAMs becomes a continuum GAM (frequency responds to local parameters), while the second decays below the noise level. The eigenmode GAMs occupy a radial range of ρ = 0.6–0.8 and 0.75–0.95, respectively. In addition, oscillations at the GAM frequency are observed for the first time in multiple plasma parameters, including ne, Te, and Bθ. The magnitude of T̃e/Te at the GAM frequency (the magnitude is similar to that of ñe/ne) and measured ne–Te cross-phase (∼140° at the GAM frequency) together indicate that the GAM pressure perturbation is not determined solely by ñe. The magnetic GAM behavior, a feature only rarely reported, is significantly stronger (×18) on the high-field side of the tokamak, suggesting an anti-ballooning nature. Finally, the GAM is also observed to directly modify intermediate-wavenumber ñe levels (kρs ∼ 1.1). The simultaneous temperature, density, flow fluctuations, density-temperature cross-phase, and magnetic behavior present a new perspective on the underlying physics of the GAM.

Linear gyrokinetic investigation of the geodesic acoustic modes in realistic tokamak configurations

“…The GAM is one of the special features of the I-mode and can be observed in the Lmode [15,40,41]. Comparison of the characteristic drive time of the GAM t RD ∼ 1/γ RD , which is given by the nonlinear coupling with the ion-temperature-mode (ITG) [8,39], with the GAM half-decay time t 1/2 confirms the results of the Ref.…”

“…The frequency expression describes only the continuum or dispersive mode in contrast to eigenmode. The latter is characterized by the GAM mode frequencies which are predicted to remain constant over a large radial extent, but a significant radial overlap in the frequency radial profile can be observed, that lead to the GAM frequency peak splitting [41].…”

Experimental observations of modes with geodesic acoustic character from the core to the edge in the TCV tokamak

“…In the 2011-2013 TCV campaign [11], GAMs were simultaneously observed as a coherent mode in four different fields: the m = 0Ẽ × B flow, the m = 1 density componentñ, the electron temperature componentT e , as well as the magnetic componentB θ , which was found to have toroidal mode number n = 0 and poloidal mode number dominated by m = 2, consistent with theoretical predictions [16]. The magnetic component of the GAM was also reported from the DIII-D [9], T-10 [12] and Globus-M [17] tokamaks. In the latter case oscillations at the GAM frequency were also observed on the D α emission intensity.…”

Geodesic modes driven by plasma fluxes during oblique NB heating in tokamaks

“…These modes may drive anomalous plasma transport and energetic ions loss, as it has been theoretically established 2,3 and experimentally observed. [4][5][6][7][8][9][10][11][12][13][14] Furthermore, they may be useful as a diagnostic tool to indicate the L-H confinement transition [4][5][6] in tokamaks. Eigenmodes in the geodesic frequency range have been experimentally observed for a wide range of tokamak ohmic discharges 7,8 and were also detected in ion cyclotron resonance (ICR) 9 and Neutral Beam (NB) heating discharges.…”

“…Eigenmodes in the geodesic frequency range have been experimentally observed for a wide range of tokamak ohmic discharges 7,8 and were also detected in ion cyclotron resonance (ICR) 9 and Neutral Beam (NB) heating discharges. [10][11][12][13][14] In the latter case, the instability is preferentially driven by counter injection in comparison to coinjection, especially during the current rump-up. [10][11][12][13][14] According to the early theoretical models, in discharges with NB injection, the GAM instability can be driven due to an inhomogeneous dependence on the pitch angle distribution [15][16][17] in velocity space.…”