The role of plasma elongation on the linear damping of zonal flows

Abstract

Drift wave turbulence is known to self-organize to form axisymmetric macroscopic flows. The basic mechanism for macroscopic flow generation is called inverse energy cascade. Essentially, it is an energy transfer from the short wavelengths to the long wavelengths in the turbulent spectrum due to nonlinear interactions. A class of macroscopic flows, the poloidally symmetric zonal flows, is widely recognized as a key constituent in nearly all cases and regimes of microturbulence, also because of the realization that zonal flows are a critical agent of self-regulation for turbulent transport. In tokamaks and other toroidal magnetic confinement systems, axisymmetric flows exist in two branches, a zero frequency branch and a finite frequency branch, named Geodesic Acoustic Modes ͑GAMs͒. The finite frequency is due to the geodesic curvature of the magnetic field. There is a growing body of evidence that suggests strong GAM activity in most devices. Theoretical investigation of the GAMs is still an open field of research. Part of the difficulty of modelling the GAMs stems from the requirement of running global codes. Another issue is that one cannot determine a simple one to one relation between turbulence stabilization and GAM activity. This paper focuses on the study of ion temperature gradient turbulence in realistic tokamak magnetohydrodynamic equilibria. Analytical and numerical analyses are applied to the study of geometrical effects on zonal flows oscillations. Results are shown on the effects of the plasma elongation on the GAM amplitude and frequency and on the zonal flow residual amplitude.

Aspect ratio effects on limited scrape-off layer plasma turbulence

“…Magnetic shaping also affects the edge propagation of geodesic acoustic modes, which extract the zonal flows energy, thus enhancing turbulence levels. 20,21 While some codes have the capability to treat arbitrary magnetic shapes, most of the studies focus on comparisons with experiments (through the coupling of a turbulence code with an equilibrium solver) rather than studying the specific effects of shaping on turbulence. The aim of this work is to study aspect ratio effects on SOL fluid turbulence.…”

Generalized expression for polarization density

“…24 They are also used to derive neoclassical polarization shielding, and residual zonal flow. 18,[25][26][27][28][29][30][31][32][33][34][35] Hinton and Robertson found that the dielectric constant is increased due to the neoclassical polarization drift. 26 Rosenbluth and Hinton (R-H) investigated collisionless zonal flow evolution 27 which is affected by neoclassical polarization.…”

“…29 A favorable role of plasma shaping in reducing ion thermal transport in ITG simulations has been attributed to the effect of shaping on the R-H residual zonal flows, [30][31][32] while an additional change in the shearing effect 36 via Geodesic Acoustic Mode frequency has been mentioned as another possibility. 32 An analytic expression for the R-H residual flow with shaping effects has been derived for a model equilibrium. 33 Theoretical progress has then extended to more detailed calculations in helical systems by Sugama and Watanabe.…”

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.…”