The mutual interactions of ion temperature gradient (ITG) driven modes, zonal flows and geodesic acoustic modes (GAM) in tokamak plasmas are investigated using a global nonlinear gyrokinetic formulation with totally unconstrained evolution of temperature gradient and profile. A series of numerical simulations with the same initial temperature and density profile specifications is performed using a sequence of ideal MHD equilibria differing only in the value of the total plasma current, in particular with identical magnetic shear profiles and shapes of magnetic surfaces. On top of a bursty or quasi-steady state behaviour the zonal flows oscillate at the GAM frequency. The amplitude of these oscillations increases with the value of the safety factor q, resulting in a less effective suppression of ITG turbulence by zonal flows at a lower plasma current. The turbulence-driven volume-averaged radial heat transport is found to scale inversely with the total plasma current. (Some figures in this article are in colour only in the electronic version)

Effects of plasma current on nonlinerar interactions of ITG turbulence, zonal flows and geodesic acoustic modes

The equilibrium plasma rotation in a general toroidal magnetic field is nearly always subsonic and is determined by the requirement that the collisional particle transport should be ambipolar in lowest order in the small-ion-gyroradius expansion. Gyrokinetic turbulence can only appreciably modify the rotation on radial length scales of order the ion gyroradius. Only in quasisymmetric fields, where collisional particle transport is intrinsically ambipolar, can the plasma rotate freely and then only in the quasisymmetry direction. Sonic rotation velocities are allowed in this case. However, the quasisymmetry gets violated when the rotation speed exceeds the diamagnetic speed appreciably, leading to reappearance of the non-ambipolar 1/ν-transport regime. Fortunately, this transport scales with the fourth power of the rotation Mach number and is expected to be modest in most plasmas of interest.

http://absimage.aps.org/image/DPP10/MWS_DPP10-2010-000353.pdf

Plasma rotation in a quasisymmetric stellarator

A new era of tokamak transport physics research has opened up with the discovery of a variety of internal transport barrier regimes. With suppression of some instabilities in the plasma, subdominant transport processes can be observed for the first time. In this paper, the theory of internal and edge transport barriers is reviewed with an emphasis on what is still needed in order to have a predictive transport theory of tokamak plasmas with barriers.

https://iopscience.iop.org/article/10.1088/0741-3335/40/5/005/pdf

Theory of internal and edge transport barriers

The mutual interactions of ion temperature gradient (ITG) driven modes, zonal flows and geodesic acoustic modes (GAM) in tokamak plasmas are investigated using a global nonlinear gyrokinetic formulation with totally unconstrained evolution of temperature gradient and profile. A series of numerical simulations with the same initial temperature and density profile specifications is performed using a sequence of ideal MHD equilibria differing only in the value of the total plasma current, in particular with identical magnetic shear profiles and shapes of magnetic surfaces. On top of a bursty or quasi-steady state behaviour the zonal flows oscillate at the GAM frequency. The amplitude of these oscillations increases with the value of the safety factor q, resulting in a less effective suppression of ITG turbulence by zonal flows at a lower plasma current. The turbulence-driven volume-averaged radial heat transport is found to scale inversely with the total plasma current.

/pdf/effects-of-plasma-current-on-nonlinear-interactions-of-itg-1j8fn3kl8q.pdf

Effects of plasma current on nonlinear interactions of ITG turbulence, zonal flows and geodesic acoustic modes

Theory of Fusion Plasmas. International School of Plasma ``Piero Caldirola''. Proceedings of the workshop held at Villa Monastero Varenna, Italy, August 27 31, 1990. Editrice Compositori Bologna 1990. Edited by E. Sindoni, F. Troyon and J. Vaclavic

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Theory of Fusion Plasmas. International School of Plasma ``Piero Caldirola''. Proceedings of the workshop held at Villa Monastero Varenna, Italy, August 27 31, 1990. Editrice Compositori Bologna 1990. Edited by E. Sindoni, F. Troyon and J. Vaclavic