Turbulence simulations of transport barriers with toroidal velocity

Abstract

The effect of a sheared toroidal velocity on a transport barrier is studied. This analysis is done by using three-dimensional global fluid simulations of electrostatic ion temperature gradient driven turbulence in tokamaks. The barrier is produced with a reversed magnetic shear. For a flat density profile, and at low collisionality, co-rotation leads to an outward motion of the barrier, whereas counter rotation leads to an inward displacement. However, the barrier displacement saturates when increasing the torque at fixed heat source. This saturation is attributed to the onset of Kelvin–Helmholtz modes. Also the central temperature is larger without external torque because the width of the transport barrier is wider. The consequence is that better confinement is obtained in absence of external torque.

Electromagnetic effects on toroidal momentum transport

“…Radial flow shear ͑dv / dr , dv / dr͒ can produce a mode shift which leads to a finite value of ͗k ʈ ͘ and this may lead to an inward flow of toroidal momentum. [7][8][9] The toroidicity, on the other hand, enhances the pinch due to symmetry breaking effects and also due to the resonant denominator.…”

“…This discrepancy could be resolved as suggested by various studies through the existence of momentum pinch ͑inward flow of momentum flux͒, which is anticipated as the result of symmetry breaking effects on toroidal momentum transport. [7][8][9][10][11][12][13][14][15][16][17][18] The first effect of this type 8,9 was identified as the effect of an asymmetric eigenfunction on the average of the parallel mode number which is needed for the parallel momentum transport. This parallel momentum can, for practical purposes, be used as an approximation for the toroidal momentum.…”

“…Recently, symmetry breaking effects of toroidicity were also found, [12][13][14]16 which are usually stronger than those caused by the flow shear on the eigenfunction. [7][8][9][10][11] A recent review on theoretical developments and experimental analysis of momentum transport and rotation can be found in Ref. 19.…”

Poloidal rotation and its relation to the potential vorticity flux

“…Of primary interest in the study of poloidal rotation is understanding the impact of rotational shear on the formation of internal transport barriers. 57,58 In this regard, a description of possible stable flow profiles should provide insight into what role poloidal rotation can play in both triggering as well as sustaining transport barriers. Future work will be oriented toward the development of a self-consistent transport model to better understand the types of flow structures which can be described within the framework of this simple description of turbulently driven poloidal flows.…”

Nonlinear flow generation by electrostatic turbulence in tokamaks

“…10,11,[28][29][30] Another symmetry breaking mechanism, which leads to an inward pinch, can come from the interplay of magnetic field curvature and ballooning mode structure in toroidal geometry. 31,32 See Table I of Ref.…”