Sheared ExB Flow Produces Self-Organized, Improved Confinement States In Magnetized Plasmas

The process of microturbulence decorrelation and stabilization by sheared ExB flow has the universality needed to explain the turbulence reduction and confinement improvement seen in edge and core transport barriers in limiter and divertor tokamaks, stellarators, and mirror machines. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. This energy confinement improvement also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of ExB shear on the growth, radial extent, and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number of ways the radial electric field E_r can change. In contrast to most neutral fluids, where turbulence is destabilized by flow shear, the quasi-two dimensional nature of the turbulence in magnetized plasmas is the key to turbulence reduction by sheared ExB flow. In 2D neutral fluids (e.g. geostrophic turbulence) similar stabilizing effects occur; an example of this will be discussed in this talk. More complex effects of ExB shear have recently been seen in wide pedestal quiescent H-mode, where simultaneous reduction in the ExB shear in the plasmas edge coupled with an increase further inside the plasma leads to improved MHD stability, due to reduced edge gradients, while still allowing an overall confinement improvement [1]. Considerable experimental work has been done to test the general picture of ExB velocity shear effects on turbulence; the experimental results are generally consistent with the basic theoretical models.

[1] K.H. Burrell et al, Phys. Plasmas 23, 056103 (2016)