Local Negative Shear and the Formation of Transport Barriers

Abstract

We present a set of 3D nonlinear equations describing drift-resistive ballooning modes in a torus including the self-consistent modification of the local magnetic shear due to the finite b shift of the flux surfaces. Simulations using these equations reveal that a bifurcation of the transport occurs when the local magnetic shear on the outside midplane reverses sign. A fully self-consistent bifurcation diagram is calculated which reveals significant hysteresis, i.e., the transport barrier is maintained at lower values of b than is required for the formation of the barrier, as expected from the experimental observations. [S0031-9007(96)00616-3]

Recent experimental studies of edge and internal transport barriers in the DIII-D tokamak

“…Increased understanding of transport barrier physics has resulted from determining the mechanisms which can stabilize plasma turbulence and, thereby, reduce turbulence-driven transport. Leading mechanisms for turbulence stabilization include: (a) reduction of plasma turbulence by E×B velocity shear nonlinear decorrelation of turbulence [1,2] or by E×B stabilization of turbulent modes [3,4]; and (b) reduction of the turbulent growth rates by the Shafranov shift (α-stabilization) [5,6] in the presence of low or negative magnetic shear, which in itself stabilizes high-n MHD modes (e.g., ballooning modes). In the DIII-D tokamak, a long succession of detailed measurements of kinetic profiles, radial electric field, E r , profiles, and plasma fluctuations have revealed important details of transport barrier physics and the critical importance of E×B velocity shear stabilization of turbulence.…”

Metamorphosis of plasma turbulence–shear-flow dynamics through a transcritical bifurcation

“…In this work we report on a bifurcation and stability probe of an economical model for L-H transition dynamics that uncovers a mechanism by which a radical change, or metamorphosis, may occur in the qualitative nature of the dynamics. We apply the results of this analysis to clarify the relationship between the structure of the model and the physics of the process that it describes, and draw comparisons with characteristics of L-H transitions observed in various experiments.Since 1988 there has been much progress in developing low-dimensional (low-order or reduced) descriptions of L-H transition dynamics and associated oscillatory phenomena (see, for example, Refs 2,3,4,5,6,7,8,9,10,11,12,13,14,15), the driving force being the potential power of a unified, low-dimensional model as a predictive tool for the design and control of confinement states. For example, a model that speaks of the shape and extent of hysteresis in the L-H transition would help engineers who are interested in controlling access to H-mode.…”

Edge transport barriers in magnetic fusion plasmas

“…Such theories invoke ideal magnetohydrodynamic (MHD) ballooning mode stability [9,10] or peeling modes [11,12] to explain the L-H transition, in which stability to the ballooning or peeling modes leads to barrier formation. Other theories invoke ideal and drift resistive ballooning modes in which diamagnetic effects cause stabilization of such modes leading to bifurcation in the edge transport [13][14][15]. Stabilization of drift resistive ballooning modes in a toroidal geometry have been studied using 3D nonlinear simulations [16,17].…”