Nonlinear translational symmetric equilibria relevant to the L-H transition

TL;DR

This paper constructs nonlinear, translationally symmetric plasma equilibria with flow, showing their relevance to the L-H transition in tokamaks, and finds that sheared flow and nonlinearity can stabilize certain states.

Contribution

It introduces a quasi-analytical method to solve a generalized Grad-Shafranov equation with flow and nonlinearity, linking equilibrium properties to L-H transition phenomenology.

Findings

Equilibria are consistent with L-H transition behavior.

Sheared flow can stabilize the H-state.

Nonlinearity and flow together influence plasma stability.

Abstract

Nonlinear z-independent solutions to a generalized Grad-Shafranov equation (GSE) with up to quartic flux terms in the free functions and incompressible plasma flow non parallel to the magnetic field are constructed quasi-analytically. Through an ansatz the GSE is transformed to a set of three ordinary differential equations and a constraint for three functions of the coordinate x, in cartesian coordinates (x,y), which then are solved numerically. Equilibrium configurations for certain values of the integration constants are displayed. Examination of their characteristics in connection with the impact of nonlinearity and sheared flow indicates that these equilibria are consistent with the L-H transition phenomenology. For flows parallel to the magnetic field one equilibrium corresponding to the H-state is potentially stable in the sense that a sufficient condition for linear stability is satisfied in an appreciable part of the plasma while another solution corresponding to the L-state does not satisfy the condition. The results indicate that the sheared flow in conjunction with the equilibrium nonlinearity play a stabilizing role.