# A New Paradigm for Turbulent Transport Across a Steep Gradient in Toroidal Plasmas

## TL;DR

This paper presents first-principle gyrokinetic simulations revealing a novel reverse trend in turbulent transport across steep gradients in toroidal plasmas, indicating a new transition mechanism to high confinement modes.

## Contribution

It uncovers critical gradients and phase transition features in turbulent transport, introducing a new paradigm independent of shear or zonal flows.

## Key findings

- Existence of linear and nonlinear critical gradients.
- Transport flux discontinuity resembles second order phase transition.
- Unconventional mode structures reduce correlation length and reverse transport trend.

## Abstract

First principle gyrokinetic simulation of the edge turbulent transport in toroidal plasmas finds a reverse trend in the turbulent transport coefficients under strong gradients. It is found that there exist both linear and nonlinear critical gradients for the nonmonotonicity of transport characteristics. The discontinuity of transport flux slope around the turning gradient shows features of second order phase transition. Under strong gradient the most unstable modes are in non-ground eigenstates with unconventional mode structures, which significantly reduces the effective correlation length and thus reverse the transport trend. Our results suggest a completely new mechanism for the low to high confinement mode transition without invoking shear flow or zonal flow.