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

First principles 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 the transport flux slope around the turning gradient shows features of a second order phase transition. Under a strong gradient the most unstable modes are in nonground 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.

Figure 1

Time history for three volume averaged physical quantities: (a) ion heat conductivity, (b) electron heat conductivity, and (c) electron particle diffusivity under three strong temperature gradients, where $t_0=0.002c_s/R_0$ is the time step size.

Figure 2

(a) Time averaged electron particle diffusivity and (b) electron particle flux for different temperature gradients. A turning point (critical gradient) is found for the trend of the transport coefficients.

Figure 3

Time history of electron heat conductivity in the nonlinear gyrokinetic simulation is shown for two cases under strong gradients: with and without zonal flow. The blue dashed line represents simulation with zonal flow self-consistently generated. For the red solid line, the zonal flow is artificially removed from the simulation.

Figure 4

Two-dimensional turbulence intensity in the poloidal plane for a nonlinearly weak gradient $R_0/L_T=10$ and a nonlinearly strong gradient $R_0/L_T=30$.

Figure 5

(a) The time history and (b) frequency spectra for the Fourier component ($n=20$, $m=51$) of electrostatic potential in the linear simulation with $R_0/L_T=75$.

Figure 6

Linear real frequency and growth rate for the most unstable mode under different temperature gradients using typical $HL\text{−}2A$ parameters with selected toroidal mode number $n=20$.

Figure 7

The poloidal spectral cascade from high to low mode number during nonlinear saturation.