  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.
