We study low confinement mode (L-mode) plasmas modeled on JT-60U reversed magnetic-shear discharges using -based global gyrokinetic simulations, which exhibit strong profile constraints arising from two spatially separated instability free energy sources in a subcritical state for triggering internal transport barriers (ITBs). We reveal a dynamical plasma response regulated by the accumulation of quasi-static zonal flows associated with undamped residues of geodesic acoustic modes, with the partition of energy evolving in time. This process produces two turbulence suppression layers sandwiching the minimum safety-factor surface, but remaining in the L-mode. These layers are expected to act as a seed for ITB formation above the threshold power.
Electromagnetic turbulence in EAST plasmas with internal transport barrier