Shafranov shift bifurcation of turbulent transport in the high β_p scenario on DIII-D

Shafranov shift stabilization of turbulence creates a bifurcation in transport, leading to multiple confinement states in the high β_p scenario on DIII-D: an H-mode confinement state with a high edge pedestal, and an enhanced confinement state with a low pedestal and an internal transport barrier (ITB). The bifurcation is observed experimentally in the ion energy transport with respect to mid-radius (ρ = 0.6) pressure gradient. Simultaneously, the electron transport exhibits a similar but less dramatic behavior with respect to pressure gradient. The Shafranov shift is found to increase at the same time as the transition to enhanced confinement, and quasilinear gyro-Landau fluid modeling shows a reduction of predicted energy flux consistent on-set of the ITB. Transient perturbations such as ELMs are likely a trigger for the transition between states by lowering the edge pressure and increasing mid-radius pressure gradient. (letter)