Shafranov shift bifurcation of turbulent transport in the high $\beta_p$ scenario on DIII-D

The Shafranov shift stabilization of turbulence creates a bifurcation in transport leading to formation of a large radius internal transport barrier (ITB) in the high $\beta_p$ scenario on DIII-D. The high $\beta_p$ scenario exhibits high confinement at high $\beta_N$ and high bootstrap fraction in the absence of rapid rotation or negative central shear. Spontaneous formation of an ITB at fixed $\beta_N$ is examined. The energy confinement improves following formation of the ITB. The improvement is associated with a decrease in the minimum mid-radius characteristic turbulence parameter associated with the Shafranov shift: $\alpha-s$, where $\alpha=q^2R d\beta/d\rho$ is a measure of the Shafranov shift, and s is the magnetic shear. After ITB formation, $\alpha-s>0$ within region of ITB and $\alpha-s<0$ outside the ITB. Before ITB formation, $\alpha-s<0$ throughout the entire core. TGLF transport simulations show a bifurcation of the transport depending on the electron pressure gradient scale length. Before ITB formation, the experimental scale length is on the high-transport side of bifurcation. After ITB formation, experimental scale length is on the low-transport side of the bifurcation in the region of the ITB.