Explaining the lack of power degradation of the energy confinement in the wide pedestal quiescent H-mode via transport modelling*

The large Shafranov shift resulting from increased near-zero-torque injected beam power (P_NBI) in the wide pedestal quiescent H-mode (WPQH) stabilizes turbulence and enables high-confinement ELM-free plasmas. Compared to the standard QH-mode, WPQH is characterized by formation of a wider and higher pedestal, and strong broadband fluctuations in the pedestal. However, unlike conventional H-modes where the energy confinement time reduces with increasing heating power, WPQH modes do not show power degradation of the energy confinement, posing a riddle. As the P_NBI was increased, reduced transport, as well as increased core diamagnetic E×B shear rate are observed, suggesting the formation of an ion internal transport barrier (ITB). In this work, TGLF code was used to predict the ITB and its stability analysis. The energy confinement time calculated from the new constructed equilibria with the modelled profiles show insensitivity to the increased P_NBI; these modelled profiles use the TGYRO transport solver with TGLF/TRANSP matched energy fluxes. Linear stability analysis reveals that drift-wave instabilities in the core are stabilized by E×B shear, the T_i/T_e ratio and the Shafranov shift; the latter was found to have the strongest effect on the turbulence suppression.