Density limit in peeling-limited pedestals at and above the Greenwald value in DIII-D high poloidal beta plasmas

Abstract High-pressure, peeling-limited pedestals with pedestal normalized beta β N,ped > 2 and pedestal top density n ped at or above the Greenwald density n G have been achieved in DIII-D high poloidal beta plasmas, together with high global normalized beta β N > 3 and energy confinement H 98 ∼ 1.2–1.7. A higher β N allows higher pedestal density above the Greenwald value and higher pedestal pressure, even with low injected torque. MHD modeling confirms that the experimental profiles lie near the peeling-mode instability boundary with high normalized pressure gradient α and high edge current density. Experimental analysis and stability calculations indicate that the high poloidal beta with strong Shafranov shift, high α , high edge q, and weak/negative magnetic shear improves the pedestal stability by decoupling the peeling and ballooning modes and stabilizing the ballooning modes, thus facilitating access to the second stability region of peeling–ballooning mode. Access to this second stability regime opens Super-H-like channels without requiring extremely strong shaping or strong torque injection. The high-pressure peeling pedestal allows the pedestal density to go beyond the Greenwald limit with strong E × B shear maintained: the pedestal pressure increases with pedestal density even when n ped > n G , until the ideal MHD instability boundary is reached, where giant ELMs occur. The giant ELMs are dominated by a strong n = 1 component and cause a large reduction of the edge pressure, but a negligible change of the core pressure, consistent with kink/peeling-mode induced instability. The pedestal recovers from the collapse and typically sustains a high baseline density, around the Greenwald limit, during the whole discharge duration. Experiments also found that internal transport barriers and peeling-limited pedestal with n ped ∼ 0.9 n G can be simultaneously achieved in high β N plasmas, revealing a feedback interaction between ITB strength and pedestal performance.