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Infinite toroidal mode number (n = ∞) ballooning mode analysis of negative triangularity discharges on DIII–D shows that the access to 2nd stability becomes strongly restricted when the top triangularity decreases even modestly from −0.18 to −0.36. This is observed in experiment to coincide with the suppression of the L–H-transition. Further theoretical analysis with ballooning mode limited pedestals shows that the threshold for opening the 2nd stability access rises from a pedestal temperature of 0.3–1 keV when the top triangularity is decreased from −0.18 to −0.36 indicating that to access the 2nd stability with the more negative triangular shape would require unrealistically high pedestal temperature. The pedestal predicted by the EPED code agrees with the experimental H-mode profile for the negative triangularity case but in contrast to positive triangular shapes the prediction shows no increase in pedestal height with increasing core pressure when triangularity is negative. This work provides a first model to predict when negative triangularity plasmas can be expected to access the H-mode. [ABSTRACT FROM AUTHOR]

The Super H-mode, a regime with high pedestal pressure and stored energy, is explored on DIII-D and combined with an ion transport barrier in the plasma core to increase performance. A significant improvement of ion temperatures and confinement is facilitated by favorable conditions such as high rotational shear and high ion pedestal temperatures. As a result of a rise in density and simultaneous decrease in rotation, the ion transport barrier disappears during the discharge evolution, leading to a transition from a very high confinement state at early times, to a reduced but still high confinement phase. Additionally, in many discharges, a global magnetohydrodynamic (MHD) event consistent with the coupling of a destabilized internal mode to an edge localized mode causes a large energy loss and leads to a reorganization of the plasma into a lower temperature, higher density state. Depending on the magnitude of the global MHD event, the plasma edge collisionality can increase significantly and shift the operational boundary from the peeling to the ballooning side, which can be understood as a drop out of the Super H-mode channel into standard H-mode. Hence, in Super H-mode discharges with ion transport barriers, both the improved pedestal height and rotational shear contribute to the high stored energy. At very low levels of rotation, the confinement factor for SH modes is still expected to exceed standard H-mode by 20%–30%. With their overall stationarity and high-performance levels, Super H-mode discharges provide an attractive regime for ITER and may enable a more compact design of future fusion power plants. [ABSTRACT FROM AUTHOR]

The pedestal structure of the high-confinement mode (H-mode) operation strongly impacts global confinement and fusion performance in tokamak devices. Studies of the pedestal structure also play an important role in better understanding the characterization of H-mode plasma discharges with type-I edge localized modes on the Experimental Advanced Superconducting Tokamak (EAST). The EPED model has been widely validated to predict the pedestal structure of several tokamak devices. The mean pedestal width correlates with the square root of the poloidal pedestal beta and is in good agreement with from experimental observations. It is also found that the scaling coefficients of and may have no dependence on the heating schemes. The comparison of predictions with measurements indicates that the REPED model could be used to predict the pedestal height for a range of experimental pedestal pressures () on EAST. [ABSTRACT FROM AUTHOR]