Investigating confinement and pedestal stability discrepancies between two high density H-mode DIII-D discharges

Operation at high pedestal pressure and density is a promising route to high performance in future fusion reactors. In this work, we investigate why a recent hybrid scenario experiment carried out on the DIII-D tokamak was unable to access the “peeling-limited” pedestal branch at high density, despite matching operational parameters and using more heating power than an earlier reference discharge that entered this regime. As a key difference between the experiments was slightly different combinations of electron cyclotron heating and co/counter-beam injection, a sensitivity analysis for confinement dependence on rotation and heating power was conducted. Transport modeling using TGYRO and TGLF predicts negligible impact from changes in heating power, or rotation on the electron temperature or density. However, ion temperature is found to increase as rotation is increased. These results are consistent with suppression of stiff long wavelength turbulence by increased shear flow. Based on computational analysis with the EPED code, the predicted impact of rotation-induced changes in core pressure on the pedestal stability will be analyzed and interpreted towards future experiments and fusion reactors.

This work was supported by USDoE via awards DE-SC0018287 and DE-FC02-04ER54698.