H-mode grade confinement with L-mode edge in negative triangularity plasmas on DIII-D*

Plasmas with Negative Triangularity (NT) shape on the DIII-D tokamak sustain H-mode level confinement and high normalized beta (H_98,y2 = 1.3, β_N = 2.6) for several energy confinement times, despite featuring edge pressure profiles typical of an L-mode plasma without Edge Localized Modes.
This work builds upon previous results from the TCV tokamak, which showed that the energy confinement time of collisionless, L-mode plasmas subject to pure electron heating doubles when reversing triangularity with other parameters held fixed. The DIII-D experiments investigated NT plasmas using both pure electron (EC) and mixed ion-electron (EC-NB) heating, thus exploring for the first time a more reactor relevant regime where T_e~T_i . Compared to matched discharges at positive triangularity (PT), in both heating regimes NT plasmas feature 30% increase in stored energy and lower intensity of density and temperature fluctuations. A linear gyrokinetic analysis indicates that these plasmas are dominated by Trapped Electron Modes at ion scale but, unlike the TCV discharges, electron scale fluctuations are active in the core. Growth rates are predicted to decrease at NT at ion scales, with the largest decrease with EC only heating. In the high power phase, NT plasmas with L-mode edge maintain confinement levels comparable to those of PT plasmas that operate in an ELMy H-mode regime at the same heating power, corroborating their H-mode grade confinement. Additionally, the plasmas at NT produced 30% more neutrons than the PT counterpart, which a TRANSP analysis shows to be due to lower impurity content.
These results indicate that NT may prove to be a promising candidate for reactor scenarios owing to high core confinement, low impurity content and ELM-free characteristics.