Overview of results from the 2023 DIII-D Negative Triangularity campaign

Negative Triangularity (NT) is a potentially transformative configuration for tokamak-based fusion energy with its high-performance core, ELM-free edge, and low-field-side divertors that could readily scale to an integrated reactor solution. Previous NT work on the TCV and DIII-D tokamaks motivated the installation of graphite-tile armor on the low-field-side lower outer wall of DIII-D. A dedicated multiple-week experimental campaign was conducted to qualify the NT scenario for future reactors. During the DIII-D NT campaign, high confinement (H98y,2 ≥ 1), high current (q95 < 3), and high normalized pressure plasmas (βN > 2.5) were simultaneously attained in strongly NT-shaped discharges with average triangularity δavg= - 0.5 that were stably controlled. The experiments with a lower outer divertor X-point shape covered a wide range of DIII-D operational space (plasma current, toroidal field, electron density and pressure) in contrast to other high-performance ELM-suppression scenarios that have narrower operating windows. Also, the following was achieved during the campaign: high normalized density (ne/nGW≤ 2), particle confinement comparable to energy confinement with Zeff ~ 2, a detached divertor without impurity seeding, and a mantle radiation scenario using extrinsic impurities. Plasmas with strong NT maintained a non-ELMing edge with an electron temperature pedestal, exceeding that of typical L-mode plasmas. These results are promising for a NT fusion pilot plant but further questions on confinement extrapolation and core-edge integration remain, which motivate future NT studies on DIII-D and beyond.