The Negative Triangularity Tokamak Path for Fusion Pilot Plants: Experimental Progress and Future Prospects

Negative Triangularity (NT), with its unique “inverted-D” plasma cross-section, offers a simple yet powerful solution for the core-edge integration challenge found in tokamaks while maintaining the high confinement needed for a viable fusion pilot plant (FPP). This presentation summarizes recent world-wide experiments that significantly advance the physics basis for the NT concept. In conventional Positive Triangularity (PT) plasmas, operation in the high-confinement (H-mode) regime achieves improved energy confinement compared to L‑mode through a high-pressure edge pedestal, which introduces serious tokamak power exhaust issues. NT offers a fundamentally different approach: high energy confinement, equivalent of H-mode, is instead obtained by both reduction of core plasma turbulence and a slightly elevated edge pedestal when compared to L‑mode. At sufficiently strong NT, consistent with the edge ballooning limit, both H-mode and the deleterious Edge Localized Mode instability are completely avoided, resolving a key power handling problem of the conventional tokamak. In fact, NT plasmas have achieved high performance power handling via a detached divertor without impurity seeding and separately high core radiation fraction, highlighting core-edge compatibility at reactor-relevant parameters. Moreover, experimental studies have shown that it is easier to achieve several additional favorable features in NT plasmas relative to H-mode PT plasmas: reliable operation at very low edge safety factor, low impurity retention with particle confinement comparable to or less than energy confinement, and access to densities exceeding the typical empirical density limit. While these results are encouraging for a NT-based FPP, outstanding viability questions remain, particularly on scaling confinement to a reactor and optimization of the divertor solution. Addressing these topics through further studies on tokamaks is essential to advancing the NT reactor concept.