Effective Transport Barriers without an H-mode Bifurcation in DIII-D Negative Triangularity Plasmas

ORAL · Invited

Abstract

Diverted negative triangularity (NT) plasmas in DIII-D achieve high thermal confinement (H98y2 > ~1) and normalized pressure βN ≤ 2.5 [1] in the absence of ELMs, potentially scaling to an attractive Fusion Pilot Plant solution. Plasmas with strong NT shaping exhibit a narrow edge transport barrier (on the order of two ion Larmor radii wide) maintained by large counter-current intrinsic edge rotation shear. This barrier forms without an H-mode bifurcation, and is sustained up to high auxiliary heating power [≥ 10 MW, much above the L-H power threshold for equivalent positive triangularity (PT) plasmas]. Global, full-f gyrokinetic simulations confirm several crucial experimentally observed properties of the NT edge barrier, such as strong edge toroidal rotation shear due to preferential loss of co-current passing and trapped ions in NT topology [2], a much deeper, narrow Er well with increased ExB shear compared to PT L-modes, and a very short (< 1 cm) radial turbulence correlation length within the barrier. Doppler Backscattering turbulence data also demonstrate that the ExB shearing rate exceeds the turbulence decorrelation rate within the narrow barrier layer. Core transport streamers/avalanches do not extend into the barrier, consistent with the observation of a locally decreased Hurst exponent. Experimental evidence and infinite-n ballooning mode stability calculations further confirm that the barrier layer cannot expand radially inward [3]. Plasmas with strong NT therefore do not approach the peeling-ballooning pedestal stability limit and avoid peak divertor heat loading associated with ELMs. Hence, NT plasmas can address major core-edge integration issues with significantly improved global confinement due to the edge barrier layer.

[1] K.E. Thome et al., Plasma Phys. Control. Fusion 66 105018 (2024).

[2] T. Stoltzfus-Dueck et al., Phys. Rev. Lett. 114 245001 (2015).

[3] A.O. Nelson, L. Schmitz et al., Phys. Rev. Lett. 131 195101 (2023).

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Presenters

  • Lothar W Schmitz

    University of California, Los Angeles

Authors

  • Andrew O Nelson

    Columbia University

  • Lothar W Schmitz

    University of California, Los Angeles

  • X. Qin

    University of California, Los Angeles, University of California Los Angeles

  • M. Becoulet

    IRFM, CEA, France

  • S.-H. Ku

    Princeton Plasma Physics Laboratory, Princeton Plasma Physics Laboratory (PPPL)

  • C.S. Chang

    Princeton Plasma Physics Laboratory

  • Patrick H. Diamond

    University of California, San Diego, University of California San Diego

  • K. J Callahan

    University of California Los Angeles

  • G. Huijsmans

    IRFM, CEA, France

  • Filipp Khabanov

    University of Wisconsin, Madison, University of Wisconsin Madison, University of Wisconsin - Madison

  • Lei Zeng

    University of California, Los Angeles, University of California Los Angeles

  • Kathreen E Thome

    General Atomics

  • Zheng Yan

    University of Wisconsin - Madison, University of Wisconsin Madison

  • Tyler B Cote

    General Atomics

  • Carlos Alberto Paz-Soldan

    Columbia University

  • George R McKee

    University of Wisconsin - Madison, University of Wisconsin Madison

  • Thomas H Osborne

    General Atomics

  • Max E Austin

    University of Texas at Austin, University of Texas Austin