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).

**This work supported by the US Department of Energy under DE-SC0020287, DE-SC0019352, DE-FG02-97ER54415, DE-FG02-08ER54999, DE-FC02-04ER54698, DE-SC0022270, DE-AC02-09CH11466, DE-SC0024651, and EUROFUSION No. 101052200

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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