Overview of results from the 2023 DIII-D negative triangularity campaign
K. E. Thome, M. E. Austin, A. W. Hyatt, A. Marinoni, A. Nelson, C. Paz-Soldan, F. Scotti, W. Boyes, L. Casali, C. Chrystal, S. Ding, Xiaodi Du, D. Eldon, D. R. Ernst, R. Hong, G. R. McKee, S. Mordijck, O. Sauter, L. Schmitz, J.L. Barr, M.G. Burke, S. Coda, T. Cote, M.E. Fenstermacher, A. M. Garofalo, P.O. Khabanov, G. Krämer, C.J. Lasnier, N.C. Logan, P. Lunia, A.G. McLean, M. Okabayashi, D. Shiraki, S. Stewart, Y. Takemura, Dinh Truong, T.H. Osborne, M. A. Van Zeeland, B.S. Victor, Huiqian Wang, J.G. Watkins, W Wehner, A.S. Welander, T. M. Wilks, J. Yang, Guanying Yu, L. Zeng, the DIII-D Team
Abstract
Abstract Negative triangularity (NT) is a potentially transformative configuration for tokamak-based fusion energy with its high-performance core, edge localized mode (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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>H</mml:mi> <mml:mrow> <mml:mn>98</mml:mn> <mml:mi mathvariant="normal">y</mml:mi> <mml:mo>,</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <mml:mo>≳</mml:mo> </mml:mrow> </mml:math> 1), high current ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>q</mml:mi> <mml:mrow> <mml:mn>95</mml:mn> </mml:mrow> </mml:msub> <mml:mo><</mml:mo> </mml:mrow> </mml:math> 3), and high normalized pressure plasmas ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> </mml:mrow> </mml:msub> <mml:mo>></mml:mo> </mml:mrow> </mml:math> 2.5) were simultaneously attained in strongly NT-shaped discharges with average triangularity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>δ</mml:mi> <mml:mrow> <mml:mi>avg</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> = −0.5 that were stably controlled. Experiments covered a wide range of DIII-D operational space (plasma current, toroidal field, electron density and pressure) and did not trigger an ELM in a single discharge as long as sufficiently strong NT was maintained; in contrast, to other high-performance ELM-suppression scenarios that have narrower operating windows. These strong NT plasmas had a lower outer divertor X-point shape and maintained a non-ELMing edge with an electron temperature pedestal, exceeding that of typical L-mode plasmas. Also, the following was achieved during the campaign: high normalized density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> / <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi>GW</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> of at least 1.7), particle confinement comparable to energy confinement with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>Z</mml:mi> <mml:mrow> <mml:mi>eff</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:math> , a detached divertor without impurity seeding, and a mantle radiation scenario using extrinsic impurities. 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.