Spherical tokamak physics research in preparation for the operation of NSTX-U

Abstract The National Spherical Torus Experiment Upgrade (NSTX-U) is preparing to resume operation, representing a crucial step toward realizing compact, cost-effective fusion pilot plants. In advance of this, extensive modeling and data analysis have been conducted to advance the physics basis for low-aspect-ratio, high-performance plasma regimes, focusing on three core objectives: confinement and stability, power and particle handling, and steady-state operation. Significant progress has been made in understanding the electron temperature flattening in high- β plasmas, which is shown to be driven by a complex interplay of magnetohydrodynamic instabilities (e.g. non-resonant infernal modes), fast-ion-driven Alfvén eigenmodes, and electron and ion-scale micro-instabilities, particularly Kinetic Ballooning Modes (KBMs), whose destabilization is strongly dependent on parallel magnetic field fluctuations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mo>∥</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ). Furthermore, a new gyrokinetic critical pedestal model was developed, accurately predicting pedestal structure by identifying KBMs as the primary stability limit, offering a critical constraint for future high-confinement scenarios. To address the challenge of high heat flux, novel liquid lithium plasma-facing components were modeled. The analysis confirmed that lithium vapor shielding is a self-regulating mechanism for heat mitigation, while also emphasizing that strong main ion parallel flow is essential to minimize core lithium contamination. Finally, progress toward steady-state operation was anchored by developing the required physics basis and control tools. This includes predictive modeling for reversed magnetic shear sustainment, demonstrating that magnetic island-induced bootstrap current reduction is negligible in STs, and advancing real-time control and disruption avoidance capabilities. The development of high-speed surrogate models (e.g. MMMNet) provides computationally efficient tools vital for non-inductive scenario optimization and integrated, low-disruptivity operations planned for NSTX-U.

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Publication Details

Journal
Nuclear Fusion
Published
2026-01-15
DOI
https://doi.org/10.1088/1741-4326/ae38f6
Citations
1
Primary Topic
Magnetic confinement fusion research
Type
article
Field-Weighted Citation Impact
27.96
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Spherical tokamak physics research in preparation for the operation of NSTX-U

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Spherical tokamak physics research in preparation for the operation of NSTX-U

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article en
1 citations

Abstract

Abstract The National Spherical Torus Experiment Upgrade (NSTX-U) is preparing to resume operation, representing a crucial step toward realizing compact, cost-effective fusion pilot plants. In advance of this, extensive modeling and data analysis have been conducted to advance the physics basis for low-aspect-ratio, high-performance plasma regimes, focusing on three core objectives: confinement and stability, power and particle handling, and steady-state operation. Significant progress has been made in understanding the electron temperature flattening in high- β plasmas, which is shown to be driven by a complex interplay of magnetohydrodynamic instabilities (e.g. non-resonant infernal modes), fast-ion-driven Alfvén eigenmodes, and electron and ion-scale micro-instabilities, particularly Kinetic Ballooning Modes (KBMs), whose destabilization is strongly dependent on parallel magnetic field fluctuations ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>δ</mml:mi> <mml:msub> <mml:mi>B</mml:mi> <mml:mrow> <mml:mo>∥</mml:mo> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ). Furthermore, a new gyrokinetic critical pedestal model was developed, accurately predicting pedestal structure by identifying KBMs as the primary stability limit, offering a critical constraint for future high-confinement scenarios. To address the challenge of high heat flux, novel liquid lithium plasma-facing components were modeled. The analysis confirmed that lithium vapor shielding is a self-regulating mechanism for heat mitigation, while also emphasizing that strong main ion parallel flow is essential to minimize core lithium contamination. Finally, progress toward steady-state operation was anchored by developing the required physics basis and control tools. This includes predictive modeling for reversed magnetic shear sustainment, demonstrating that magnetic island-induced bootstrap current reduction is negligible in STs, and advancing real-time control and disruption avoidance capabilities. The development of high-speed surrogate models (e.g. MMMNet) provides computationally efficient tools vital for non-inductive scenario optimization and integrated, low-disruptivity operations planned for NSTX-U.

Nuclear FusionVol. 66(3)
ITER (FR), Lawrence Livermore National Laboratory (US), Oak Ridge National Laboratory (US), University of Wisconsin–Madison (US), Seoul National University (KR), University of California, Los Angeles (US), Lehigh University (US), Princeton University (US), University of Washington (US), Marathon Oil (United States) (US), Princeton Plasma Physics Laboratory (US), Tokamak Energy (United Kingdom) (GB), CompX (United States) (US), Energy Center of Wisconsin (US), Nova Photonics (United States) (US), Boston Fusion (United States) (US), United Kingdom Atomic Energy Authority (GB), École Polytechnique Fédérale de Lausanne (CH), General Atomics (United States) (US), Massachusetts Institute of Technology (US), University of Tennessee at Knoxville (US), Columbia University (US), The University of Texas at Austin (US)
Openalex Percentile: Top 2%
Magnetic confinement fusion research
27.96
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