S. Munaretto, Promise Oluwagbope Adebayo-Ige, Himank Anand, G. Avdeeva, S.G. Baek, Kstish Barada, Emily A. Belli, Elena Belova, J.W. Berkery, T. N. Bernard, Nicola Bertelli, P. J. Bonofiglo, Paul T Bonoli, Mark D. Boyer, Jeff Candy, Choong Seock Chang, Huiwon Chung, C. Clauser, Mihai Comanescu, D. Corona, N. A. Crocker, Ricardo Antonio De Levante Rodriguez, Earl William DeShazer, Ahmed Diallo, Julien Dominski, V. N. Duarte, F. Ebrahimi, Eric Emdee, S Ethier, N Ferraro, E L Foley, eric fredrickson, M. Galante, Kaifu Gan, Stefan P Gerhardt, Robert James Goldston, M. Gorelenkova, W. Guttenfelder, R Hager, Federico David Halpern, Jacob M. Halpern, D. R. Hatch, Frank Hoffmann, Md. Shahinul ISLAM, Stephen C Jardin, S. Kaye, Andrei Khodak, Jon Kinsey, A. Kleiner, Egemen Kolemen, Seung-Hoe Ku, M. Lampert, Brian Leard, Benoit P LeBlanc, Jeff Lestz, F. M. Levinton, PingYu Li, N.C. Logan, Nicolas Lopez, Robert Lunsford, T. Macwan, R Maingi, Joseph McClenaghan, Adam McLean, J. Ménard, Yong-Su Na, A. Nelson, Masayuki Ono, Andres Pajares, Alexei Y Pankin, Jason F Parisi, F. I. Parra, Matthew S Parsons, Bhavin S Patel, M. Podestá, F. Poli, M. Porcelli, Tariq Rafiq, Roger Raman, Juan Riquezes, S.A. Sabbagh, Álvaro Sánchez-Villar, Eugenio Schuster, Syun'ichi Shiraiwa, Tajinder Singh, Sterling P Smith, David Smith, V. Soukhanovskii, Gary M Staebler, B. Stratton, Kathreen E Thome, Matthew Tobin, I U Uzun-Kaymak, B. Van Compernolle, Weixing Wang, W. Wehner, A.S. Welander, Brian D. Wirth, J. Yang, V. Zamkovska
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 ( δ B ∥ ). 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.