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Sep 23

Off-normal simulations of radiative collapse events in Alcator C-Mod

Alex Ryan Saperstein, Jonathan Citrin, Aaron Ho, Ryan Sweeney, Conor Perks, Mark D Boyer, Gregorio Luigi Trevisan, Yumou Wei, Cristina Rea

Nuclear FusionSep 23, 2026Plasma & Confinement Control & Diagnostics AI, Modeling & Simulation

In preparation for ARC, supplementary offline tools are being developed alongside a real-time warning system in order to reduce its disruptivity. This paper introduces Off-Normal SIMulations (ONSIMs) to the MOSAIC pulse-simulating framework with that intent in mind. These modules probe simulated equilibria with a library of distinct perturbations, and can characterize disruptivity based on the scenario's resiliency to them. This paper addresses the development of a module for simulating radiative collapses, with a specific focus on validating for the first time the power balance dynamics characteristic of these events. The simulations are performed using the TORAX transport simulator and are benchmarked against radiative collapse precursors on Alcator C-Mod. It is found that transport models that include sufficient turbulence effects, like a surrogate of the QuaLiKiz transport model, are capable of reproducing the relevant radiative collapse dynamics for an L-mode discharge. It is also found that these dynamics are relatively insensitive to the shape of the collapse-inciting impurity profile, which bodes well for the robustness when probing various equilibria. The benchmarking reported here is part of a larger ongoing validation effort, which lays the groundwork for radiative collapse ONSIMs. Future work will be done to translate validations like this to a general implementation of ONSIMs to SPARC and ARC scenarios.

Commonwealth Fusion SystemsMassachusetts Institute of Technology

Sep 18

Core-edge integrated modeling of ARC: on the effect of impurity transport and detachment conditions.

Marco Muraca, Pablo Rodriguez-Fernandez, Nathan T Howard, Jo Hall, Giovanni Tardini, Davide Silvagni, Thomas A Body, Jon Hillesheim

Nuclear FusionSep 18, 2026Plasma & Confinement Materials & Plasma-Facing Components AI, Modeling & Simulation

Integrated modeling of the flat-top phase of ARC H-modes has been conducted to assess the feasibility of high-performance scenarios compatible with divertor detachment. The analysis incorporates self-consistent evolution of impurity radiation and density profiles, demonstrating that fusion power levels approaching a GW can be achieved while maintaining divertor temperatures below 2 eV with Ar seeding. Sensitivity studies reveal a strong dependence of fusion power on the separatrix density, with performance spanning 750–1000 MW, and a weaker dependence on enrichment factor and pedestal density. Alternative seeding strategies using Neon have also been explored. Plasmas with Argon seeding consistently access H-mode, providing the highest fusion power and detached divertor operation, whereas Neon seeding leads to lower performance (600–850 MW) and less robust H-mode access, due to excessive core impurity accumulation. A small W impurity peaking has been found, with decreasing values at higher Zeff. Further analyses incorporate reduced momentum transport modeling, and sensitivity studies of neoclassical impurity transport, confirming the robustness of the results. Overall, these findings support the viability of high-performing H-mode operation in ARC, ensuring divertor protection, enabled through Argon and Neon impurity seeding.

Massachusetts Institute of TechnologyMax Planck Institute for Plasma PhysicsCommonwealth Fusion Systems

Aug 24

On the feasibility of model-based feedback control of vertical instability growth rate using out-vessel coils in ARC-like scenarios

Arunav Kumar, Cesar Clauser, Theodore Golfinopoulos, Jon C. Hillesheim

arXiv (physics.plasm-ph)Aug 24, 2026Plasma & Confinement Control & Diagnostics Fusion Plant Engineering AI, Modeling & Simulation

In this work, we propose a model-based feedback controller that regulates the vertical instability growth rate ($γ_{gr}$) of a high-elongation, double-null tokamak directly, using only out-vessel poloidal field (PF) coils. High elongation raises the achievable plasma current and fusion performance but makes the plasma vertically unstable, and in a fusion power plant the in-vessel coils that present devices rely on for stabilization may be absent, leaving only distant out-vessel circuits. The controller couples a machine learning surrogate of non-rigid, profile agnostic vertical instability metric to a constrained quadratic program: the surrogate supplies real-time $γ_{gr}$ estimates and, via automatic differentiation, the actuator sensitivities, while the program allocates coil voltages to track a target growth rate, maintain double-null divertor balance, and respect electromechanical limits. We tested this method on the ARC~V3A power plant design configuration across 24 closed-loop simulations spanning equilibrium variations, actuator degradations, and transient disturbances. We achieved full or marginal success in 83\% of these cases (full in 50\%, marginal in a further 33\%) and lose control in the remaining 17\%; the failures map the boundary of out-vessel controllability (occurring at the highest growth rates) and under actuator limits. The controller does not regulate boundary shape explicitly: separatrix geometry follows indirectly from growth rate and flux balance control and would require a separate shape control layer for sustained scenario evolution.

Commonwealth Fusion SystemsMassachusetts Institute of Technology

Aug 17

Mesh-based multiphysics coupling acceleration for fusion neutronics clustering for fusion blanket applications

Jin Whan Bae, Arpan Sircar, Katarzyna Borowiec, Vittorio Badalassi, Cami Collins

Nuclear FusionAug 17, 2026Blankets & Neutronics AI, Modeling & Simulation

Accurate modeling of particle transport within fusion blankets is essential for predicting performance metrics such as heat deposition and the tritium breeding ratio (TBR). However, high-fidelity coupling of thermal fluids from computational fluid dynamics (CFD) to neutronics simulations often incurs significant computational costs due to the complexity of surface intersection calculations in Monte Carlo codes. This paper presents an accelerated multiphysics coupling method for neutronics that utilizes hierarchical agglomerative clustering to map complex material property distributions to a neutronics model. Implemented within the Fusion Reactor Design and Assessment (FREDA) framework, the method leverages existing Python packages to automate the creation of clustered geometries for OpenMC. The approach is demonstrated on a sector model of an ARC-class tokamak with an immersion molten salt blanket, and an simple geometry with varying isotopic concentrations. Results show that the clustering method significantly reduces computational burden without compromising fidelity, providing a foundation for agile iteration of neutronics simulations involving multiple coupled material properties.

Oak Ridge National Laboratory

Aug 10

Steady state, core, operational optimization of an ARC-like tokamak via plasma composition and shape

A. Saltzman, P. Rodriguez-Fernandez, A. Ho, G. Snoep, J. Han, J. Hall, M. S. Anastopoulos Tzanis, J. Hillesheim, A. J. Creely, P. Snyder, et al.

arXiv (physics.plasm-ph)Aug 10, 2026Plasma & Confinement Fusion Plant Engineering AI, Modeling & Simulation

Impurity composition, plasma shape, and pedestal density all provide strong levers on fusion power. Here, we explore the ways in which their variation changes fusion power and seek to find the optimum of these parameters. The key impacts of these variables are through changes in the core turbulent transport, the density of the fuel species, the pedestal pressure, and the plasma volume. ITG stabilization due to increased amounts of impurities is observed. The dependence of all of these parameters on the pedestal pressure is especially complicated because of the separate impacts on the peeling and ballooning modes, which can each limit the pedestal. Optimization of this multidimensional operating space is enabled by the use of Bayesian optimization, resulting in an operating point similar to ARC V3A with ~30% more fusion power and a higher fusion power density. Increased shaping parameters, including elongation, triangularity, and squareness are all beneficial, as is high Zeff. When elongation is also allowed to vary, a ~65% increase in fusion power can be achieved. While not commonly considered, we find squareness is an important lever on fusion power. The plasma performance is limited by the Greenwald density limit constraint. This workflow developed here and demonstrated with the example of ARC V3A can readily be applied to other tokamak designs.

Jul 28

Enhanced alpha channeling with spin-polarized fuel

J. F. Parisi, A. Diallo, J. W. S. Cook

arXiv (physics.plasm-ph)Jul 28, 2026Plasma & Confinement AI, Modeling & Simulation

The nuclear spin state of deuterium-tritium (D-T) fuel sets both the D-T fusion cross section and the emission direction of the fusion-born alphas and neutrons. We show two ways that spin-polarized fuel (SPF) could enhance alpha channeling, the wave-mediated damping of alpha power onto fuel ions rather than electrons, which is predicted to increase fusion power significantly. First, the enhanced SPF cross section produces more alphas, and second, the perpendicular (to the magnetic field) bias of the alphas' kinetic energy couples more efficiently to the perpendicular-resonant channeling waves. The birth anisotropy survives slowing-down and appears as a population inversion of the bulk alpha distribution over a broad region of velocity space, so resonant alphas can drive a suitably tuned channeling wave rather than damp it. Without channeling, SPF roughly doubles the fusion power density through the cross-section boost and its temperature feedback on the reactivity, a well-known result. Our velocity-space calculations find the channeling efficiency about 1.5 times higher for vector-aligned fuel than for unpolarized fuel, and channeling raises the fusion power enhancement to three or four times as the channeling efficiency improves, provided the waves do not depolarize the fuel. A transport model of an ARC-class equilibrium with stiff critical-gradient transport gives an enhancement of 2.2, rising to 3.4 for less stiff transport and to 4.7 in the zero-dimensional model when the critical gradients rise with the hotter ions. Channeling also transports helium quickly to the divertor: at fixed pumping the core helium fraction nearly halves, and a divertor pump several times less selective for helium supports the same core helium dilution. Spin-polarized fuel thus enhances fusion power through the anisotropic alpha distribution, beyond its increase of the reactivity.

Marathon FusionProxima FusionPrinceton Plasma Physics LaboratoryUnited Kingdom Atomic Energy Authority

Jul 24

Predictions of high field side lower hybrid current drive in positive and negative triangularity DIII-D-class and ARC-class plasmas

Grant Rutherford, Paul T Bonoli, Collin Dunn, Ethan Peterson, Stephen J Wukitch

Nuclear FusionJul 24, 2026Plasma & Confinement Heating & Current Drive AI, Modeling & Simulation

Negative triangularity (NT) offers a potential solution to the high heat fluxes incident on the divertor of reactor-class tokamaks by blocking access to H-mode. An important step in assessing how NT may scale to a reactor is determining the behavior of existing current drive schemes in NT. Lower hybrid current drive (LHCD) is an attractive technology due to its high efficiency. The effect of triangularity on high field side (HFS) LHCD was evaluated with the ray-tracing/Fokker-Planck codes GENRAY/CQL3D using positive triangularity (PT) and NT equilibria with otherwise identical shaping parameters generated by the Grad-Shafranov solver TokaMaker. Two classes of devices were considered: DIII-D-class and ARC-class. Triangularity was found to significantly affect LHCD physics, with strong absorption being achieved for a wider variety of launcher configurations in NT than PT. Larger driven currents were predicted in PT than NT for some DIII-D-class cases and the majority of the ARC-class cases. The LH current density profiles were more narrow on average in NT than PT. Finally, the neutron flux incident on the launcher was considered for the ARC-class plasmas. Overall, HFS LHCD is expected to function well in NT.

Massachusetts Institute of Technology