Recent Publications

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

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

Oak Ridge National Laboratory

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.

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.

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.

Enhanced alpha channeling with spin-polarized fuel

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

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.

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

Massachusetts Institute of Technology

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.