Pellet injection scoping for the Infinity Two fusion pilot plant

Type One Energy is designing a high-field stellarator fusion pilot plant, Infinity Two, which uses modular high-temperature superconducting magnets optimized to minimize thermal transport and alpha particle losses among other metrics[1].

Key to suppressing transport from ion-temperature-gradient turbulence is a density gradient supported by core fueling from pellet injection. This approach has already been shown to improve ion thermal confinement on the W7-X stellarator[2].

Using the neutral gas shielding equations, an initial study found the ablation profile of 2.3-2.8 mm radius pellets injected at 0.3-1 km/s from the outboard midplane to be consistent with the fueling profile required to support the density gradient in the outer core[3].

After ablation, the resulting partially-ionized plasmoid can be transported by a grad-B drift before homogenizing with the background plasma. This work calculates ablation and estimates drift acceleration using the magnetic equilibrium and a variety of anticipated profiles both during ramp-up and steady-state operation phases to identify a fueling strategy. Operational limits, model uncertainties, and aspects requiring investigation with higher-fidelity models are also discussed.

[1] CC Hegna et al. Journal of Plasma Physics 91 (2025) E76.

[2] J Baldzuhn et al. Plasma Phys. Control. Fusion 62 (2020) 055012.

[3] W Guttenfelder et al. Journal of Plasma Physics 91 (2025) E83.