Speaker
Description
The ARC tokamak, currently under design by Commonwealth Fusion Systems, is expected to produce over 1 GW of fusion power in a deeply burning plasma regime with Q > 50.[1] Fusion-produced alphas will carry over 200 MW of power and act as the dominant source of heating in ARC, and if poorly confined, can cause melting or sputtering of the first wall. This work presents an overview of alpha particle physics in ARC.
First, simulations of ripple-induced alpha losses and resulting steady-state heat loads to an axisymmetric first wall were performed using the Monte Carlo orbit-following code ASCOT5, including a scan over TF coil misalignment magnitude, and found 1-2% of alpha power crossing the last closed flux surface (LCFS) with most realistic misalignments. With perfectly aligned TF coils, alpha wall loads were on the order of 100 kW/m^2, concentrated just below the outer midplane.
Next, the effects of MHD instabilities, including toroidicity- and ellipticity-induced Alfvén eigenmodes (TAEs and EAEs), tearing modes, and sawtooth oscillations, on alpha-particle transport were investigated using reduced energetic-particle transport models, such as the Resonance-Broadened Quasilinear (RBQ) model and ORBIT-Kick. These studies predicted generally benign alpha-particle transport and losses. Nevertheless, the cumulative impact of these processes over slowing down timescales may alter the alpha-particle current drive, requiring self-consistent analysis with integrated transport codes such as TRANSP.[2]
Finally, the reduction of ITG turbulence by TAE-induced zonal flows in a reduced plasma current version of ARC was explored using nonlinear gyrokinetic code CGYRO. Significant suppression of ion-scale turbulence was observed near the r/a = 0.35 flux surface in the presence of unstable TAEs, effectively raising the ITG critical gradient by 25%[3]. These effects suggest that in a full-current ARC plasma, we may see performance improvement via this mechanism, as has been predicted for SPARC.[4]
This work is supported by Commonwealth Fusion Systems, and is based on work supported by the U.S. Department of Energy, Office of Science, Fusion Energy Sciences, under the Milestone-Based Fusion Development Program. This work has been carried out within the framework of the EUROfusion Consortium, partially funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). The Swiss contribution to this work has been funded by the Swiss State Secretariat for Education, Research and Innovation (SERI). This research uses resources of the National Energy Research Scientific Computing Center (NERSC), a Department of Energy User Facility.
[1]: J. C. Hillesheim et al. “Overview of the physics basis for the ARC fusion power plant.” J. Plasma Phys. 92 (2026) E69.
[2]: P. J. Bonofiglo et al. “Reduced Transport Modeling of Alpha Particle Physics in ARC Burning Plasmas.” Under review (2026).
[3]: J. Hall et al. “Gyrokinetic simulation of fast ion turbulence stabilization in the ARC tokamak.” In preparation (2026).
[4]: A. Di Siena et al. “How Fusion-Born Alpha Particles Suppress Microturbulence in Burning Plasmas.” Under review (2026).