CFS is building the SPARC tokamak (R=1.85 m, a=0.57 m, B=12.2 T, Ip=8.7 MA). The 1st
objective of SPARC will be demonstration of Q>1, and then focus will shift to retiring physics
risks for ARC. The ARC fusion power plant (ARC V3A: R=4.62 m, a=1.18 m, B=11.4 T, Ip=12.0
MA) is a tokamak facility designed to generate 400 MWe,net from Pfus=1.13 GW, targeting
operation by the early 2030’s. The ARC physics basis[1-5] has identified key uncertainties and
risks to be retired via SPARC operation. Multiple levels of physics model fidelity are deployed
across topics to facilitate rapid iteration of the ARC design, while also incorporating key
constraints from state-of-the-art simulations. This enables a strategy of entering into SPARC
operation with a reference ARC design, along with the necessary models and workflows to
update the design in response to SPARC results. Low fidelity reduced models are used for
scoping of large parameter ranges, and higher fidelity modeling such as CGYRO and M3D-C1
are used to determine constraints on lower fidelity, faster models. Analysis for the current
design shows that for the target plasma scenario, ARC exceeds the shaping criterion for access
to the QCE regime, but access requirements for the separatrix collisionality at high magnetic
field need to be demonstrated on SPARC; other scenarios such as XPR, RMP ELM
suppression, and buffering of small ELMs in a long legged divertor are also considered. ARC is
predicted, based on non-linear gyrokinetic profile prediction with PORTALS-CGYRO, to be in a
regime of extremely stiff core transport compared to most present day experiments, requiring
caution in application of empirical confinement scalings, and emphasizing the sensitivity of
pedestal predictions for overall fusion performance. Integration of high core performance with a
power exhaust solution is assessed with the X-Lengyel model [6], finding access to detachment
at impurity seeding levels similar to present day experiments. Vertical stability, kink and tearing
mode analysis, and design of error field correction coils find viable ARC operational space, and
identify risks to investigate in SPARC such as NTM onset conditions. Alpha particle interactions
with TF ripple, sawteeth, NTMs, and quasi-linear TAE transport show benign internal
redistribution and small losses. Scaling of relative disruption forces and runaway electron
metrics are comparable between SPARC and ARC. Together, work across these topics
progresses to both address key physical constraints and understand ARC plasma conditions,
while also implementing simple constraints and heuristics that enable rapid iteration and
incorporation of new results.

This work was funded by Commonwealth Fusion Systems.