The Helios cryostat, maintenance schemes, and cryogenic production system

“Helios”, the Thea Energy, Inc. stellarator fusion power plant described by Swanson (2025), operates cryogenically, in vacuum, and requires repeatable access for scheduled maintenance. This paper outlines the preconceptual design of the Helios cryostat, the cryogenic cooling system, and the considerations made in mechanical design and remote handling to reduce plant downtime during planned maintenance cycles. The represented design state is a major piece of the stellarator system that is presented in whole by this, and its partner papers. A majority of the concepts introduced are not novel to fusion. However, the compilation of these concepts, in a single stellarator, provides novel insight into the design for maintenance and high availability that the planar coil stellarator architecture permits. The presented modular cryostat provides access to inner stellarator components through easily removable and replaceable panel port covers. The plasma vessel is segmented into 24 sections that are sealed and joined with bolted joints to promote ease of maintenance. This method of assembly allows repeatable access to internal components without the need for destructive ingress methods. Remote handling equipment is used to gain access to these components after operation as the surrounding area remains off limits to personnel due to material activation. An 84 day, 88% capacity factor, biennial maintenance cycle is outlined with supporting analysis for decay heat and equivalent dose after Helios shut down. Helios’ cryogenic cooling requirements are broken into three distinct stages; cool down, operational ramp, and power generation. The cool down phase is supplemented with a supply of liquid nitrogen for the higher power requirements. Total designed cooling power is 1 MW at 80 K and 50 kW at 20 K.