A 20-K, 600-W, Cryocooler-Based, Supercritical Helium Circulation System for the SPARC Toroidal Field Model Coil Program

Abstract

From June 2019 to July 2021, the MIT Plasma Science and Fusion Center in collaboration with Commonwealth Fusions Systems (CFS) designed, built, and commissioned a test facility at MIT to evaluate the performance of a REBCO-based, 2.9-m tall, 1.9-m wide Toroidal Field Model Coil (TFMC) for the SPARC tokamak. This paper presents the facility’s supercritical helium (SHe) circulation system design and measured performance. The facility employed a forced-flow SHe-circulation loop cooled by cryocoolers to provide a nominal cooling power of 600 W at 20 K and up to 70 g/s SHe flow to the TFMC at an absolute pressure of 20 bar. The reliance on cryocoolers as the facility’s cooling source was an ideal arrangement. Procurement costs were modest, acquisition time was reasonably, and siting requirements were minimal. Steady improvement in cryocooler design provided a simple to use system with sufficient cooling capacity for our needs. Extensive, closed-loop analyses were preformed both to support this procurement and to finalize the overall design of the SHe cooling circuit. The SHe system worked reliably, permitting flexible operation of the TFMC test facility at all working conditions.