High temperature superconducting (HTS) tapes, which have emerged over the last decade, are enablers for new technologies. Specifically, high temperature superconducting magnets to enable commercial fusion energy are arguably the most tantalizing and potentially impactful. Fusion energy would be a long-envisioned source of firm, safe, carbon-free electricity whose successful development requires creative, innovative cryogenic engineering. 

Fusion power plants use high temperatures and pressures to create the conditions found in stars in which light atoms such as hydrogen fuse and produce helium and enormous amounts of energy that can be converted to drive generator turbines. One leading approach, being pursued by MIT spinout Commonwealth Fusion Systems (CFS), leverages the powerful fields of HTS-based magnets to create a “star in a bottle” capable of containing and managing a steady-state plasma; the company’s SPARC device aims to be the first system to achieve net energy from magnetically confined fusion and a proof of concept for subsequent commercialization. 

SPARC has several operating modes that pose a wide range of cryogenic requirements and duty cycles. These include steady-state static loads, elevated loading during multiday high temperature cleaning processes and, most demanding of all, the peak dynamic loads associated with an intensive daily schedule of fusion pulses that will begin after expected commissioning in 2025. 

These pulses create as much heat as a small commercial power plant. SPARC is not designed to produce electricity, so instead of being sent to a generator, the heat must be removed by the cooling systems.

Some of the system’s HTS components require cooling to 8 K and some to 15 K; other non-HTS elements require 80 K. Many design aspects of SPARC are still being finalized, so requirements can be imprecise and subject to change. CFS operates in an especially urgent environment because of fusion energy’s potential as a game-changing technology to mitigate climate change. 

“One of my colleagues used the term ‘just-in-time engineering’; a lot of work happens in parallel in order to accelerate the timeline to get SPARC operational as soon as possible,” says Adam Weiner, Cryogenics Lead for SPARC, who worked for over a decade at the Fermi National Accelerator Laboratory.

To meet these challenges, the SPARC cryogenics team is building a unique cooling plant and distribution system using supercritical helium coolant, which offers better thermodynamic efficiency and simpler single-phase heat transfer than liquid helium. Two main compressors provide redundancy, while an architectural combination of series and parallel turbines delivers operational flexibility. 

But the system’s most distinctive element is its blowdown system, which handles the intense dynamic fusion-pulse cooling loads. Described by Weiner as a “cryogenic rocket engine,” it’s capable of delivering a short-term burst of coolant to SPARC’s toroidal field (TF) magnets for total equivalent cooling power of 2.9 MW over 10 seconds. 

The team initially envisioned handling pulse cooling with a large pump, cold buffer and cold/warm expansion vessel, but instead opted for tanks pre-filled with 8 K helium pressurized up to 20 bar. During a pulse, peak flow of over 200 kg/s passes through the TF magnets and into recovery tanks. Re-cooling of the helium takes about four hours — fast enough to support the planned schedule of four high-power pulses per day.

Weiner notes that the tank-based blowdown strategy has made it easier to adapt to changes in flow requirements as SPARC’s design has evolved. “Also, our site isn’t that big, so we appreciate being able to use the tanks to store helium inventory.” Additionally, 100% of the cryoplant cooling capacity can be quickly delivered if needed to protect SPARC’s TF magnets from serious damage. 

A powerful enabler for the SPARC cryo work has been effective collaboration with supplier companies. “There’s a lot of excitement among their technical teams, and they’ve worked with us very well, despite us pushing hard and asking them to do things they’re not used to, like designing while building,” says Weiner. “The fast pace of our procurement cycle can be tough, but it does result in quicker payment and project turnover, which we hope is good for their businesses.” 

CFS’s roadmap envisions SPARC’s successor, ARC, being the first on-grid fusion power plant by the early 2030s, followed by rapid commercial uptake. Cryogenics is playing a pivotal role in that endeavor and ultimately in decarbonization of the world’s electrical supplies. www.cfs.energy

Image 1: The Commonwealth Fusion System's (CFS) team is working on a high temperature superconducting magnet for SPARC, the CFS commercial fusion demonstration facility. Credit: Commonwealth Fusion Systems

Image 2: The SPARC facility on the Commonwealth Fusion Systems campus in Devens, Mass. will house the SPARC tokamak, which will
demonstrate commercially relevant net energy from fusion for the first time in history. Credit: Commonwealth Fusion Systems