Nature, Published online: 27 March 2025; doi:10.1038/d41586-025-00938-y More than 1,600 readers answered our poll; many said they were looking for jobs in Europe and Canada.

March 25, 2025by Stephen Shankland [@stshank] This month, Commonwealth Fusion Systems began a new chapter of fusion energy progress: assembling the SPARC tokamak. We can now see the beginnings of the actual machine we’ll use to prove the commercial viability of our technology. In March, we installed the SPARC tokamak’s first element, a disc-shaped stainless steel construction called the cryostat base. We rolled it in on tracks, lowered it into place with a crane, leveled it, and fixed it in place with bolts and grout. It’s now nestled into its home in Devens, Massachusetts, surrounded by systems to power and cool it. “With the cryostat base now in place, we’ve begun building the heart of our fusion energy system,” said Samer Hamade, Vice President of Projects at CFS. “This is a very visible example of how the CFS fusion energy project has shifted into a new phase, tokamak assembly. It’s really energizing to see the first part of SPARC filling what was a circular hole in the floor — a true testament to the hard work and dedication of the team.” We’ve been pushing SPARC ahead step by step for years. It began with research on the best way to take the monumental step of demonstrating net fusion energy. We fostered a supply of the high-temperature superconductors (HTS) key to SPARC’s compact design. We began manufacturing SPARC’s magnets based on large-scale prototypes. We built SPARC’s building. CFS is moving fast to build SPARC. Less than four years ago, what’s now the SPARC facility was just a dirt lot. SPARC is a donut-shaped type of fusion device called a tokamak that uses powerful electromagnets to produce the right conditions for fusion energy, including an interior temperature surpassing 100 million degrees Celsius. Fusion, the power source of the sun, combines small elements into heavier ones, releasing tremendous amounts of energy in the process. Because it’s clean, with virtually unlimited fuel and no greenhouse gas emissions, it could be the last new energy source humanity needs. SPARC’s job is to demonstrate net fusion energy: generating more power from fusion than is needed to start and maintain the process. That’s a crucial step toward our later ARC power plants that actually put power on the grid starting in the early 2030s at a site in Chesterfield County, Virginia. ## **What’s a cryostat?** To keep SPARC’s superconducting magnets cold enough to perform well, CFS houses them inside a larger chamber called cryostat that insulates them from the outside world with a vacuum. It’s the same way a Thermos uses a flask with no air between its walls to keep your drinks hot or cold — but on a much larger scale. _What it looks like underneath the cryostat base._ The cryostat base is the bottom of this structure. It’s got a few other jobs besides maintaining that vacuum to help keep the tokamak interior cold: * Support SPARC’s 1,000-ton weight — about as much as two loaded Boeing 747 jets * Absorb some of the neutrons the fusion process creates * Accommodate conduits for helium coolant, magnet power, and communication links to internal sensors The cryostat base is 24 feet in diameter and weighs 75 tons. It’s a carefully designed and manufactured article, a piece of steel made to our exacting specifications then shipped with a protective blue metal cap. A police escort shepherded the cryostat base along its preplanned route to Devens. ## **SPARC assembly begins** The cryostat base installation began a months-long, carefully choreographed process of assembling SPARC like a 3D game of Tetris. In coming months, we’ll also drop D-shaped toroidal field (TF) magnets into two orange stands, insert SPARC’s vacuum vessel into the interior of those TF magnets, add the circular poloidal field (PF) magnets that loop around the structure, drop the cylindrical central solenoid (CS) magnets down the center of the tokamak, and seal the whole assembly with the cryostat sides and top. _The cryostat base in its final location inside Tokamak Hall._ CFS already has been working hard to install the equipment around SPARC that’ll make it work. That includes the systems to power and cool the tokamak’s super-strong magnets, the diagnostic sensors to monitor the fusion process, and the heating system to turn SPARC’s hydrogen fuel into a plasma for the fusion process. “We completely designed the base simultaneously with all SPARC system interfaces like magnet supports, power, cryogenics, vacuum pumping, and instrumentation. And we received it on time after speedy fabrication,” said Moji Safabakhsh, a Director of Engineering at CFS. “Installation of the cryostat base is a watershed moment starting the assembly process and interconnecting SPARC to the balance of the plant.”

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The stakes couldn’t be higher. No one is coming to save us—not politicians, not the media.