On a 60-acre campus in Massachusetts, 18 superconducting magnets chilled to 8 kelvin are being assembled around a chamber that will bottle plasma 6 times hotter than the sun’s core, and the cryogenic plant hit its first commissioning target
A cryogenic plant beneath a converted industrial building in Devens, Massachusetts, hit minus 445 degrees Fahrenheit on September 16.
That is 8 kelvin, colder than the surface of Pluto.
It was not a malfunction.
The 20 tesla field these magnets can generate was first demonstrated in a prototype test several years ago and stood as a world record for a magnet of its class.
Six feet from that cold point, the machine being assembled around it will generate plasma at 180 million degrees Fahrenheit, hotter than the sun’s core.
How a single building holds both extremes at once, and why that gap is the design itself?
Why the machine must be the coldest and hottest place in the room
A superconducting magnet carries electrical current with zero resistance, but only below a critical temperature. For the tape wound into these coils, that threshold sits near 20 kelvin. The design targets 8 kelvin operating temperature, giving engineers a buffer so a small heat leak cannot push the wire out of its superconducting state and collapse the field.
Because resistance vanishes, a 20 tesla magnetic field can be sustained at a fraction of the power a conventional copper wound magnet would consume. That field strength is the lever that changes everything about reactor size. Older fusion machines needed to be enormous to confine plasma at lower fields, so a stronger field means a smaller machine that can be built and tested on a commercial timeline.
The extreme cold is not an inconvenience. It is the precise mechanism that makes the whole reactor possible.
What the machine looks like at human scale
The tokamak under assembly at Devens is a donut shaped chamber roughly 12 feet across at its widest point. Before a single magnet could be installed, workers lowered a 48 metric ton vacuum vessel half into position on the cryostat base, a disc of stainless steel designed to support a finished machine weighing 1,000 metric tons.
That base had to be leveled to tolerances measured in fractions of an inch before anything else could follow. Eighteen toroidal field magnets will eventually wrap around that vessel in a ring, each one a curved slab of wound superconducting tape made from a compound containing yttrium and barium, with each layer thinner than a human hair.
By late June, three magnets had been publicly confirmed in place, though assembly of the full ring has taken longer than the original schedule projected. The September cryogenic milestone signals the support infrastructure is now ready to receive the remaining units.
The numbers behind the field strength
The 20 tesla field these magnets can generate was first demonstrated in a prototype test several years ago and stood as a world record for a magnet of its class. That figure is nearly double the field strength of the large international benchmark machine in France, and the advantage allows a reactor roughly 50 times smaller while maintaining equivalent plasma performance.
The tokamak is designed to achieve a fusion gain of Q=11, generating 50 to 100 MW of fusion power from just 25 MW of input heating. A gain greater than one means the machine produces more energy than it consumes, the threshold no fusion device has crossed at commercially relevant scale. Separately, a parallel program in Hefei, China, passed final tests on the world’s largest superconducting magnet for a fusion reactor in June.
The two programs use different approaches and different field strengths, yet both cleared major engineering gates within the same quarter.
The complication inside the timeline
Keeping 18 magnets at 8 kelvin while assembling them in an ambient temperature building requires the cryogenic plant to run continuously and respond to every heat load introduced by the installation work. A single magnet quench, where superconducting tape warms above its threshold and suddenly develops resistance, can release stored energy as heat fast enough to damage the coil.
Managing that risk across an 18 magnet system is where the engineering difficulty concentrates. The chief executive of the company building the reactor has been direct about the stakes, telling reporters that “if you only build one fusion power plant, we have utterly failed.” The goal is a design repeatable enough to manufacture in quantity, which means every installation decision made in Devens is also a template for future commercial plants.
A 400 MW commercial plant is already in the permitting stage in Chesterfield County, Virginia, and a July funding round brought total capital raised to roughly $3 billion.
What the 8-kelvin milestone opens next
The immediate next step is a dry dress rehearsal that runs every support system, including power, radiofrequency heating and the cryogenic plant, under conditions that mimic a real fusion pulse without the tokamak present. The company’s own account of reaching this commissioning goal describes a bypass pipe used to practice cooldowns before the tokamak itself was ready to receive the cold.
The distinction between cold hardware and working hardware is still real. Engineers tracking adjacent programs, including hydrogen turbine work approaching net output, note that components behaving well in isolation often surprise teams once the full thermal and electromagnetic environment is present.
What is not in doubt is the ratio itself: plasma at 180 million degrees Fahrenheit, pinned by a magnetic field generated at temperatures colder than any naturally occurring place in the solar system, inside a building on a former military base in rural Massachusetts. Alongside other extreme temperature energy efforts pushing deep into the earth, it marks a moment when the physical limits of temperature are being used as tools rather than obstacles.
First plasma is still projected for sometime next year, and whether the remaining magnets can be installed and commissioned on that schedule will determine whether Devens earns the title its designers intended: the first compact machine to prove fusion energy beyond the laboratory.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
