Tests show high-temperature superconducting magnets are ready for fusion

The test was immediately declared successful, having met all the standards established for the design of the brand new fusion device, dubbed SPARC, for which the magnets are the important thing enabling technology. Champagne corks popped because the weary team of experimenters, who had labored long and hard to make the achievement possible, celebrated their accomplishment.

But that was removed from the top of the method. Over the following months, the team tore apart and inspected the components of the magnet, pored over and analyzed the info from a whole lot of instruments that recorded details of the tests, and performed two additional test runs on the identical magnet, ultimately pushing it to its breaking point with a view to learn the main points of any possible failure modes.

All of this work has now culminated in an in depth report by researchers at PSFC and MIT spinout company Commonwealth Fusion Systems (CFS), published in a set of six peer-reviewed papers in a special edition of the March issue of IEEE Transactions on Applied Superconductivity. Together, the papers describe the design and fabrication of the magnet and the diagnostic equipment needed to judge its performance, in addition to the teachings learned from the method. Overall, the team found, the predictions and computer modeling were spot-on, verifying that the magnet’s unique design elements could function the inspiration for a fusion power plant.

Enabling practical fusion power

The successful test of the magnet, says Hitachi America Professor of Engineering Dennis Whyte, who recently stepped down as director of the PSFC, was “crucial thing, for my part, within the last 30 years of fusion research.”

Before the Sept. 5 demonstration, the best-available superconducting magnets were powerful enough to potentially achieve fusion energy — but only at sizes and costs that might never be practical or economically viable. Then, when the tests showed the practicality of such a robust magnet at a greatly reduced size, “overnight, it mainly modified the price per watt of a fusion reactor by an element of just about 40 in at some point,” Whyte says.

“Now fusion has a probability,” Whyte adds. Tokamaks, essentially the most widely used design for experimental fusion devices, “have a probability, for my part, of being economical because you have got a quantum change in your ability, with the known confinement physics rules, about having the ability to greatly reduce the scale and the price of objects that may make fusion possible.”

The excellent data and evaluation from the PSFC’s magnet test, as detailed within the six latest papers, has demonstrated that plans for a brand new generation of fusion devices — the one designed by MIT and CFS, in addition to similar designs by other industrial fusion corporations — are built on a solid foundation in science.

The superconducting breakthrough

Fusion, the technique of combining light atoms to form heavier ones, powers the sun and stars, but harnessing that process on Earth has proved to be a frightening challenge, with many years of exertions and lots of billions of dollars spent on experimental devices. The long-sought, but never yet achieved, goal is to construct a fusion power plant that produces more energy than it consumes. Such an influence plant could produce electricity without emitting greenhouse gases during operation, and generating little or no radioactive waste. Fusion’s fuel, a type of hydrogen that could be derived from seawater, is virtually limitless.

But to make it work requires compressing the fuel at extraordinarily high temperatures and pressures, and since no known material could withstand such temperatures, the fuel should be held in place by extremely powerful magnetic fields. Producing such strong fields requires superconducting magnets, but all previous fusion magnets have been made with a superconducting material that requires frigid temperatures of about 4 degrees above absolute zero (4 kelvins, or -270 degrees Celsius). In the previous couple of years, a more moderen material nicknamed REBCO, for rare-earth barium copper oxide, was added to fusion magnets, and allows them to operate at 20 kelvins, a temperature that despite being only 16 kelvins warmer, brings significant benefits by way of material properties and practical engineering.

Making the most of this latest higher-temperature superconducting material was not only a matter of substituting it in existing magnet designs. As an alternative, “it was a rework from the bottom up of just about all of the principles that you just use to construct superconducting magnets,” Whyte says. The brand new REBCO material is “extraordinarily different than the previous generation of superconductors. You are not just going to adapt and replace, you are actually going to innovate from the bottom up.” The brand new papers in Transactions on Applied Superconductivity describe the main points of that redesign process, now that patent protection is in place.

A key innovation: no insulation

One among the dramatic innovations, which had many others in the sector skeptical of its probabilities of success, was the elimination of insulation across the thin, flat ribbons of superconducting tape that formed the magnet. Like virtually all electrical wires, conventional superconducting magnets are fully protected by insulating material to stop short-circuits between the wires. But in the brand new magnet, the tape was left completely bare; the engineers relied on REBCO’s much greater conductivity to maintain the present flowing through the fabric.

“Once we began this project, in for example 2018, the technology of using high-temperature superconductors to construct large-scale high-field magnets was in its infancy,” says Zach Hartwig, the Robert N. Noyce Profession Development Professor within the Department of Nuclear Science and Engineering. Hartwig has a co-appointment on the PSFC and is the top of its engineering group, which led the magnet development project. “The cutting-edge was small benchtop experiments, not likely representative of what it takes to construct a full-size thing. Our magnet development project began at benchtop scale and ended up at full scale in a brief period of time,” he adds, noting that the team built a 20,000-pound magnet that produced a gentle, even magnetic field of just over 20 tesla — far beyond any such field ever produced at large scale.

“The usual method to construct these magnets is you’ll wind the conductor and you’ve got insulation between the windings, and you wish insulation to cope with the high voltages which can be generated during off-normal events comparable to a shutdown.” Eliminating the layers of insulation, he says, “has the advantage of being a low-voltage system. It greatly simplifies the fabrication processes and schedule.” It also leaves more room for other elements, comparable to more cooling or more structure for strength.

The magnet assembly is a rather smaller-scale version of those that may form the donut-shaped chamber of the SPARC fusion device now being built by CFS in Devens, Massachusetts. It consists of 16 plates, called pancakes, each bearing a spiral winding of the superconducting tape on one side and cooling channels for helium gas on the opposite.

However the no-insulation design was considered dangerous, and so much was riding on the test program. “This was the primary magnet at any sufficient scale that basically probed what’s involved in designing and constructing and testing a magnet with this so-called no-insulation no-twist technology,” Hartwig says. “It was very much a surprise to the community after we announced that it was a no-insulation coil.”

Pushing to the limit … and beyond

The initial test, described in previous papers, proved that the design and manufacturing process not only worked but was highly stable — something that some researchers had doubted. The following two test runs, also performed in late 2021, then pushed the device to the limit by deliberately creating unstable conditions, including a whole shutoff of incoming power that may result in a catastrophic overheating. Often called quenching, this is taken into account a worst-case scenario for the operation of such magnets, with the potential to destroy the equipment.

A part of the mission of the test program, Hartwig says, was “to really go off and intentionally quench a full-scale magnet, in order that we are able to get the critical data at the proper scale and the proper conditions to advance the science, to validate the design codes, after which to take the magnet apart and see what went flawed, why did it go flawed, and the way can we take the following iteration toward fixing that. … It was a really successful test.”

That final test, which ended with the melting of 1 corner of one in all the 16 pancakes, produced a wealth of latest information, Hartwig says. For one thing, they’d been using several different computational models to design and predict the performance of assorted facets of the magnet’s performance, and for essentially the most part, the models agreed of their overall predictions and were well-validated by the series of tests and real-world measurements. But in predicting the effect of the quench, the model predictions diverged, so it was essential to get the experimental data to judge the models’ validity.

“The very best-fidelity models that we had predicted almost exactly how the magnet would warm up, to what degree it might warm up because it began to quench, and where would the resulting damage to the magnet can be,” he says. As described intimately in one in all the brand new reports, “That test actually told us precisely the physics that was occurring, and it told us which models were useful going forward and which to go away by the wayside because they don’t seem to be right.”

Whyte says, “Mainly we did the worst thing possible to a coil, on purpose, after we had tested all other facets of the coil performance. And we found that almost all of the coil survived with no damage,” while one isolated area sustained some melting. “It’s like just a few percent of the amount of the coil that got damaged.” And that led to revisions within the design which can be expected to stop such damage within the actual fusion device magnets, even under essentially the most extreme conditions.

Hartwig emphasizes that a serious reason the team was capable of accomplish such a radical latest record-setting magnet design, and get it right the very first time and on a breakneck schedule, was because of the deep level of data, expertise, and equipment gathered over many years of operation of the Alcator C-Mod tokamak, the Francis Bitter Magnet Laboratory, and other work carried out at PSFC. “This goes to the guts of the institutional capabilities of a spot like this,” he says. “We had the potential, the infrastructure, and the space and the people to do this stuff under one roof.”

The collaboration with CFS was also key, he says, with MIT and CFS combining essentially the most powerful facets of an instructional institution and personal company to do things together that neither could have done on their very own. “For instance, one in all the key contributions from CFS was leveraging the facility of a non-public company to determine and scale up a supply chain at an unprecedented level and timeline for essentially the most critical material within the project: 300 kilometers (186 miles) of high-temperature superconductor, which was procured with rigorous quality control in under a yr, and integrated on schedule into the magnet.”

The combination of the 2 teams, those from MIT and people from CFS, also was crucial to the success, he says. “We considered ourselves as one team, and that made it possible to do what we did.”