Pushing the Envelope with Magnet Fusion

David Fischer sits next to the experimental vacuum chamber (illuminated in blue), where high-temperature superconducting strips will be mounted for proton radiation and measuring the transport current in the site. His laptop shows data taken in such measurements – the basis for determining the critical current. Credit: Zoe Fisher

with Energy collaborator David Fischer irradiates high-temperature superconducting tape to test its resistance and prepare for the union’s first pilot plant.

“At the age of 12 to 15 I was drawing; I was making plans for smelting equipment. “

David Fischer recalls growing up in Vienna, Austria, imagining how best to cool the oven used to contain the ion hot soup known as plasma in a coupling device called a tokamak. With the plasma hotter than the sun’s nucleus generated in a donut-shaped vacuum chamber, just one meter away from these magnets, what temperature range might be possible with different coolers, he thought.

“I was drawing these plans and telling them to my father,” he recalls. “Then I somehow forgot about the idea of ​​merging.”

Now beginning his second year at the MIT Plasma Science and Fusion Center (PSFC) as a postdoctoral fellow and a new EIT-sponsored MIT Energy contributor, Fischer has clearly reconnected with the “merger idea.” And his research revolves around concepts that engaged him so much as a young man.

Fischer’s early designs explored a popular approach to generating stable carbon-free fusion energy, known as “magnetic closure”. Because plasma responds to magnetic fields, tokamak is created with magnets to keep melting atoms inside the container and away from metal walls, where they can cause damage. The more effective the magnetic closure the more stable the plasma can become and the longer it can be held inside the device.

Fischer is working on ARC, a concept of the pilot coupling plant that employs thin cassettes with high temperature superconductors (HTS) in the melting magnet. HTS allows magnetic fields much higher than would be possible with conventional superconductors, enabling a more compact tokamak design. HTS also allows melting magnets to operate at higher temperatures, greatly reducing the cooling required.

Fischer is particularly interested in how to keep HTS tapes from degrading. Coupling reactions generate neutrons, which can damage many parts of a coupling device, with the strongest effect on the components closest to the plasma. Although superconducting bands may be as much as a meter away from the first tokamak wall, neutrons can still reach them. Even in reduced numbers and after losing most of their energy, neutrons damage the microstructure of the HTS band and over time change the properties of superconducting magnets.

Much of Fischer’s focus is devoted to the effect of radiation damage on critical currents, the maximum electrical current that can pass through a superconductor without dissipating energy. If radiation causes critical currents to degrade too much, the coupling magnets can no longer produce the high magnetic fields needed to limit and compress the plasma.

Fischer notes that it is possible to reduce magnet damage almost completely by adding more protection between the magnets and the melting plasma. However, this would require more space, which comes at a higher price in a compact compact power plant.

“You can’t just put an endless shield in the middle. You have to first learn how much damage this superconductor can tolerate and then determine how long you want the melting magnets to last. And then project around these parameters.”

Fischer’s expertise in HTS tapes stems from his studies at the Technische Universität Wien (Vienna University of Technology), Austria. Working in his master’s degree in the low-temperature physics group, he was told that a doctoral position was available by studying radiation damage in coated conductors, materials that could be used for fusion magnets.

Recalling the drawings he shared with his father, he thought, “Oh, this is interesting. I was drawn to the union more than 10 years ago. Yes, let’s do it. “

The resulting research on the effects of neutron radiation on high-temperature superconductors for fusion magnets, presented at a seminar in Japan, caught the attention of PSFC nuclear science and engineering professor Zach Hartwig and Chief System Officer of the Union Systems of the Commonwealth, Brandon Sorbom.

“They seduced me,” he laughs.

Like Fischer, Sorbom had explored in his dissertation the effect of radiation damage on the critical current of HTS tapes. What no researcher was able to examine was how the cassettes behave when irradiated at 20 kelv, the temperature at which the HTS coupling magnets will operate.

Fischer now finds himself overseeing a proton radiation lab for PSFC Director Dennis Whyte. He is building a device that will not only allow it to irradiate superconductors at 20 K, but also immediately measure changes in critical currents.

He is pleased to be back in the NW13 lab, affectionately known as “The Vault”, working confidently with student assistants of the graduate and university study opportunity program. During his Covid-19 blockage, he was able to work from home to program a measurement program, but he lost the daily connection with his colleagues.

“The atmosphere is very inspiring,” he says, noting some of the questions his work has stimulated recently. “What is the effect of radiation temperature?” What are the mechanisms for critical current degradation? Can we create HTS tapes that are more resistant to radiation? Is there a way to heal the damage from radiation? “

Fischer may have the chance to explore some of his questions as he prepares to coordinate the planning and modeling of a new neutron radiation structure at MIT.

“It’s a great opportunity for me,” he says. “It’s great to be in charge of a project now and to see that people have confidence that you can make it work.”

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