Master's candidate explores ways to cool high-temperature superconductors used in fusion research
March 23, 2017
“Where I grew up, it is below zero degrees Celsius five months of the year.”
Julien Barber, a first-year graduate student in the Department of Mechanical Engineering at MIT, is describing Winnipeg, Manitoba, where average temperatures do not rise above freezing from November to March. It may not be surprising, then, that he is currently keeping things cool at the MIT Plasma Science and Fusion Center (PSFC), studying cryogenic methods of preventing superconducting magnets used in fusion research from overheating.
As an undergraduate at the University of Manitoba, Barber became accustomed to extreme cold. He spent time conducting climate change research in the high Arctic aboard the Canadian Icebreaker Amundsen, and first harnessed the cold, not for fusion devices, but for grocery stores.
“In Manitoba you have giant grocery stores, or data centers, that are burning carbon and spending money to cool things when the temperature outside is often colder than that of the component that requires cooling. My thesis partner, my advisor and I were attempting to find some way to transfer that cooling potential indoors.”
His work resulted in the design of a passive two-phase thermosyphon that could extend outside the roofs of buildings. Cool air passing the exposed thermosyphon would condense, liquefy, and fall to the bottom of the apparatus inside the building. The thermal energy from the source would then boil the liquid, sending it back up to the top as vapor, providing a passive means of cooling a given component.
“It would go back and forth. We designed an energy-saving system with no energy generation. We didn’t move a single electron. All we did was offset electricity by thermal transport. This is where I started to understand the role of thermodynamics, and how all energy is connected to heat transfer.”
At the PSFC, Barber works with Joe Minervini, Division Head of the Magnets and Cryogenics division. The group is trying to integrate high temperature superconductors (HTS) into field coils for a new generation of fusion devices that could be smaller and less expensive than previously envisioned. The tokamak design Barber studies uses a donut-shaped vacuum chamber surrounded by a system of magnets to confine hot plasma fuel long enough to create fusion energy. The center of the plasma burns at temperatures hotter than the sun, while only meters away the superconducting cables must be maintained at -253 °C. This temperature gradient is one of the largest known to man, and represents a major engineering challenge.
Tokamaks have typically used low temperature superconductors (LTS) (a material that provides zero resistance to electrical current) in order to create the magnetic fields required to confine the burning plasma. However, these LTS materials must be cooled to extremely low temperatures for them to remain superconducting. The high-temperature superconductors used in the new conceptual design present opportunities for fusion research.
“With LTS you were restricted to helium, which has limited thermal properties and ability to transfer heat from the magnetic coils. With HTS you can operate at higher temperatures, which allows you to explore new realms of cryogens for cooling, such as hydrogen.”
Little work has been done on using liquid hydrogen as a cooling mechanism for this kind of application. Barber’s work so far has focused on analyzing the thermal properties and the heat transfer potential of supercritical hydrogen, and exploring how this cryogen might improve the performance of the magnetic coils in fusion devices. Experiments are being designed to test his modeling. Despite hydrogen’s promising heat transfer potential, the fluid comes with its own set of challenges. Hydrogen is a volatile substance, most commonly used as rocket fuel, and requires extreme care in handling and implementation.
Barber’s attraction to fusion, a potentially endless source of carbon-free energy, developed naturally from his early interest in climate change. He credits his father, a professor and climate researcher at the University of Manitoba, not only for generating his interest in clean energy and conservation, but for providing him with a global perspective, a consequence of living in foreign countries during multiple sabbaticals.
“My upbringing fostered the desire to tackle big problems that have a global impact. Fusion fits the bill of a global remedy for both energy needs and some environmental problems. It’s not trying to solve the problem of one specific person. It’s trying to change the game.”
Barber is passionate about contributing to this change. A member of the MIT Energy Club, he formed a group called NRG, for students interested in developing ideas for carbon-free energy.
“We met weekly, brainstormed, and defined sectors of energy space ripe for the picking. We wanted to find ideas we could all work on together.”
Barber is hoping some of the ideas they have submitted to clean energy competitions will receive development funds. Beyond that, NRG is helping Barber approach energy on what he calls “a more holistic level,” not focusing on only one topic.
“I want to be sure I don’t just know about hydrogen and magnets. I’m interested in exploring the intersection between business and engineering. I love being in a lab, I like crunching numbers and solving problems with pen and paper – very academic – but I also like involving myself on the business side of things, engaging with people, pitching ideas,and figuring out how to implement new clean energy projects."
He’s already a convincing salesman for fusion, a challenge that could occupy him for decades.
“I’ve always been of the mindset, go for the hard things. Tackle what’s difficult. Make the biggest impact you can.”