Handling magnets in search of fusion | MIT News

“You get the high field, you get the performance. “

Principal scientist Brian LaBombard summarizes what could be considered a guiding philosophy behind the design and engineering of fusion devices at MIT’s Plasma Science and Fusion Center (PSFC). From 1972 with the Alcator A tokamak, passing through Alcator C (1978) and Alcator C-Mod (1991), the PSFC used magnets with high fields to confine the hot plasma in compact and efficient tokamaks. Joining what was then the Plasma Fusion Center as a graduate student in 1978, just as Alcator A was finishing its run, LaBombard is one of the few to have worked with every iteration of the high-field concept. Now he has turned his attention to PSFC’s latest fusion venture, a fusion energy project called SPARC.

Designed in conjunction with Commonwealth Fusion Systems (CFS), an MIT spinoff company, SPARC uses new high-field high-temperature superconducting (HTS) magnets to achieve fusion that will produce net energy gain. Some of these magnets will wrap toroidally around the donut-shaped vacuum chamber of the tokamak, confining fusion reactions and preventing damage to the walls of the device.

The PSFC has spent three years researching, developing and manufacturing a scale version of these toroidal field (TF) coils – the Toroidal Field Model Coil, or TFMC. Before TF coils can be built for SPARC, LaBombard and his team must test the model coil under the conditions they will experience in this tokamak.

HTS magnets must be cooled to remain superconducting and to be protected from the heat generated by the current. For the tests, the TFMC will be enclosed in a cryostat, cooled to the low temperatures necessary for the eventual operation of the tokamak and charged with current to produce a magnetic field. How the magnet reacts when current is supplied to the coil will determine if the technology is in place to build the 18 TF coils for SPARC.

A success story

That LaBombard is part of the next PSFC merger project is not unusual; that he is involved in the design, engineering and testing of magnets. Until 2018, when he was leading the R&D research team for one of the magnet designs being considered for SPARC, LaBombard’s more than 30 years of famous research focused on other areas of the question of fusion.

As a graduate student, he was acclaimed early on for the research he reported in his doctoral thesis. Working on Alcator C, he made groundbreaking discoveries in plasma physics in the “borderline” region of the tokamak, between the edge of the fusion core and the wall of the machine. With typical modesty, LaBombard attributes part of its success to the fact that the subject was not well studied and that the Alcator C provided measurements not possible on other machines.

“People knew the border, but no one really studied it in detail. On Alcator C, there were some interesting phenomena, like marfes [multifaceted asymmetric radiation from the edge], being detected for the first time. This prompted me to perform very detailed boundary layer measurements that no one had ever seen before. It was a whole new territory, so I made a splash.

This splash established him as a leading investigator in the field of limit plasmas. After a two-year stint at the University of California at Los Angeles working on a plasma wall test facility called PISCES, LaBombard, who grew up in New England, was happy to return to MIT to join the new PSFC Alcator C-Mod project. .

Over the next 28 years of the construction and operation phase of C-Mod, LaBombard continued to make revolutionary contributions to the understanding of tokamak on-board and divertor plasmas, and to the design of internal components capable of survive harsh conditions and control plasma, including the vertical target of C-Mod. plate divertor and a unique divertor cryopump system. This experience led him to design the “X-point target divertor” to handle extreme fusion power exhausts and to offer a national advanced derivative tokamak (ADX) experience to test such ideas.

From the start, LaBombard’s real passion was to create groundbreaking diagnostics to unveil boundary layer physics and guide graduate students to do the same: an Omegatron, to measure the concentrations of impurities directly in boundary plasma, solved with respect to charge / mass; fast scanning Langmuir-Mach probes for measuring plasma fluxes; a Shoelace antenna to provide insight into on-board plasma fluctuations; the invention of a Langmuir mirror probe for real-time measurements of high bandwidth plasma turbulence.

Change side

His expertise established, he could have continued this focus on the edge of plasma through collaborations with other laboratories and the PSFC. Instead, he finds himself on the other side of the vacuum chamber, immersed in the design and technology of magnets. When challenged to find an effective HTS magnet design for SPARC, he and his team were able to come up with a winning strategy, one that seemed most likely to achieve the high compact field and high performance for which PSFC tokamaks are known.

LaBombard is spurred on by his new leadership and excited about the upcoming TFMC test. His new role takes advantage of his training in physics, electricity and magnetism. It also supports his passion for designing and building objects, which he perfected as an apprentice at his machinist father’s high school and professionally explored building systems for Alcator C-Mod.

“I consider my main role to be to make sure that the TF coil operates electrically, as it is supposed to,” he says. “So it produces the magnetic field without damaging the coil. “

A successful test would validate understanding of how the new magnet technology works and prepare the team to build magnets for SPARC.

Among those overseeing TFMC test hours will be graduate students, current and former, who will remind LaBombard of his own days as a student working on Alcator C and his years supervising students on Alcator C-Mod.

“These students were directly involved in the Alcator C-Mod. They would step in, make things happen – and as a team. This team spirit really allowed everyone to surpass themselves.

“And looking at when SPARC was taking shape, you could see that at all levels, from new to young, they really got involved in the spirit of Alcator – recognizing the plasma performance that can be made possible by a high magnetic fields. “

He laughs, looking to the past and to the future.

“And they bring it to SPARC.”

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