A closer look at SPARC’s burning plasma ambitions

October 5, 2020, 3:00PM|Nuclear News Staff

Cutaway of the SPARC engineering design. Image: CFS/MIT-PSFC, CAD rendering by T. Henderson

Seven open-access, peer-reviewed papers on the design of SPARC, Commonwealth Fusion Systems’ (CFS) fusion tokamak, written in collaboration with the Massachusetts Institute of Technology’s Plasma Science and Fusion Center, were published on September 29 in a special edition of the Journal of Plasma Physics.

The papers describe a compact fusion device that will achieve net energy where the plasma generates more fusion power than used to start and sustain the process, which is the requirement for a fusion power plant, according to CFS.

The timeline for this planned device sets it apart from other magnetic confinement fusion tokamaks: Construction is to begin in 2021, with the device coming on line in 2025.

CFS expects the device to achieve a burning plasma—a self-sustaining fusion reaction—and become the world’s first net energy (Q>1) fusion system. The newly released papers reflect more than two years of work by CFS and the Plasma Science and Fusion Center to refine their design. According to CFS, the papers apply the same physics rules and simulations used to design ITER, now under construction in France, and predict, based on results from existing experiments, that SPARC will achieve its goal of Q>2. In fact, the papers describe how, under certain parameters, SPARC could achieve a Q ratio of 10 or more.

“These are concrete public predictions that when we build SPARC, the machine will produce net energy and even high gain fusion from the plasma,” said CFS’s chief executive officer, Bob Mumgaard. “That is a necessary condition to build a fusion power plant for which the world has been waiting decades. The combination of established plasma physics, new innovative magnets, and reduced scale opens new possibilities for commercial fusion energy in time to make a difference for climate change. This is a major milestone for the company and for the global clean tech effort as we work to get commercial fusion energy on the grid as fast as possible.”

The plan, in brief: CFS was spun out of MIT in spring 2018 to take decades of fusion research into the private sector. The company continues to collaborate with MIT’s Plasma Science and Fusion Center.

CFS envisions SPARC coming on line in 2025 as the world’s first net energy–producing fusion machine, fusing hydrogen isotopes deuterium and tritium and paving the way for the first commercial fusion power plant. To achieve fusion, the deuterium-tritium fuel must be heated to about 100 million degrees centigrade. Magnetic fields confine the charged plasma, insulating it from ordinary matter, and the stronger the magnetic field the stronger the confining force on the plasma.

SPARC is being designed as a pulsed experiment and would not generate electricity, although CFS plans to follow SPARC with a net electricity–producing fusion pilot plant called ARC, which stands for affordable, robust, and compact.

First, however, the team needs to build the magnets that would contain the plasma, an effort that is already under way. The team plans to demonstrate a 20-tesla, large-bore magnet next year, the same year that construction on SPARC would begin.

About those magnets: While both ITER and SPARC would be magnetic confinement fusion tokamaks, SPARC would use high-temperature superconducting magnets made of rare earth barium copper oxide. According to CFS, the new high-temperature superconducting magnets can “enable a similar performance as ITER, but built more than 10 times smaller and on a significantly faster timeline.”

The operational limits for plasma pressure, density, and current increase with magnetic field, yielding better performance. The SPARC design would be about twice the size of MIT’s now-retired Alcator C-Mod experiment but would achieve fusion performance comparable to that expected in the much larger ITER reactor.

Martin Greenwald, deputy director of MIT’s Plasma Science and Fusion Center and one of the project’s lead scientists, wrote an editorial, titled “Status of the SPARC physics basics,” that was published in the journal. “The design for the ITER experiment explicitly required the highest possible magnetic field achievable with the niobium-based technology available at the time,” Greenwald said. “The use of a newer, higher field magnet technology enables similar levels of plasma performance in devices of considerably smaller size and thus lower capital cost.”

The papers: The seven papers, for which 47 researchers from 12 institutions participated, summarize progress and outline the key research questions that SPARC is expected to help answer. The papers also identify the research needed to complete the final elements of the machine design and the operating procedures and tests that will be involved as work progresses toward a fusion power plant.

In his editorial, Greenwald wrote, “Leveraging the broad progress in tokamak physics, the SPARC design has been fundamentally informed and optimized not only by empirical scaling (all data, no physics), but also by ‘first principles’ modeling (all physics, no data). Both approaches result in essentially the same prediction of overall plasma performance and fusion gain, thereby increasing confidence in the projections. Ongoing work features the use of state-of-the-art codes for calculation of ICRH [ion cyclotron resonance] heating, turbulent transport, pedestal structure, edge profiles, magnetohydrodynamics stability, and ripple losses of fast alphas.”

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