Advances in magnet technology have enabled researchers at MIT to propose a new design for a practical compact tokamak fusion reactor that might be realized in as little as a decade: the ARC (affordable, robust, compact) reactor. The stronger magnetic field makes it possible to produce the required magnetic confinement of the superhot plasma—the working material of a fusion reaction—but in a much smaller device than those previously envisioned.
The reduction in size, in turn, makes the whole system less expensive and faster to build, and also allows for some ingenious new features in the power plant design. The proposed tokamak (donut-shaped) reactor is designed to have 500 MW fusion power at 3.3 m major radius and is described in a paper in the journal Fusion Engineering and Design.
The compact, simplified design allowed by high magnetic fields and jointed magnets, enabled by the use of high temperature superconductors. A liquid immersion blanket and jointed magnets greatly simplify the tokamak reactor design.
The ARC reactor, as described in Sorbom et al.
ARC is a ∼200–250 MWe tokamak reactor with a major radius of 3.3 m, a minor radius of 1.1 m, and an on-axis magnetic field of 9.2 T [tesla, a measure of magnetic field strength]. ARC has rare earth barium copper oxide (REBCO) superconducting toroidal field coils, which have joints to enable disassembly. This allows the vacuum vessel to be replaced quickly, mitigating first wall survivability concerns, and permits a single device to test many vacuum vessel designs and divertor materials. The design point has a plasma fusion gain of Qp ≈ 13.6, yet is fully non-inductive, with a modest bootstrap fraction of only ∼63%. Thus ARC offers a high power gain with relatively large external control of the current profile.
32 T superconductor magnet
Researchers at the National High Magnetic Field Laboratory are designing a 32 T superconducting magnet, due for completion in 2016. At 32 tesla, it will be ~8.5 tesla stronger than the current record.
In June 2015, a test for the 32 tesla magnet set a new world record of 27 tesla for an all-superconducting magnet.
Begun in 2009, the project represents a breakthrough in superconducting magnet technology on many fronts. Among other innovations, it combines low-temperature superconductors commonly used in today’s superconducting magnets—niobium tin and niobium titanium—with “YBCO,” a superconducting ceramic composed of yttrium, barium, copper and oxygen.
The finished, 2.3-ton magnet system will feature about 6 miles of YBCO tape, formed into 112 disc-shaped pancakes. Two inner coils of YBCO, fabricated at the MagLab will be surrounded by a commercial outsert consisting of three coils of niobium-tin and two coils of niobium-titanium.
This highly attractive combination is enabled by the ∼23 T peak field on coil achievable with newly available REBCO superconductor technology. External current drive is provided by two innovative inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting efficient current drive provides a robust, steady state core plasma far from disruptive limits. ARC uses an all-liquid blanket, consisting of low pressure, slowly flowing fluorine lithium beryllium (FLiBe) molten salt. The liquid blanket is low-risk technology and provides effective neutron moderation and shielding, excellent heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits an output blanket temperature of 900 K, single phase fluid cooling, and a high efficiency helium Brayton cycle, which allows for net electricity generation when operating ARC as a Pilot power plant.—Sorbom et al.
The new reactor is designed for basic research on fusion and also as a potential prototype power plant that could produce significant power. The basic reactor concept and its associated elements are based on well-tested and proven principles developed over decades of research at MIT and around the world, the team says.
Fusion, the nuclear reaction that powers the sun, involves fusing pairs of hydrogen atoms together to form helium, accompanied by enormous releases of energy. The hard part has been confining the superhot plasma while heating it to temperatures hotter than the cores of stars. The magnetic fields effectively trap the heat and particles in the hot center of the device.
While most characteristics of a system tend to vary in proportion to changes in dimensions, the effect of changes in the magnetic field on fusion reactions is much more extreme: The achievable fusion power increases according to the fourth power of the increase in the magnetic field. Thus, doubling the field would produce a 16-fold increase in the fusion power.
While the new REBCO superconductors do not produce quite a doubling of the field strength, they are strong enough to increase fusion power by about a factor of 10 compared to standard superconducting technology, said PhD candidate Brandon Sorbom, the lead author of the paper. This significant improvement leads to a cascade of potential improvements in reactor design.
ITER, the world’s most powerful planned fusion reactor currently under construction in France, is expected to cost around $40 billion. Sorbom and the MIT team estimate that the new design, about half the diameter of ITER (which was designed before the new superconductors became available), would produce about the same power at a fraction of the cost and in a shorter construction time.
But despite the difference in size and magnetic field strength, the proposed reactor, called ARC, is based on the same physics as ITER, said Dennis Whyte, a professor of Nuclear Science and Engineering and director of MIT’s Plasma Science and Fusion Center.
Another key advance in the new design is a method for removing the the fusion power core from the donut-shaped reactor without having to dismantle the entire device. That makes it especially well-suited for research aimed at further improving the system by using different materials or designs to fine-tune the performance.
In addition, as with ITER, the new superconducting magnets would enable the reactor to operate in a sustained way, producing a steady power output, unlike today’s experimental reactors that can only operate for a few seconds at a time without overheating of copper coils.
Another key advantage is that most of the solid blanket materials used to surround the fusion chamber in such reactors are replaced by a liquid material that can easily be circulated and replaced, eliminating the need for costly replacement procedures as the materials degrade over time.
As currently designed, the reactor should be capable of producing about three times as much electricity as is needed to keep it running, but the design could probably be improved to increase that proportion to about five or six times, Sorbom said. So far, no fusion reactor has produced as much energy as it consumes, so this kind of net energy production would be a major breakthrough in fusion technology, the team said.
The research was supported by the US Department of Energy and the National Science Foundation.
- B.N. Sorbom, J. Ball, T.R. Palmer, F.J. Mangiarotti, J.M. Sierchio, P. Bonoli, C. Kasten, D.A. Sutherland, H.S. Barnard, C.B. Haakonsen, J. Goh, C. Sung, D.G. Whyte (2015) “ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets,” Fusion Engineering and Design doi: 10.1016/j.fusengdes.2015.07.008