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The ARC fusion reactor (affordable, robust, compact) is a theoretical design for a compact fusion reactor developed by the Massachusetts Institute of Technology (MIT) Plasma Science and Fusion Center (PSFC). The ARC design aims to achieve an engineering breakeven of three (to produce three times the electricity required to operate the machine) while being about half the diameter of the ITER reactor and cheaper to build.[1]

The ARC has a conventional advanced tokamak layout, as opposed to other small designs like the spherical tokamak. The ARC design improves on other tokamaks through the use of rare-earth barium copper oxide (REBCO) high-temperature superconductor magnets in place of copper wiring or conventional low-temperature superconductors. These magnets can be run at much higher field strengths, 23 T, roughly doubling the magnetic field on the plasma axis. The confinement time for a particle in plasma varies with the square of the linear size, and power density varies with the fourth power of the magnetic field,[2] so doubling the magnetic field offers the performance of a machine 4 times larger. The smaller size reduces construction costs, although this is offset to some degree by the expense of the REBCO magnets.

The use of REBCO also raises the possibility of allowing the magnet windings to be flexible when the machine is not operational. This offers the significant advantage that they can be "folded open" to allow access to the interior of the machine. Accomplishing this would greatly lower the cost of maintenance, which with other designs generally requires the maintenance to be carried out through small access ports using remote manipulators. If realized, this could improve the reactor's capacity factor, an important metric in power generation costs.

There is a plan to build a scaled-down demonstration version of the reactor, named SPARC, by the company Commonwealth Fusion Systems, with backing from Eni, Breakthrough Energy Ventures, Khosla Ventures, Temasek, and Equinor, among others.[3][4][5][6]

History

The design of the reactor was declared in 2014 in an article available on arXiv[2] and subsequently also distributed in the journal Fusion Engineering and Design, in 2015.[7]

In the official brochure of the prototype SPARC project, it is explained that ARC concept was born as "a project undertaken by a group of MIT students in a fusion design course. The ARC design was intended to show the capabilities of the new magnet technology by developing a point design for a plant producing as much fusion power as ITER at the smallest possible size. The result was a machine about half the linear dimension of ITER, running at 9 Tesla and producing more than 500 megawatt (MW) of fusion power. The students also looked at technologies that would allow such a device to operate in steady state and produce more than 200 MW of electricity." [8]
Design features

The ARC design has several major departures from traditional tokamak-style reactors. The changes occur in the design of the reactor components, whilst making use of the same D–T (deuterium - tritium) fusion reaction as current-generation fusion devices.
Magnetic field

To achieve a near tenfold increase in fusion power density, the design makes use of rare-earth barium-copper-oxide (REBCO) superconducting tape for its toroidal field coils.[2] The intense magnetic field allows sufficient confinement of superhot plasma in such a small device. In theory, the achievable fusion power density of a reactor is proportional to the fourth power of the magnetic field intensity.[1] The most probable candidate in this class of materials is Yttrium barium copper oxide, with a design temperature of 20 K suitable for using other coolants (e.g. liquid hydrogen, liquid neon, or helium gas) instead of the much more complex liquid helium refrigeration required by ITER magnets.[2] In the cited official SPARC brochure, there is an YBCO cable section design which is commercially available and which design should be suitable to fields up to 30T.

ARC is a 270 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.[2]

The design point has a fusion energy gain factor Qp ≈ 13.6 (the plasma produces 13 times more fusion energy than is required to heat it), yet is fully non-inductive, with a bootstrap fraction of ~63%.[2]

The design is enabled by the ~23 T peak field on coil. External current drive is provided by two inboard RF launchers using 25 MW of lower hybrid and 13.6 MW of ion cyclotron fast wave power. The resulting current drive provides a steady-state core plasma far from disruptive limits.[2]
Removable vacuum vessel

The design includes a removable vacuum vessel (the solid component that separates the plasma and the surrounding vacuum from the liquid blanket) that does not require dismantling the entire device. That makes it well-suited for research on other design changes.[1]
Liquid blanket

Most of the solid blanket materials used to surround the fusion chamber in conventional designs are replaced by a fluorine lithium beryllium (FLiBe) molten salt that can easily be circulated/replaced, reducing maintenance costs.[1]

The liquid blanket provides neutron moderation and shielding, heat removal, and a tritium breeding ratio ≥ 1.1. The large temperature range over which FLiBe is liquid permits blanket operation at 800 K with single-phase fluid cooling and a Brayton cycle.[2]
See also

List of fusion experiments

References

"Advances in magnet technology could bring cheaper, modular fusion reactors from sci-fi to sci-reality in less than a decade". Retrieved 2015-08-12.
Sorbom, B. N.; Ball, J.; Palmer, T. R.; Mangiarotti, F. J.; Sierchio, J. M.; Bonoli, P.; Kasten, C.; Sutherland, D. A.; Barnard, H. S. (2014-09-10). "ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets". Fusion Engineering and Design. 100: 378–405. arXiv:1409.3540. doi:10.1016/j.fusengdes.2015.07.008.
Devlin, Hannah (9 March 2018). "Nuclear fusion on brink of being realised, say MIT scientists". the Guardian. Retrieved 16 April 2018.
Rathi, Akshat. "In search of clean energy, investments in nuclear-fusion startups are heating up". Quartz. Retrieved 2020-09-08.
Systems, Commonwealth Fusion. "Commonwealth Fusion Systems Raises $115 Million and Closes Series A Round to Commercialize Fusion Energy". www.prnewswire.com. Retrieved 2020-09-08.
Systems, Commonwealth Fusion. "Commonwealth Fusion Systems Raises $84 Million in A2 Round". www.prnewswire.com. Retrieved 2020-09-08.
Sorbom, B. N.; Ball, J.; Palmer, T. R.; Mangiarotti, F. J.; Sierchio, J. M.; Bonoli, P.; Kasten, C.; Sutherland, D. A.; Barnard, H.S.; Haakonsen, C. B.; Goh, J.; Sung, C.; Whyte, D. G. (2015). "ARC: A compact, high-field, fusion nuclear science facility and demonstration power plant with demountable magnets". Fusion Engineering and Design. 100: 378–405. arXiv:1409.3540. doi:10.1016/j.fusengdes.2015.07.008.

SPARC project official brochure, p. 19

External links
Markiewicz, W. D.; Larbalestier, D.C.; Weijers, H. W.; Voran, A. J.; Pickard, K. W.; Sheppard, W. R.; Jaroszynski, J.; Xu, Aixia; Walsh, R. P. (2012-06-01). "Design of a Superconducting 32 T Magnet With REBCO High Field Coils". IEEE Transactions on Applied Superconductivity. 22 (3): 4300704. Bibcode:2012ITAS...2243007M. doi:10.1109/TASC.2011.2174952. ISSN 1051-8223.
Larbalestier, David (March 15, 2010). "Transformational Opportunities of YBCO/REBCO for Magnet Technology" (PDF). Superpower Inc. Retrieved 12 August 2015.

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