Compact Fusion Reactor Design Validated

At a facility in Devens, Massachusetts, people are currently assembling a device that is lighter than a commercial airliner with the objective of replicating what the Sun does. This is not just a figure of speech; it is actual fact. Commonwealth Fusion Systems is building a machine which will get hydrogen isotopes up to a temperature of 150 million degrees Celsius so that their atomic nuclei fuse and a part of their mass is changed into energy. Moreover, five new peer-reviewed studies show that the physics supports this method.

The research papers published in the Journal of Plasma Physics set out the scientific basis for the company’s ARC compact fusion reactor, as first reported by IEEE Spectrum; ARC stands for affordable, robust and compact. This kind of description is uncommon in the field of fusion engineering, since that field has previously been associated with cost overruns and long development times. The company, however, believes that its approach actually does achieve these characteristics.

How a compact fusion reactor shrinks the problem

What is important is the use of magnets. For instance, conventional fusion reactors such as the International Thermonuclear Experimental Reactor (ITER), which is being built in southern France, employ low-temperature superconducting magnets and these have to be cooled down to near absolute zero. While the magnets work correctly, they are extremely large; the tokamak of ITER, that is, the doughnut-shaped vessel which contains the plasma, weighs 23,000 tonnes.

CFS has adopted a different method; their compact fusion reactor employs high-temperature superconductor (HTS) magnets made from a material called rare earth barium copper oxide. In this instance, the term ‘high-temperature’ is relative. The magnets still operate within the temperature range of minus 200 to minus 250 degrees Celsius. Nevertheless, this temperature is high enough to enable the cooling system to be greatly reduced, which in turn allows all the other components to be reduced as well. The result is a reactor that can generate 1.1 gigawatts of fusion power and supply 400 megawatts of net electricity to the grid - sufficient for about 280,000 homes - from a device which is only a small fraction of the size of ITER.

Brandon Sorbom, who is one of the founders of CFS and holds the position of chief science officer, states that the fuel could be carried in a pickup truck. Deuterium is derived from seawater and tritium, the rarer isotope of hydrogen, will later be produced inside the reactor when the neutrons from the plasma strike the lithium in the molten salt blanket that surrounds the reactor.

Taming a miniature star

The five papers concern the part which Sorbom calls the most complicated section of the machine - the plasma. Plasma physics is notoriously difficult because the superheated gas, carrying an electrical current of 12 million amperes, is inherently unstable; disruptions occur when the plasma spirals out of control and strikes the reactor wall, and at 150 million degrees that is no gentle collision.

A group of scientists from MIT, Columbia University, and the Max Planck Institute for Plasma Physics have described ways of reducing the damage resulting from disruptions. One of these methods involves rapidly introducing large quantities of gas into the reactor as a sort of cushion. The engineers have not only designed ARC with the aim of preventing disruptions but also so that it can survive them. The reactor is built to withstand one disruption per day and will restart the plasma within a minute while continuing to produce electricity. The molten salt blanket situated around the plasma retains enough heat to keep the turbines running during the brief interruption.

Two-thirds of the 58 authors who took part in the papers are not from CFS. This is significant since it demonstrates that the validation does not rest merely on internal optimism. The fusion science community as a whole looked at the calculations and confirmed them independently by means of peer review.

The prototype reactor which CFS is developing, called SPARC, has now reached a stage beyond 75% at its Devens site, with the objective of achieving first plasma in 2027 in order to show that net energy gain is possible; should this goal be reached, ARC could then begin feeding power into the grid in Virginia in the early 2030s. There remain significant engineering problems to be solved, for example identifying materials that can endure continuous neutron bombardment and there is still the issue of the economics of tritium breeding. Nevertheless, at least the physics side of the matter appears to be settled.