By Dr. Jonathan Tennenbaum
In recent years, technological breakthroughs and the growing role of private investors have brought new promise to nuclear fusion – the effort to bring the energy source of the Sun and other stars down to Earth.
Launched in 2018, the Fusion Industry Association (FIA) comprises 22 companies, most of them startups, pursuing a variety of innovative approaches to realizing nuclear fusion as a commercial energy source.
In an October 6, 2020 article, FIA executive director Andrew Holland noted, “So far, fusion entrepreneurs and their investors have invested more than US$1.5 billion in private fusion energy start-ups.”
I have written about one of those companies, LPP Fusion in one of a series of articles in Asia Times. LPP Fusion is developing the Dense Plasma Focus, which generates fusion reactions in a pulsed regime by intense, self-focusing electrical discharges.
This article is devoted to a very different approach being pursued by FIA member Commonwealth Fusion Systems.
Commonwealth Fusion Systems was launched in March 2018 by a group of researchers and students from the Plasma Science and Fusion Center at the Massachusetts Institute of Technology (MIT), long a focal point of fusion research in the United States.
CFS intends to generate profits through the commercialization of fusion energy via what it calls the High-Field Path to Practical Fusion Energy. CFS will fund the relevant research and development work at MIT.
So far CFS has raised over $200 million in private capital, including a large chunk of investment from the Italian industrial giant ENI.
CFS’s approach is based on a device known as a compact high-field tokamak, which utilizes super-powerful magnetic fields to contain and stabilize the fusion process in a donut-shaped chamber.
Experimental devices of this sort have already been built. The three most important of them are Alcator A, B and C-mod at MIT.
Building on that experience, CFS’s innovative edge lies in its strategy to utilize for the first time in a fusion test reactor magnet systems based on revolutionary high-temperature superconducting materials known as rare-earth barium copper oxides (ReBCOs).
Among other things, this technology will make it possible to realize magnetic field strengths two or more times higher than those which have been used in comparable fusion experiments until to now.
The superior properties of the REBCO-based magnet systems open the door to a whole series of other innovations, which CFS intends to introduce into the design of a test reactor called SPARC and a follow-on project called ARC. (SPARC stands for “Soonest/Smallest Private-Funded, Affordable, and Robust Compact” reactor.)
The construction of SPARC should begin in 2021. Commonwealth Fusion’s leaders are confident that SPARC will deliver, by the middle of this decade, the world’s first demonstration of so-called “scientific breakeven.”
That means that more energy is released by fusion reactions than has been inputted into the fuel by the external heating systems that are needed to achieve and maintain the required 100 million-degree-temperatures.
In this respect, SPARC would leapfrog ITER (originally the International Thermonuclear Experimental Reactor), the giant nuclear fusion reactor now under construction in France. ITER’s first experiments with deuterium-tritium fusion fuel are not expected to begin until 2035.
It is important to note, however, that achieving “scientific breakeven” by no means implies that the reactor system, taken as a whole, produces more energy than it consumes.
Firstly, the reactor expends much more energy than what is finally delivered to the fuel in the form of heating. A great deal of energy is lost due to the low efficiency of present heating methods. Plus, the reactor has many other systems that consume energy.
Finally, a large portion of the energy from the fusion reactions is lost in the process of conversion into electricity via cooling systems, heat exchangers and turbines.
Achieving net power production will be the task of ARC, the follow-on project to SPARC. According to Commonwealth Fusion’s plans, ARC will be a full-fledged prototype fusion power plant with a net electrical output of 200-250 MW.
This sounds rather optimistic. But along the way, the CFS-MIT partnership has already made a breakthrough in the area of high-temperature superconductor technology, whose importance extends well beyond the domain of fusion per se.
A paper just published in the journal Superconductor Science and Technology describes the design, fabrication and impressive test results of a new, high-temperature superconductor cable called VIPER. I shall go more into the details of SPARC, ARC and VIPER in a later article. Here are some essentials.
The tokamak strategy
The SPARC is an advanced variant of the tokamak, a donut-shaped device originally invented by Soviet scientists in the late 1950s. In contrast to pulsed systems such as the plasma focus or laser fusion, tokamak reactors are intended to operate in a stable, sustained regime.
Expressed in the simplest possible terms, the tokamak strategy is the following:
Nuclear fusion reactions of the type under consideration occur in useful amounts only at temperatures of 100 million degrees and higher. The “easiest” fusion reaction is that between the hydrogen isotopes deuterium (D) and tritium (T). Other reactions, such as D-D or hydrogen-boron require far higher temperatures.
Practically all tokamaks that are intended to actually produce energy run on D-T.
At million-degree temperatures, the fuel mixture is transformed into a hot plasma, a medium in which electrons and nuclei are no longer bound to each other but move around independently, interacting via electric and magnetic forces.
Tokamaks use magnetic fields to confine and compress the hot plasma to a core region inside the reactor’s combustion chamber, suspended away from the chamber’s walls.
The basic principle is that charged particles, when placed in a magnetic field, experience forces that oblige them to spiral around the magnetic field lines. The stronger the field, the smaller the radius of motion of the particles, which thereby become more and more entrapped in the field lines.
Obviously, the magnetic field must be strong enough to counteract the natural tendency for the million-degree plasma to expand.
The first attempt at a magnetic bottle would be a cylindrical configuration with the magnetic field parallel to the axis. Such a field can be generated by coils wound around the outside of the cylinder.
The most obvious problem with this setup is that the plasma is still free to move along the axis and leak out from the ends of the bottle. Solution: Bend the cylinder around and join the ends to form a donut shape. (See above)
This toroidal configuration lends itself to utilizing an additional trick: By applying a second, variable magnetic field one can induce electric currents in the plasma. If the currents are strong enough, the so-called “pinch effect” occurs, causing the currents of plasma particles to contract (“pinch”) and thereby opposing the forces of expansion. Induced currents also heat the plasma.
Initially, it was hoped that this would be enough, together with magnetic compression, to achieve so-called ignition, in which fusion temperatures are maintained by the reaction itself.
It didn’t turn out that way, however. Today, tokamak devices that are intended to reach fusion conditions must provide additional heating in the form of electromagnetic radiation and/or particle beams injected into the plasma. So far not one has come near to ignition.
There is much more to the story, but this will suffice for my present purposes. Interested readers can find a good overview here.
Over 200 tokamaks have been built and operated in laboratories around the world. Apart from steady advances in knowledge of the underlying physics, the history of the tokamak has been marked by disappointments and setbacks that have been overcome only gradually.
The biggest problem is that plasmas are very difficult to “tame.” They behave completely unlike gases that you can simply store in a bottle in an inert state. They also tend to lose a lot of energy by radiation (so-called Bremsstrahlung), particularly when impurities are present.
Fusion plasmas exist in states very far from equilibrium. Not only are they hot, but they can contain a great deal of energy in the form of electric and magnetic fields – energy that they can concentrate and release in such a way as to break out of confinement and damage the reactor.
These are the dreaded “instabilities” and “disruptions” that have plagued tokamaks and similar devices since the beginning of fusion research.
After more than a half-century-long struggle between plasmas and tokamak scientists, I think it is fair to say that the behavior of plasmas in these devices is quite well understood, and methods have been devised to prevent or control their misbehavior under various circumstances.
These capabilities are nowadays enhanced by the enormous advances in computer simulation techniques and, most recently, artificial intelligence.
Against this background, realizing fusion using tokamaks is more an engineering problem than a physics problem. But the engineering challenges remain very formidable. Several fusion scientists believe that realizing a viable commercial fusion power plant on the basis of a tokamak will prove impossible; or, if it is possible, only in the distant future.
Superconductors to the rescue
Commonwealth Fusion aims to change that with a new strategy utilizing high-temperature superconductors.
Magnets lie at the heart of a tokamak. The performance of the magnet system has a decisive influence on the design of the whole reactor and on its physical regime of operation.
Among other things, theoretical analysis indicates that the achievable power density of a tokamak reactor – energy generated by fusion reactions per unit of volume and per unit of time – increases roughly as the fourth power of the magnetic field intensity.
The higher the power density, the smaller the reactor has to be to achieve a given power output. Generally speaking, the smaller we can make the reactor the less costly and the more efficient it will be.
A functioning tokamak power plant is unimaginable without superconducting magnets. But until recently all viable options required cooling the material down to around the temperature of liquid helium (around 4 degrees Kelvin, or minus 269 degrees Celsius), which creates great difficulties for the design and economical operation of the reactor.
More importantly, conventional superconductors are limited in terms of both the maximum field strengths at which they can operate and their capability to withstand the intense mechanical forces produced by those fields.
It is here that the recently-available REBCO superconductors create a new situation. Magnets wound from the new VIPER cable not only will work at much higher temperatures, but they also can operate reliably at far higher field intensities and higher mechanical stresses than ever before possible.
This permits a drastic reduction in reactor size, greatly reduces the demands on the cooling system and provides much more flexibility in reactor design. SPARC and ARC are intended to take full advantage of this new, game-changing technology.
Will Commonwealth Fusion’s strategy work out? Naturally, there can be no guarantee but I think the proper attitude was expressed by Eric Lerner in a soon-to-be-published Asia Times interview in which he stressed the need for a crash program approach to fusion.
“A crash program is a program where the priority is so high that you don’t choose between a method A, B and C, but you do all of A and B and C and you see which one gets you there sooner,” Lerner said.
Jonathan Tennenbaum received his PhD in mathematics from the University of California in 1973 at age 22. Also a physicist, linguist and pianist, he’s a former editor of FUSION magazine. He lives in Berlin and travels frequently to Asia and elsewhere, consulting on economics, science and technology. This article was originally published on Asia Times.