In the world of fusion energy, bigger is simply better. Decades of experience with tokamaks shows that if you want more power from your plasma (hot, charged fuel) than you put in, you need to have a bigger machine. Which is why the international fusion project ITER in France, the most advanced ever designed, is twice the size of its European predecessor JET in the UK.
Where JET was never quite able to break-even and produce more fusion power than the heat needed to keep the reaction going in its plasma, ITER’s 500 megawatts of fusion power will be ten times more than the heat sent into the artificial star’s plasma. A world record power multiplication, although as an experimental facility ITER will not convert the heat it produces into electricity. Factoring in all power requirements on site, ITER would about break even if it did so.
Commonwealth Fusion Systems’ SPARC, in contrast, will be four times smaller than ITER’s 12 meter diameter; at 3 meters across, it will be just half the size of JET while still aiming to beat it by generating twice the fusion power than the heat going into the plasma. You might assume that the fusion community thinks that SPARC – like ITER, not intended to produce electricity – is a pipe dream, but researchers are actually cautiously optimistic. The key to the magic performance claim: a more concentrated plasma thanks to stronger, more advanced superconducting magnets than were available for either competitor.
Before we dig into the SPARC design, how do tokamaks work in general?
“A tokamak is a donut-shaped magnetic cage in which one can confine a very hot hydrogen plasma (a gas of 100 million degrees Celsius or more) so that it does not touch the wall of the container. At these temperatures, hydrogen nuclei can fuse to form helium and energy is released, just like in the heart of the sun.”
“The magnetic cage is provided by strong electromagnets, which need to be superconducting so that the current flowing in them does not experience any electric resistance. Even the small resistance of copper magnets would consume too much energy for fusion reactors to be efficient.”
Are tokamaks the best design we have to realize fusion power?
“The tokamak is the best developed concept with the highest proven performance. Over the years, only one other concept has survived, the stellarator. That design also has a donut shape, but with carefully-shaped magnetic fields for a more stable plasma. In the EU, we are presently perfecting the tokamak as the main line with work on stellarators like Wendelstein 7-X as a backup option.”
How could SPARC’s high-temperature superconducting magnets make it more efficient?
“The new superconductors could allow the team to double the magnetic field strength of the electromagnets, which would help making the device more compact and in the long term also less expensive. However success, as with any research project, is not guaranteed. These magnets still have to be developed, and there are a number of open questions which the team will address. For example, at double the magnetic field the forces on the magnets will increase by a factor of four, and a credible concept to cope with the mechanical loads still has to be shown.”
Do you think the SPARC design will be able to go beyond break-even?
“SPARC aims for Q = 2, which means two-fold power multiplication of the heat sent into the plasma. If they manage to construct it to specification, with a magnetic field in the plasma centre twice the value foreseen for the ITER experiment, then yes. The physics base they use is sound and the assumptions they make about their performance are pretty conservative.”
“I am less confident about the ARC design that the MIT / CFS team have presented for a step after SPARC. That machine would need a big step forward in fusion plasma physics and fusion technology because the design has made quite aggressive assumptions in some of the parameters.”
If SPARC is successful, what would then be needed to develop it further?
If you want to follow the trajectory proposed by the US colleagues from SPARC to ARC, you will have to strongly improve plasma confinement under the conditions of high magnetic field with respect to our present asssumptions. Commonwealth Fusion Systems assumes an 80% increase relative to the assumptions that ITER is based on. On the technology side, they propose a completely new design for the main magnets, which would allow to remove parts in order to serve the device. That implies joints in the coils that have not been demonstrated to work on large scale up to now and are viewed as a really critical element of the ARC technology. Both items, by the way, are not part of the SPARC design, so they imply that the development from SPARC to ARC is huge, something we try to avoid in the EU by building on ITER for our DEMO design.
What would SPARC imply for the future of fusion energy?
“The Q = 2 value does not imply a big breakthrough in fusion plasma physics. For that, you need to go to Q = 10 or higher, when the plasma is dominantly heating itself by the energy generated through the fusion processes. Hence the Q = 10 value for ITER, which aims at this breakthrough in fusion plasma physics.”
“Technologically speaking, operating SPARC at its nominal design parameters will be a big step towards the demonstration of the use of high temperature superconductors. That would pave the way to their use in future fusion power plants beyond ITER, which would make those devices more compact and possibly cheaper as well.”
Professor Hartmut Zohm (1963, Freiburg, Germany) has been a Scientific Fellow of the Max-Planck-Institute of Plasma Physics (IPP Garching, Germany) and Head of IPP’s Tokamak Scenario Development Division since 1999. He is especially interested in the physics of the plasma core and how it extrapolates from present day experimental devices via ITER to a DEMOnstration reactor.
After studying physics at Karlsruhe, Hartmut Zohm joined IPP for his PhD research on the topic of magnetic modes in the ASDEX tokamak. After a stay with General Atomics in San Diego, Hartmut Zohm taught as Professor of Electrical Engineering and Plasma Research at the University of Stuttgart and returned to IPP in 1999.
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