Nuclear fusion suffers from something of an image problem. While it is often cited as the ultimate form of safe, clean and sustainable energy, the progression towards commercialisation has been tortuously slow. Subsequently, many sceptics deride it as a ruinously expensive pipe-dream that will always be 30 years away.
The Iter nuclear fusion project is a case in point. Begun in 1985, Iter aims to make the long-awaited transition from experimental studies of plasma physics to full-scale, electricity-producing power plant. Now, 30 years on, and with hundreds of millions of dollars already spent, construction of the facility in Cadarache in southern France has only just commenced. A conservative estimate of when it will become operational? Early 2030s – possibly. And even then, it will be for demonstration purposes only.
But what if there was another way? What if a ‘miracle’ technology came along to make nuclear fusion possible on much shorter timescales, and at far lower cost? And what if it was a British firm, employing a small but select group of nuclear scientists and engineers, that was leading the charge?
That’s the exciting scenario being played out at Tokamak Energy, based at the Culham Innovation Centre in Oxford, which promises a ‘faster way to fusion’ by combining two emerging technologies – spherical tokamaks and high-temperature superconductors (HTS).
Spherical tokamaks have a central column which is reduced to the minimum possible size, giving the plasma the appearance of a cored apple. Efficiency and stability are a hallmark of the spherical tokamak design, but the lack of space in the narrow central column has so far prevented it from being used as a fusion device – until the development of HTS.
Room-sized plant
These superconductors are available in thin 0.1mm tapes, where the superconducting layer is only 1µm thick. Although it becomes superconducting at around 90K, operation at 20K improves performance by an order of magnitude – so that a magnet made of HTS can carry much larger currents than a conventional 4K ‘low temperature’ superconductor. Theoretical calculations show that a spherical tokamak using high fields produced by HTS magnets could be significantly smaller than other fusion machines proposed.
For example, a compact spherical tokamak’s power plant would have a volume up to 100 times smaller than Iter, so would be approximately room-sized rather than aircraft-hangar-sized. This development creates a substantial commercial opportunity, should the company be able to demonstrate net energy gain, promising the development of smaller, cheaper nuclear fusion plants located anywhere in the world.
So far, Tokamak Energy has built and demonstrated a small tokamak which produces plasma pulses of up to 20 seconds. And it has built a second smaller device, the world’s first tokamak with exclusively HTS magnets, which has demonstrated 29 hours’ continuous plasma. Now the firm plans to refine its technology, to achieve energy gain in controlled fusion within five years, produce its first electricity within 10 years, and construct a 100MWe plant within 15 years. This is a vastly accelerated timeline compared with that of Iter.
“HTS spherical tokamaks allow much higher plasma pressure for a given magnetic field,” says David Kingham, chief executive of Tokamak Energy. “The HTS lets you pass high currents in a small magnet under huge magnetic fields. It’s a far more robust method of producing a magnet at a compact scale. That’s the breakthrough
that HTS offers.
Rapid development
“So, potentially, compact fusion devices could be developed much more quickly than would be expected with mainstream fusion, which involves huge tokamaks or large-scale laser inertial fusion devices. And once developed, the devices could be built and deployed more rapidly.
“Moreover, they are better suited to the evolving distributed power generation model, and will be designed to use abundant fuel – deuterium plus tritium bred from lithium inside the device. The installed base could therefore increase dramatically, from an initial 100MWe to more than 3GWe in five years.”
The history of the tokamak – a magnetic confinement system for fusion reactors – goes back a long way. Invented in the Soviet Union in the 1960s, it was soon adopted by researchers around the world. The operational principles are therefore well understood.
Plasma, contained in a vacuum vessel maintained by external pumps, is created by letting in a small puff of gas, which is then heated by driving a current through it. The hot plasma is contained by a magnetic field, which keeps it away from the machine walls. The combination of two sets of magnetic coils – known as toroidal and poloidal field coils – creates a vertical and horizontal directional field, acting as a magnetic cage to hold and shape the plasma.
Large power supplies are used to generate the magnetic fields and plasma currents. Plasma current can be induced by a transformer effect, with a central solenoid acting as the primary winding and the plasma as the secondary winding. The heating provided by the plasma current – known as ohmic heating – supplies up to a third of the 100 million°C temperature required to make fusion occur. Additional plasma heating is provided by neutral beam injection. In this process, neutral hydrogen atoms are injected at high speed into the plasma, ionised and trapped by the magnetic field. As they are slowed down, they transfer their energy to the plasma and heat it.
Radio-frequency heating is also used to heat the plasma. High-frequency oscillating currents are induced in the plasma by external coils or waveguides. The frequencies are chosen to match regions where the energy absorption is very high (resonances). In this way, large amounts of power may be transferred to the plasma.
The Joint European Torus, located at Culham Centre for Fusion Energy, is the largest and most powerful tokamak operating. Back in 1997, under a full experimental campaign, JET achieved a world record peak fusion power of 16MW, but needed 24MW of power to heat the plasma. The aim, of course, is to develop a tokamak that can produce net power gain.
The conventional view is that tokamaks have to be huge to produce power. However, Tokamak Energy’s work with HTS shows that compact tokamaks can achieve high energy gain with power of less than 100MWe. This level of performance changes the proposition of nuclear fusion, says Alan Sykes, Tokamak Energy’s technical director, and one of the world’s most respected designers of spherical tokamaks.
“JET has operated for over 30 years – and it has produced superb results,” he says. “Other designs in Japan and the US have also worked well. So then the industry thought that all it needed to do was build bigger and bigger devices and, lo and behold, we would get more power out than we put in.”
But the trouble with tokamaks such as the one proposed at Iter is that they have got so big that they have become worldwide mega-projects. On that basis, various companies are building systems and components all over the world, and fitting the system together is proving arduous. “And so you get delays,” says Sykes. “Even though Iter started in 1985, it looks like 2030 before fusion is achieved. That in itself won’t produce electricity, so people are despairing that this is a mammoth project that is getting impractical. We want to use the breakthrough provided by HTS to design smaller, simpler tokamaks that are faster to deliver.”
Sykes’ principal message is that, through the use of HTS, spherical tokamaks can be designed with much higher-pressure plasma for a given magnetic field. “You don’t have to have enormous machines – efficient machines are better. What you want is an output somewhere in the region of 50-100MW, provided you get more power out than you put in. We believe this can be done with small devices. We are at the stage where it’s not the physics that is limiting us, it’s the engineering. That’s the challenge.”
That engineering effort is ongoing, as was confirmed by a recent scientific paper addressing one of the main challenges – heat deposition into the superconducting central column of a spherical tokamak plant.
The central column effectively does the legwork on generating the toroidal magnetic field. It needs to be superconducting because copper couldn’t pass the current required up the central column without the whole device turning into a molten mess. HTS can handle the high-current density, and produce a high magnetic field. Therefore, a key challenge in designing a fusion power plant is to manage the heat deposition into the central core containing superconducting toroidal field coils. Spherical tokamaks have limited space for shielding the central core from fast neutrons produced by fusion and the resulting gamma rays.
The recent technical paper, published by Tokamak Energy and nuclear fusion colleagues at Culham Electromagnetics and the York Plasma Institute, outlines recent advances in developing a series of three-dimensional computations to more accurately calculate the heat deposition into the superconducting core.
“HTS machines have a lot of challenges to do with electrical protection,” says Dr David Hawksworth, chairman of Tokamak Energy’s magnet advisory panel. “Super-conductors have no resistance. If there was some form of thermal disturbance – known as a quench – you could get a huge current flowing up the central column, but it’s now flowing through a resistor and generating a lot of heat. It would have the potential to do an awful lot of damage.
“We now have calculations that show it should be possible to have protection systems in place to deal with such matters, but we have to demonstrate that in practice. We have a lot of ideas, but they need to be developed.”
Another issue is the joining of the superconducting tapes. Such joints would have an element of resistance: the lower Tokamak Energy’s engineers can get that resistance, the better.
“We are talking about passing currents of many tens of kilo-amps,” says Hawksworth. “That could put a lot of heat load into the system, which means potentially those joints could get hotter and hotter, and no longer superconduct. So we need to do more development work around jointing.”
Technical challenges aside, the other obstacle to Tokamak Energy achieving its ambitions is cold, hard cash. The company is backed mainly by private individuals, and, although the IMechE recently announced an investment through its new Stephenson LP fund, Tokamak Energy is seeking additional monies to accelerate the plans.
In the short term, it wants to expand its engineering centre in Oxfordshire and employ a larger team. It also wants to strengthen its international collaborations with organisations such as the Massachusetts Institute of Technology and form a joint venture with a leading supplier of HTS, to improve the supply and reduce the cost of this crucial material. “Additional investment would allow us to tackle more problems in parallel,” says Kingham.
Funding is linked to public opinion on nuclear fusion, which also needs to be addressed, he says.
“One blockage that has still to be removed is that of scepticism about new ideas, and resistance to innovation of the type we are proposing. We are just now at the point of having sufficient published evidence to rebut the scepticism, but it can take a long time to overcome institutional resistance to disruptive innovation.”
Kingham insists that, with adequate funding, Tokamak Energy could produce a device that could provide energy gain within five years, and supply its first electricity within 10 years.
The business model is to build fusion devices that one day could be safely located on the edge of towns and cities – for example, to replace existing coal-fired power plants; to be supplied for remote off-grid environments, such as mining complexes; or to provide emergency power during disaster relief efforts.
Another big selling point is that nuclear fusion brings no risk of meltdown or weapons proliferation, and no emissions of carbon dioxide or any other pollutants, says Kingham.
“Two years ago, people would have said this was ‘pie in the sky’ or that we were being ridiculously optimistic,” he adds. “But advances in the physics and the engineering mean there are no technical show-stoppers. Yes, there’s still a huge amount of work to do. But it’s now a matter of how we best go about tackling the challenges that lie ahead.”
‘Fusion will gain back its excitement’: parallel research at MIT
Across the pond in the US, 3,000 miles away from Tokamak Energy’s site in Oxford, is an academic who also believes that high-temperature superconductors are the game-changer for nuclear fusion.
Professor Dennis Whyte, director of the Plasma Science and Fusion Centre at the Massachusetts Institute of Technology, has for the past six years been working on the design of a compact, high-field, fusion demonstration tokamak plant. He says: “And then, a couple of years ago, I was at a conference and watched a presentation from Alan Sykes at Tokamak Energy and I thought ‘We are working on similar things’. They also believed that the application of the HTSs was a game-changer. It ripples through the entire design of a tokamak. It’s not just about plasma physics; it’s about everything in terms of how you would put together, maintain and operate such a device.”
The strong similarities between the work at MIT and Tokamak Energy have led to some collaboration between the two organisations, although they are working on differing design proposals. “We are working in parallel with each other – them as a private company and us as a large research institution. I admire the fact that Tokamak Energy has put its money where its mouth is. They have built a tokamak with HTS.”
Specifically, MIT has been developing what it calls an affordable, robust, compact (ARC) reactor that would significantly reduce the size, cost and complexity of a nuclear fusion demonstration plant. ARC is a 200-250MWe tokamak reactor, with a major radius of 3.3m and an on-axis magnetic field of 9.2T. MIT’s design has rare earth barium copper oxide superconducting toroidal field coils, which have joints to enable disassembly. This feature allows the vacuum vessel to be replaced quickly, so a single device can test many vessel designs and materials.
Whyte believes HTS will have an enormous impact on the speed at which nuclear fusion can be developed. “HTS is the breakthrough technology,” he says. “We all have different approaches – that’s fantastic. This is where fusion will gain back its excitement, because people will see progress again and not this very long timeline.”