To do this requires an enormous pulse of electric current and so Sandia researchers use the Z Machine which can store charge in huge banks of capacitors and then release it quickly. The 37m wide machine can create current pulses of 27 million amps lasting a ten-millionth of a second. In 2013 the Sandia team will start trying to achieve fusion using the Z Machine and simulations suggest they might be able to reach break-even. But to really put the idea to the test and produce genuine energy gain they reckon that they need a new machine – Z-IFE – able to produce current pulses up to 70 million amps.
As a potential power source, the Z Machine has the drawback that it takes longer to charge up than a laser and the metal can targets are bigger and more cumbersome than the fuel capsules of laser fusion. So the Sandia scheme would work at a slower repetition rate – 1 shot every 10 seconds. To make that rate economic, each explosion would have to be bigger to generate more power, so this scheme has the added challenge of developing a reaction chamber that can withstand a much bigger blast and be ready for the next one every 10 seconds. Nevertheless, the team believes that their sledgehammer approach of big pulses, big bangs and slow repetition is much simpler to implement than the high speed and pinpoint accuracy required for laser fusion energy.
The NAS panel’s next port of call was Rochester and the Laboratory of Laser Energetics. Here the main topic of discussion was other ways to achieve fusion with lasers that avoid some of the drawbacks of NIF’s indirect drive using neodymium-glass lasers. Researchers at Rochester and the Naval Research Laboratory in Washington, DC, argue that direct drive would be a better approach for laser fusion energy generation. Shining the laser directly onto the fuel capsule avoids the energy lost in the hohlraum, so a less powerful laser would be needed. It would be simpler too, since instead of having to construct a target with a fuel capsule carefully positioned inside a gold or uranium can for each shot, only the fuel capsule would be needed. Since future laser fusion power plants are expected to have to perform around 10 shots per second they will consume slightly less than a million targets a day, so simplicity – and just as importantly, low cost – will be a key factor.
Livermore researchers abandoned direct drive because you need laser beams of very high quality to make it work; any imperfections in the beam and the capsule will not implode symmetrically. But Rochester and the Naval Research Lab stuck at it and developed ways to smooth over beam imperfections. They have tested these techniques using Rochester’s Omega laser but it doesn’t have the power of NIF so they have not been able to test direct drive to ignition energies.
The Naval Research Lab team have also made another innovation. They have developed a laser that emits light that is already ultraviolet so it doesn’t need to be stepped down to shorter wavelengths like Omega and NIF, which avoids the energy loss of conversion using KDP crystals. Instead of using neodymium-doped glass as the medium to amplify light, their laser uses krypton- fluoride gas which is pumped full of energy using electron beams. The Naval researchers have built demonstration models that have a high repetition rate but low power, and ones with high power but are only capable of single shots. They haven’t yet won funding to develop a high-power, high-repetition version ready for fusion experiments.
The NAS panel heard about other approaches as well, such as imploding targets using beams of heavy ions. Accelerating ions is a much more energy-efficient process than making a laser beam and there is no problem with creating a high repetition rate. Focusing also uses robust electromagnets rather than delicate lenses which can get damaged by blasts or powerful beams. But creating beams with the right energy and sufficiently high intensity to implode a target is still a challenge. Researchers at the Lawrence Berkeley National Laboratory near San Francisco have built an accelerator to investigate those challenges but the project is desperately short of funds.
Then there is another laser-based technique called fast ignition. This separates out the two functions of the laser pulse – compressing the fuel and heating it to fusion temperature – and uses a different laser for each. In a conventional laser fusion facility these two jobs are achieved by carefully crafting the shape of the laser pulse during its 20-nanosecond length: the first part of the pulse steadily applies pressure to implode the capsule and compress the fuel, then an intense burst at the end sends a shockwave through the fuel which converges on the central hotspot, heating it to the tens of millions of °C required to ignite. Getting that pulse shape right is complicated and requires a very high-energy laser. By separating the two functions a fast ignition reactor can make do with a much lower-energy driver laser because it only has to do the compression part. Once the implosion has halted and the fuel has reached maximum density, a single beam with a very short but very high intensity pulse is fired at the fuel and this heats some of it to a temperature high enough to spark ignition.
Researchers at Osaka University in Japan have pioneered the fast ignition approach and have been joined recently by Rochester’s Omega laser which was upgraded with a second laser for fast ignition experiments. Although these efforts are making progress towards understanding how fast ignition works, neither is powerful enough to achieve full ignition. Researchers in Europe are keen to join this hunt and have drawn up detailed plans for a large fast ignition facility that would demonstrate its potential for power generation. Known as the High Power Laser Energy Research facility (HiPER), it would have a 200-kilojoule driver laser – one-tenth the energy of NIF’s – and a 70-kilojoule heater laser. Calculations suggest HiPER should be able to achieve a much higher energy gain than NIF but its designers are waiting to finalise the design. They want to wait for the achievement of ignition at NIF in case it provides any useful lessons.
The problem for all these possible alternatives – with the possible exception of Rochester’s Omega laser – is that they have been starved of funding. For the past two decades while the Department of Energy has been pouring money into NIF other approaches have been neglected. The fear for the proponents of these alternative approaches was that history was about to repeat itself: Livermore’s design for the LIFE reactor is very thorough and very persuasive; would it persuade the NAS panel that the bulk of any future ICF funding should be channelled straight to Livermore?
But the panel was not seduced. Its mandate had been to come up with a future research programme on the assumption that ignition at NIF had been achieved. Ignition’s stubborn refusal to cooperate at NIF has knocked some of the lustre off the plans for LIFE – it no longer seemed an obvious shoo-in for the next big inertial fusion project. The panel’s report, released in February 2013, says that many of the technologies involved in inertial fusion are at an early stage of technological maturity and that it is too early to pick which horse to back. It suggested a broad programme of research that would provide the information needed to narrow the field in the future. This was good news for the alternative approaches, but with a faltering US economy forcing cutbacks to many areas of research funding and with Livermore researchers seemingly losing their way on the road to ignition, prospects for a generous research programme do not look good.
With the construction of ITER going at full throttle and with NIF working towards ignition, is fusion closing in on the big breakthrough that researchers have been dreaming of for more than six decades? Many thousands of those researchers would like to believe so but there have been false dawns before: Ronald Richter’s Argentine fusion reactor that never was; ZETA and all the enthusiasm generated by the 1958 Geneva conference; the astonishing temperatures achieved by the first Russian tokamaks in 1968; and the heat generated by TFTR and JET in the 1990s which got close to break-even but didn’t quite get there. Each time, the press has excitedly published accounts of the promise of fusion for solving the world’s energy problems but then unexpected technical problems, lack of funding or simply the slow pace of research has meant that fusion has faded again from the public consciousness. Many have grown cynical that fusion will ever deliver on its promises.
Remember the jibe: fusion is the energy of the future, and always will be.
But there are legitimate concerns that fusion will ever provide an economic source of energy – even if high gain is achieved – and those concerns are usually expressed by engineers. They argue that fusion scientists’ fixation on developing a reactor that will simply produce more energy than it consumes ignores the very serious hurdles such a reactor would still have to overcome before it could compete with existing sources of energy.
In 1994 the Electric Power Research Institute (EPRI) – the R&D wing of the US electric utility industry – asked a panel of industry R&D managers and senior executives to draw up a set of criteria that a fusion reactor would have to meet in order to be acceptable to the electricity industry. They came up with three. The first is economics: to compensate for the increased risk of adopting a new technology, a new fusion plant would need to have lower life-cycle costs than competing technologies at the time. The second criterion was public acceptance: it would need to be something the public wanted and had confidence in. Finally the industry panel would want fusion to have a simple regulatory approval process: if the nuclear regulator required a lengthy investigation of the design or required that the reactor be sited far from population centres or be encased in a containment building, fusion’s prospects could be seriously damaged.
One of the first to question the viability of fusion power generation was Lawrence Lidsky, a professor of nuclear engineering at the Massachusetts Institute of Technology and an associate director of its Plasma Fusion Center. By 1983 Lidsky had been working in plasma physics and reactor technology for twenty years and had formed some serious concerns about fusion’s future. Colleagues at the Plasma Fusion Center were reluctant to talk about it, so Lidsky wrote an article for MIT’s magazine Technology Review entitled ‘The Trouble with Fusion.’ Lidsky argued that, because of the inescapable physics of deuterium-tritium fusion, any fusion power plant is going to be bigger, more complex and more expensive than a comparable nuclear fission reactor – and so would fail EPRI’s economic and regulatory criteria – and that complexity would make it prone to small breakdowns – failing the public acceptance criterion.
Lidsky’s first criticism was with the choice of the fuel itself. When the fusion pioneers of the 1940s and ’50s realised how difficult it was going to be to get to temperatures high enough to cause fusion, they naturally sought the fuel that would react the most easily – a mixture of deuterium and tritium. Reacting any other combination of light nuclei, such as deuterium and deuterium or hydrogen and helium-3, was just not conceivable with the technology of the day. So D-T became the focus of fusion research and scientists chose to ignore the fact that the reaction produces copious quantities of high-energy neutrons that would be a major headache for any working power reactor. Fission reactors produce neutrons too but the ones in a fusion reactor have a higher energy and so penetrate the structure of the reactor itself where they can knock atoms in the steel out of position. Over years of operation this neutron bombardment makes the reactor radioactive and weakens it structurally, which limits its life and means that any maintenance or repair is difficult or impossible to carry out with human beings.
Key parts of the reactor could instead be made with other metals that are more resistant to neutrons, such as vanadium, but that would increase the cost. Fusion scientists have long been aware of this issue and have sought other more exotic neutron-resistant materials but such efforts have always played second fiddle to the drive towards break-even. In any event, testing the materials would require a very intense source of neutrons and today no such source exists. US researchers have proposed building fusion reactors that are optimised to produce lots of neutrons rather than energy and one of these could be used as a testbed for new materials. But with the country’s fusion budget severely squeezed it was never a high priority.
Another option is to produce neutrons using a high-intensity particle beam in a purpose-built accelerator facility. The agreement between the European Union and Japan over where to build ITER, the so-called broader approach, provided money to start work on such a test rig, dubbed IFMIF (International Fusion Materials Irradiation Facility), but at the time of writing this project was still working on a design and testing technology – far from actually bombarding any materials with neutrons. So the effort to find suitable materials for a fusion reactor lags behind the rest of the fusion enterprise and is unlikely to provide any useful data for ITER, though it could for ITER’s successors.
Lidsky also contended that a D-T fusion reactor would be unavoidably large and complex and therefore unacceptable to the electricity industry. For a start, the reactor must cope with a range of temperatures that drops over the distance of a few metres from roughly 150,000,000°C – hotter than anywhere else in our solar system – to -269°C, a few degrees from absolute zero temperature, which is the operating temperature of the superconducting magnets. Managing these temperature gradients and heat flows will be a major challenge. And, he argued, a power-producing D-T reactor cannot avoid being large and therefore expensive. History backs up his assertion: as tokamaks have developed they have got bigger and bigger. The plasma vessel of ITER is 19m across and this is just the start: outside it are several metres more including the first wall – the initial line of defence against heat and neutrons – the ‘blanket’ through which liquid lithium will pass so that it can be bred into tritium by the neutrons, a thermal shield to protect the super-cold magnets from the heat of the reactor and finally the magnets themselves. Altogether a huge structure, considerably larger than the core of a fission reactor and in power engineering, size = cost. Anyway, ITER is not designed to generate electricity; the demonstration power reactor that is proposed to come after it, known as DEMO, will by some estimates be 15% larger in linear dimensions.
If fusion research continued on its current course, Lidsky concluded in 1983, ‘the costly fusion reactor is in danger of joining the ranks of other technical “triumphs” such as the zeppelin, the supersonic transport and the fission breeder reactor that turned out to be unwanted and unused.’ His Technology Review article was followed by an adapted version in the Washington Post entitled ‘Our Energy Ace in the Hole Is a Joker: Fusion Won’t Fly.’ These public criticisms caused a furore in fusion research and led to a war of letters between Lidsky and PPPL director Harold Furth lasting many months. Shortly after the articles were published, Lidsky was stripped of his associate director title at the Plasma Fusion Center and he became a pariah in the fusion community.
But Lidsky’s message was not entirely negative. He acknowledged the attractions of limitless fuel and minimal radioactive waste from fusion, but in essence he thought that fusion had taken a wrong turn and needed to start again by focusing on a different reaction that produces no neutrons: the fusion of hydrogen and boron-11. This seems ideal, but boron has five times the positive charge of hydrogen, making fusion much harder to achieve. Although some schemes for fusing hydrogen and boron have been proposed – including using Sandia’s Z Machine – none have yet been tested.
Although these concerns were expressed three decades ago, many of them hold true today. Better materials and techniques exist now, but the basic physics remains the same. Fusion enthusiasts concede that there are some major challenges ahead but it is not that they can’t be solved; they just haven’t been solved yet. Just because something is hard, it doesn’t mean we shouldn’t attempt it. But even many of the most ardent fusion enthusiasts concede that commercial fusion power is unlikely before 2050. That view is supported by another more recent report from EPRI which sought to find out if any fusion technologies existing now might be useful to the power industry in the near term. Published in 2012, it examined magnetic as well as inertial confinement fusion approaches and some of the alternative schemes but concluded that all were in an early stage of technical readiness and none would be ready for use in the next thirty years. It suggested that fusion research efforts ought to pay more atte
ntion to the problem of generating electricity instead of fixating on the scientific feasibility of producing excess energy.
So is fusion the energy dream that is destined never to be fulfilled? Fusion is often compared to fission, both having been born out of the postwar enthusiasm for all things technological. Fission proved its worth astonishingly fast: the splitting of heavy elements was discovered in 1938; the first atomic pile produced energy in 1942; and the first experimental electricity-producing plant started in 1951. Thirteen years from discovery to electricity. But the comparison with fission is misleading: it hardly takes any energy at all to cause a uranium-235 nucleus to split apart and release some of its store of energy; and as a fuel, solid uranium is easy to handle. Fusion, in contrast, requires temperatures ten times those in the core of the Sun and its fuel is unruly plasma. When fusion research began, scientists had little real knowledge of how plasmas behave and, although great strides have been taken, there is still a lot we don’t know about plasma physics.
Piece of the Sun : The Quest for Fusion Energy (9781468310412) Page 28