Last October the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF), which houses the largest laser in the world, achieved an important step towards the commercialisation of fusion energy – for the first time a fuel capsule gave off more energy than was absorbed by the fuel. This isn’t quite the goal of ‘ignition’, whereby the fusion reaction generates as much energy as is put in to the whole system, but it’s getting close. By the end of 2014, a very similar facility, the Laser Mégajoule (LMJ), will start experiments at the Commissariat à l’Énergie Atomique’s Cesta Centre, near Bordeaux, France, although its prototype Ligne d’intégration laser (LIL) has been open to the scientific community since 2005. Gareth Jones, CEO of Gooch and Housego, and Professor Roland Smith, head of Plasma Physics at Imperial College London, both agree that once the proof-of-concept has been achieved by one or other of these projects, Laser Inertial Confinement Fusion (ICF) which is the technique used by both facilities, will be all over the front pages and become of critical importance to both governments and to industry.
A large part of the driving force behind these research facilities has been the Comprehensive Nuclear Test-Ban treaty which was adopted in 1996 by the United Nations General Assembly. It means that no nuclear weapons tests can be carried out by any state that has signed the treaty, so nuclear nations have had to find alternative methods of testing fusion reactions. This has led to facilities being commissioned that are supported by governments which aim to better understand fusion reactions and to be able to create them in safe, manageable environments.
Nuclear weapon development is not held in too high regard by large parts of the public, meaning that spending taxpayer’s money on the large facilities required to increase a weapon’s effectiveness is not always popular.
NIF is quoted to have cost $3.54 billion and took 15 years to build and was started just a year after the test ban, but the advances that could be made here could be used for more than weapons of mass destruction. The most commercially valuable application is in the energy sector where the market constantly asks for cleaner, less expensive and safer methods of producing electricity.
Maximising the energy out
The basic ICF method employed at facilities such as NIF is to take a large amount of energy and heat a pellet, also known as a target, of deuterium and tritium to similar temperatures to that of the core of the Sun in order to get energy out. The aim is to eventually generate more energy from the process than the energy put in.
The fuel is readily available and abundant: sea water provides the deuterium, and the tritium can be produced from the by-products of the fusion reaction. NIF bases its laser inertial confinement fusion on a process called indirect drive, whereby heat is generated from a fuel pellet imploding. The 192 laser beams at the NIF don’t heat the pellet directly, but focus their energy onto a hohlraum – essentially a housing made of gold or uranium holding the pellet. This generates soft X-rays which break down the coating of the fuel pellet in such a way as to cause the pellet to implode and ignite the deuterium-tritium fuel core to release energy.
The smooth and even expansion of the casing is crucial to ensure that the pellet compresses uniformly, as Dr Smith explained: ‘Imagine squeezing a ball of jelly in your hands – it doesn’t squeeze elegantly and goes very unstable, things get messy.’ This uneven ‘squeezing’ means the whole process becomes very inefficient at heating the fuel and almost definitely results in a failed attempt at getting the elements to fuse.
Indirect drive is less than one per cent efficient and so very energetic lasers are required. Here, a single relatively low laser pulse is split and directed to separate laser bays where they are pre-amplified and then split again until 192 separate parallel beams are created. These beams are sent to the main amplifier units. Direct drive, whereby the pellet is heated directly by a laser, is potentially more efficient, but may also be harder to engineer.
Other planned facilities such as Dipole (see box overleaf) were designed initially to use a process called fast ignition that requires much lower powered lasers. The casing is heated the same but with lower energy lasers before a very high-powered petawatt laser delivers a single burst of energy to one side. This, however, is yet to be put into practice.
The laser system at NIF is actually based on quite an old technology; the amplification is achieved by passing the light through neodymium doped glass that has been excited by an array of flash lamps. The glass lases at a wavelength of around 1µm.
Only a certain amount of added energy can be gained each time the laser travels through the glass. The amount of energy per unit area that can be gained from a solid-state material, like glass, is around 10 joules per square centimetre. ‘You can get a few joules of energy out of a rod but its gain is only about a factor of 10,’ Smith explained. Increasing the efficiency is difficult: ‘The saturation fluence (energy per unit area) is a fundamental atomic property of the material decided by lasing and transition rates. This is easy to make worse but hard to make better,’ he said. ‘This immediately outlines a problem with laser fusion: if you can get 10 joules per square centimetre for a laser and you want a megajoule or more, you are going to need a lot of area,’ explained Smith.
In order to improve the efficiency of the process the laser is passed through the glass multiple times in a process called regenerative amplification. A relatively small rod is pumped with light from the flash lamps and by passing the laser through the rod multiple times, meaning more energy can be extracted. Once the beam has increased in energy it is passed to a slightly bigger rod where the process is repeated, and repeated again in larger and larger rods until the beam is pumped into giant slabs of glass.
However, the glass gain medium collects a great deal of energy during this process and as the size of the glass and the amount of energy passing through it increases, problems are caused for the engineers. They need to avoid thermal lens focusing which occurs when the glass gets too hot causing the refractive index to change. This will focus the laser prematurely and cause large amounts of very expensive damage to the system. To combat this they use glass slabs because it allows them to insert coolants between the surfaces and reduce the temperature to a manageable level. The system also requires a few hours to cool down in between shots.
Once the laser has reached an acceptable level it is frequency doubled, or sometimes tripled, to reach the correct wavelength for laser fusion. The frequency manipulators used at NIF are made using potassium dihydrogen phosphate (KDP) crystals provided by Gooch and Housego and are the largest of their type to be grown. The laser is then diverted and aimed through the holes at the top and bottom of the hohlraum where, hopefully, fusion occurs and ignites a sustained set of reactions that produce more energy than was initially put in. However, these are proof of concept experiments with most of the interest being dependent on the nuclear weapon and nuclear safety aspect of successful fusion. The question has to be asked: ‘How do we get energy out of this?’
In the UK, the people that are asking this are the Inertial Fusion Energy Network (IFEN), headed by Smith. They are a group of experts that are trying to devise the best way to harness the power from these reactions and to turn it into a commercial product while also figuring out how to ensure that the process is safe. They are also trying to find companies that could manufacture the parts required to make the fusion plants.
A pane in the glass
The equipment used for these proof-of-concept projects is often specialised and unique for the application. Smith gave an example of the glass that was manufactured for NIF: two factories were built to make it – and the glass, if stacked, would be about a foot wide and 1.5 miles long.
These large projects, such as NIF, often involve government funded research programmes, which turn to industry to build the components. According to Jones, the relationship between research and industry is especially strong in the UK: ‘Today, the UK has both in the research community and in industry, with companies like Gooch and Housego, a very strong position in the world; the UK is world leading in several of these areas.’ He continued: ‘There is a challenge in the UK for the government to decide if laser fusion is going to be a large contributor to energy supply in the second half of the century, and if so the UK should clearly be involved it that.’
In order to further laser fusion, government interest needs to be kept high in order to capitalise on the eventual success of either NIF or LMJ. Jones stated: ‘Maintaining an adequate level of industrial and academic capability is going to be critical because if the UK does that and takes it forward it would be a major contributor to wealth creation in the UK.’
But with current austerity measures and reduced government funding, the UK could lose impetus in laser fusion research. This could mean that the UK becomes reliant on other nations instead of providing components and expertise to them.
Jones has concerns over the future of ICF and the UK. ‘If the UK lets its lead go and doesn’t continue to invest in research and industry in the intervening years it could be that when, finally, laser fusion does make it into the energy supply and becomes a multi-billion dollar industry, the risk is that the UK won’t benefit from the leadership it has today.’
Over the anticipated timeframe for ICF to be ready for commercial use, maintaining this level of interest and technological knowhow could prove difficult and, yet, it is essential. Jones explained: ‘It’s on a [financial] scale, and on a time scale, that is not consistent with investment from small companies such as Gooch and Housego, you’re talking about investing for returns that are waiting for decades to come. Similarly, the plants that you would need to produce the parts for these very big laser systems are large and expensive. You can’t take this sort of proposition to your shareholders and say “we would like you to invest in this; it will be great in 10 or 20 years time”. They would laugh at you.’
This is where the UK government comes in, it needs to recognise and stick with ICF as a technology of the future for the sake of, what Jones refers to as ‘UK PLC’. He reaffirmed: ‘If it is not regarded as a priority, the UK could lose its lead which would mean that if ICF takes off as a leading technology, the UK would be a customer and not a supplier.’
Jones uses the model that was instigated in the US to show how the government could keep industry interested: ‘The use of industry that took place in the US shows that it can be involved and benefit from doing the “big science”. So they have not just created a great scientific research facility, they have also created an industrial capability along the way.’ This is something that Jones thinks the UK should be trying to do as well: ‘There is no use in generating this research base without the industrial capabilities that go with it.’
He concluded: ‘It’s going to be critical that if the next generation of systems are developed there needs to be a way to ensure that the UK industry will benefit,’ adding that industry is not going to jump in and take a 20-year risk all by itself – which is why the government needs to help. However, both Jones and Smith feel that this is unlikely to happen until facilities like NIF have proven that ICF can successfully break even in terms of energy.
Tom Eddershaw is a technical writer for Electro Optics, Imaging & Machine Vision Europe and Laser Systems Europe.
You can contact him on tom.eddershaw@europascience.com or on +44 (0) 1223 275 478.
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