A project led by STFC’s Central Laser Facility (CLF) is exploring the possibility of tapping the power of the Sun by creating a ‘star in a lab’. The Sun is powered by nuclear fusion reactions in which lighter atomic nuclei are fused to form a single heavier element. With a recently proposed method of laser-driven fusion called ‘shock ignition’, scientists are hoping to recreate the Sun’s nuclear engine here on Earth. The process, which uses a form of hydrogen found in seawater called deuterium, has the potential to unlock a limitless source of cheap, carbon-free energy.
Ever since scientists first unlocked the power of the atom, there have been limited ways to exploit the power of nuclear fusion. One is to use photovoltaic panels to capture the radiation emitted by the Sun and convert it into electricity, which has the advantage of being clean and safe, but the disadvantage of only being able to convert a relatively small percentage of the captured sunlight into energy.
A second option is to detonate a quantity of highly-radioactive enriched uranium within a confined space containing ‘heavy’ hydrogen fuel and then use that energy to induce a fusion reaction between the hydrogen nuclei, which has the advantage of unleashing the energy equivalent of many kilotonnes of TNT in just a fraction of a second, but the disadvantage of being a hydrogen bomb – the most destructive weapon ever conceived by mankind.
For decades, scientists have been searching for a middle-ground – something that exploits the process that powers the Sun more directly than solar panels, but without the potential to destroy everything with a ten kilometre radius.
Nuclear fusion differs from nuclear fission (the process used in conventional nuclear reactors) in that, rather than tearing apart the atomic nuclei of heavy elements to create lighter elements and liberate energy, fusion reactions fuse together lighter elements to create heavier elements. The newly created heavier element has less mass than the combined mass of the two lighter elements, which means that some of that mass has been lost. Einstein’s famous equation, E=mc2 tells us that this lost mass is converted into energy. Most of this energy is carried by a neutron, which can be captured and used to heat water, drive a steam turbine and generate electricity.
Of course, there’s a reason why it takes either the crushing gravitational mass of an entire star or the catastrophic power of an atom bomb to trigger the process of nuclear fusion: atoms really don’t want to fuse together. Fusion is possible because the ‘strong’ nuclear force has the ability to bind atomic nuclei together to create heavier atoms, but, for the ‘strong’ force to work, you have to overcome a force that wants to keep those nuclei as far away from each other as possible: electromagnetism.
Interior of the NIF target chamber where 192 high-power laser beams are focused onto the target.
(Credit: Lawrence Livermore National Laboratory)
Anyone who has ever tried to push together the matching poles of two magnets can attest to the astonishing repulsive power of the electromagnetic force. The atomic nucleus is made up of two particles – the proton and the neutron. As their name would suggest, neutrons carry no electromagnetic charge and, as such, have no problem getting together, but protons are a very different matter. These particles carry a positive charge and, as such, really (really) don’t want to get anywhere near each other. Only in the extreme pressure and temperatures found in the heart of a star (or nuclear bomb) can they be forced to overcome their mutual repulsion.
If scientists were to exploit nuclear fusion’s potential, they would need to find a way to recreate a star on Earth, but on a much smaller and more controllable scale. Luckily, towards the end of the 1950s, some bright sparks invented the laser and it didn’t take long (only three days to be precise) for scientists to start pondering how its ability to deliver controllably, focused energy might be used to unlock the power of fusion.
Six decades on and the laser-driven fusion energy problem remains unsolved, but one promising method is Inertial Confinement Fusion (ICF), which uses multiple highly-focused, extremely powerful, laser beams to blast a pellet of hydrogen fuel. The lasers deliver enough energy to create a plasma around the pellet that expands, creating a rocket-like inward force that compresses and heats the fusion fuel to tens of millions of degrees – providing the heat and pressure needed to initiate a fusion reaction in which two hydrogen nuclei fuse to form a heavier helium nucleus and thus release energy. In essence, the technique creates (for a fraction of a second at least) a teeny tiny star.
However, for ICF to be a viable source of power the technique has to be able to liberate more energy than is put into initiating the reaction in the first place. This ‘more out than in’ problem is a big deal, after all, an energy source that uses more energy than it produces is no energy source at all. As it stands, efforts to achieve energy gain via laser fusion are yet to be successful, but a recently proposed method called 'shock ignition' is looking very promising.
Dr Robbie Scott, a theoretical plasma physicist at STFC’s Central Laser Facility, has been playing a leading role in the design of shock ignition experiments performed at the National Ignition Facility (NIF), California, home of the highest-energy laser in the world. Dr Scott has been at STFC for 14 years and originally worked in ISIS Neutron and Muon Source (ISIS) where he designed the neutron-producing core for the facility’s Target Station 2. Now working at CLF, he began to wonder whether laser fusion could be harnessed as the ultimate neutron source for ISIS, which led him to his current work with laser fusion simulations and experiments.
The focus of his work, which is part of a collaboration between NIF, CLF, University of York, University of Warwick, and the University of Rochester’s Laboratory for Laser Energetics (LLE), has been to break down the shock ignition ‘puzzle’ down to its component pieces and address each piece in a series of individual experiments.
In one experiment, Dr Scott’s team looked at how the shockwave required for shock ignition is initiated by the interaction between the lasers and the plasma, how the laser is absorbed, and the level of high-energy electrons it generates. This is important because, in order for the fusion process to start, the pellet has to be subjected to precisely the right level of pressure when it implodes and high-energy electrons can disrupt the implosion. However, as the team observed during the experiment, if done correctly, the electrons might actually help with the implosion process.
In a follow-on experiment, performed earlier this year, Dr Scott chose to investigate the shape of the shockwave generated by the laser-plasma interaction as it propagates through a fuel capsule. During the shock ignition process, it is essential that the fuel pellet is subjected to equal levels of pressure throughout – so that it implodes into a sphere and not a sausage or disc shape – not an easy task when, in the case of NIF, there are 192 laser beams that need to be precisely targeted onto a sphere only two millimetres wide.
For this experiment, instead of a fuel pellet, they used a solid plastic sphere that allowed them to use X-rays to observe the shape of the shock as it travelled through the target. At the moment the shock reached the centre of the sphere, they calculated the pressure reached an impressive 40 billion bars (a well-inflated car tyre, by comparison, is a mere 30 bars). Although the results are still being analysed, the team hope they will reveal whether their predictions were matched by the results of the experiment.
When complete, these results will be combined with the previous experiments and with computer models and simulations to access the viability of the shock ignition approach to laser-driven fusion.
‘This represents a real opportunity for UK science to demonstrate that advanced laser fusion schemes, such as shock ignition, are a better way to achieve fusion energy gain,’ says Dr Scott. 'Laser fusion has the potential to provide a carbon-free, base-load electricity generation platform and, if it could be realised in time, offers a genuine solution to global warming, which is probably the single greatest challenge facing mankind today.’
Last updated: 22 July 2019