A new experiment in nuclear fusion, envisaged by many as an affordable, safe and low-carbon energy source for the future, has achieved a new record in energy yield, making an unexpectedly large improvement on previous attempts and reaching an output energy of almost 70% of the input energy.
"This is a really big step forward for our field," says Debbie Callahan, lead researcher on the project at the Lawrence Livermore National Laboratory (LLNL) in California.
In the experiment, laser beams are concentrated in a hot spot, which can be thought of as the tip of a match catching fire. What the scientists were trying to achieve is an outward spread of the hot spot burn, similar to when the flame of a match burns down the stick. "For the first time, we managed to obtain a propagating burn, which resulted in a much larger energy output," says Callahan.
When two atoms merge, Einstein's famous energy-mass equation E = mc^2 tells us that energy is released. This is because the mass of the fused atom is lower than the sum of the masses of the original atoms: the resulting difference is emitted as energy. The problem is that merging atoms together is very difficult. When they come close, their positively charged nuclei repel each other. The only place in the whole Universe where there is enough pressure for fusion to occur is at the centre of stars like our Sun.
Or at least, that was the case until about 70 years ago, when scientists figured out how to make hydrogen atoms fuse, here on Earth. At the LLNL, scientists use a technique called ‘inertial confinement fusion’. There are few scientific experiments in the world more awe-inspiring than this. The hydrogen atoms, which the researchers call the fuel, are frozen inside a spherical capsule as big as a grain of sand. The capsule is made of a material similar to diamond, which is the hardest material on the planet. Around this capsule is a hollow cylinder made entirely of gold. This is about 1 cm long and sits at the centre of a 10-meter wide aluminium sphere that is covered in sensors to detect the energy coming out during fusion.
So far so good, but how do you get the atoms to fuse? Well, you need to press them together very vigorously, usually at extremely high temperatures. To do that, scientists use the biggest laser in the world, called the National Ignition Facility. It's as large as three American football fields, and shoots 192 laser beams straight at the diamond capsule: 96 of them enter the top of the cylinder, 96 enter the bottom. The golden cylinder acts as an x-ray oven, transforming the laser light into thermal x-rays which heat up the capsule so much that it explodes. At that stage, the capsule becomes a sort of spherical rocket: its explosion generates an equal and opposite force that makes the fuel inside it implode.
Because the energy required to achieve this is very high, usually scientists make the capsule small. "But this time we made the capsule larger, and although the laser power remained the same, we managed to achieve fusion, which resulted in higher energy than previously measured," says Callahan. "What we need now is more laser energy, and our team is working on that as well as on other aspects of the engineering design to achieve even higher energy yields".
The amount of energy released during fusion was 1.3 megajoules, which converts to about 300 kcal, or the energy of a candy bar. This may not sound like a lot, but it was released in an extremely short amount of time (a fraction of a second), which makes the power burst more similar to the power a space rocket develops when taking off.
“We have made a lot of progress over the last decade, and this result shows we are going in the right direction,” concludes Callahan.
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