more on nuclear fusion: towards ignition!
I recently wrote about and tried to get a handle on the nuclear fusion facility, ITER, being built in southern France, but I barely mentioned the importance of magnets, and I didn’t mention another essential feature or factor in nuclear fusion – called ignition. That’s because I’m still a learner after all these years. But some news broke recently regarding a completely different experimental fusion facility in the USA, which uses lasers rather than magnets to control and focus the energy, which, as previously described, needs to be – a lot.
The National Ignition Facility (NIF) at the Lawrence Livermore National Lab in California is designed, it seems, to try and achieve exactly that – ignition. The term is kind of self-explanatory, as when you ignite something you get a burst of energy, seemingly more than you put into the igniting, like when you strike a match. But ignition in nuclear fusion is a really difficult thing to achieve, which is presumably why they had to build a whole national facility around it. They’ve been trying to achieve it for decades.
I did write that to achieve fusion – ignition? – required temps of around 150,000,000 celsius, and obviously to sustain such temperatures requires a fair amount of energy, ten times that at the sun’s centre. Did I get that figure wrong? Pressure comes into it too (there’s a direct proportionality between temperature and pressure at any given volume).
I’ve found a great video explainer of the ignition breakthrough, presented by Anton Petrov, and a recent New Scientist podcast (no 81) also discusses it. So basically the possibilities of nuclear fusion as an energy technology have been on the cards since the development of the H-bomb in the late forties and early fifties. The energy required to set off an H-bomb, and for subsequent neutron bomb technology, was derived from nuclear fission. So that’s a lot of energy to make more energy. Since then, the aim, the holy grail, has been to find a way to create ignition, an energy output that is greater than, and preferably much greater than, the energy input. This is, of course,, essential for real-use thermonuclear energy. A number of technologies for creating thermonuclear fusion have proved successful, except insofar as the input-output ratio is concerned. Out of all these experiments chasing this elusive ignition, two models seemed most promising. Firstly, the toroidal fusion reactor (eg ITER), which is a magnetic confinement reactor, in which super-heated plasma is spun very quickly around a magnetically confined chamber, to create higher-than-the-centre-of-the-sun energy/temperatures. A number of these reactors, or tokamaks, have been built around the world and have successfully created fusion, but not ignition.
The second model is very different. It’s called inertial confinement fusion, and it uses tiny hydrogen pellets. The idea came from observation of the H-bomb: a small enough hydrogen pellet would require a minimum energy of 1.6 megajoules (million joules) of energy to initiate an explosion – essentially, an ignition. This energy could be provided by lasers. Now this process is complicated – it’s not simply a matter of fusioning hydrogen into helium because, as described in my previous post about ITER, there are isotopes involved. These isotopes (deuterium and tritium) are used to overcome the electrostatic repulsion which would normally occur when using proteum, the common form of hydrogen. This repulsive force between protons is known as the Coulomb force. The attractive force between protons and neutrons, called the nuclear force, acts against the electrostatic repulsion force, and this helps in overcoming the Coulomb barrier, and facilitating a fusion energy greater than that inside our sun, where plasma particles may not fuse at all over long periods. We’re basically looking at creating a more efficient kind of fusion, which requires the kinds of temperatures and pressures found inside much larger stars than our sun.
The key to the elusive status or point known as ignition is a concept called the Lawson criterion. Wikipedia describes it thus:
The Lawson criterion ….compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is higher than the rate of loss, and enough of that energy is captured by the system, the system is said to be ignited.
We haven’t achieved ignition yet, but it seems another baby step has been taken. One of the researchers at the NIF has described it as a ‘Wright brothers moment’, which has led to a bit of head-scratching. Basically, what was achieved at the NIF was a ‘momentary’ ignition – very momentary, and still only releasing some 70% to 80% of the energy input. Yet this was the most significant achievement in 60 years of work – a proof of concept achievement, which is built on previous experiments yielding increasing levels of energy. The process involved almost 200 super-amplified lasers confining and directing energy at a tiny hydrogen pellet for a period of 3 nanoseconds. That’s 3 billionths of a second. This required excruciating accuracy, coordination and timing, with everything – the lasers, the amplifiers, the pellet, the hohlraum chamber (holding the pellet) and so forth, being executed precisely. The precision level has improved markedly in recent times, leading to this breakthrough moment (after all, the ‘Wright brothers moment’ wasn’t exactly the first commercial passenger flight). The 1.3 megajoules released in this most recent ignition experiment was some 25 times what the facility could muster only three years ago. So there doesn’t seem far to go.
And yet. The energy input required is enormous. The lasers would need to fire more or less constantly – machine-gun-like – to produce the output required for human use (the current record of 1.3 megajoules has been described as ‘just enough to boil a kettle’. So we’re talking orders of magnitude, not just for the laser energy but for the hydrogen pellets, which need to be produced en masse at a teeny fraction of current costs. And so on.
This not to minimise the achievement. The publicity already being generated augurs well for the future of a technology that has for so long failed to live up to expectations. Those at ITER and other labs around the world will receive a great fillip from this, not to mention some small mountains of cash. Looking forward to it.
References
Major Breakthrough in Nuclear Fusion After Decades of Research (Anton Petrov video)
https://en.wikipedia.org/wiki/Lawson_criterion
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