A powerful fusion reactor is working right now – it currently provides just over 3.5% of the world’s energy…it’s called the Sun, and we can only tap it indirectly through solar panels. In the Sun, hydrogen atoms are slammed together at high speeds and join together – fusing to form helium atoms; energy is released when they do so. Of course, the conditions on the Sun – high pressures, high temperatures – do not normally exist on Earth, but we can create them; it just takes a lot of technology.
What makes it worth the effort is that there are many hydrogen atoms – a one-gallon bucket of water contains enough hydrogen to power more than 2000 homes for a year (unless I misplaced a decimal point or two). On a kilowatt-hour per kg basis, fusion power is more efficient than virtually anything else we know – which is why we’ve been pursuing it for over a half-century. We’ve made it happen – but only in thermonuclear weapons and very briefly in fusion reactors. It’s creating sustained fusion power to produce usable energy that’s always been a generation in the future.
Theory and Practice
In theory, hydrogen fusion sounds easy – if you slam two hydrogen atoms together hard enough, they’ll stick together, releasing energy in the process; that’s what happens in the Sun and every star in the universe. The details are tricky – to stick together, the hydrogen atoms need to be moving fast enough to overcome the electrical forces pushing them apart, which requires heating it to millions of degrees and keeping it there. We’ve made it happen, so we know it’s not impossible – but it’s not easy. And harder still is to use this phenomenon to produce energy.
Those pesky details
“Ignition is a state where the fusion plasma can begin ‘burn propagation’ into surrounding cold fuel, enabling the possibility of high energy gain.” - Physical Review Letters.
The reason it’s hard to get two hydrogen atoms to fuse is that it’s the atomic nuclei that need to fuse – and the atomic nuclei have a positive electrical charge, so they repel each other. In fact, the closer they are, the stronger the force keeping them apart; it takes 100 times as much energy to hold two positive charges 1 mm apart compared to holding them 1 cm apart; forcing them to within a millionth of a millionth of 1 mm apart (where the strong nuclear force begins to have an effect) requires a trillion trillion (or, in scientific notation, 1024) times as much energy as holding them at a distance of 1 mm.
Fusion is possible because there’s another force at work – the strong nuclear force – that’s even more powerful than the electromagnetic force pushing the nuclei apart. The problem is that the strong nuclear force has a very short range – so short that bringing two hydrogen nuclei within that range requires a lot of energy to overcome the electrostatic force pushing them apart. The only way to reach this high energy level is to crank up the speed – since speed is related to an atom’s temperature this means heating the hydrogen to incredibly high temperatures. The atoms in a hot material move far faster than those in something cold; this means cranking up the temperature…to tens of millions of degrees.
While heating atoms to such high temperatures is not easy, keeping them at such high temperatures is even more challenging. The two primary approaches involve using a powerful magnetic field to confine the plasma (magnetic confinement) and using powerful lasers to initiate fusion – this is called “inertial confinement.” With the lasers, the high temperatures generated by the initial reactions maintain the fusion for a short period. The ultimate goal is for the initial burst of fusion energy to cause enough heating to set off secondary fusions producing sufficient heating for the fusion to continue.
At the moment, our work with fusion power has been like trying to start a fire with damp wood. You can force the wood to smolder and maybe even burn for short periods, but the igniter is putting more energy into the wood than the fire is producing, and as soon as the igniter is turned off, the fire starts to go out. That’s about where we are with fusion at the moment – we have to put more energy in than it produces to keep a fusion reaction going. What we need with our fusion reactors – as with our fireplaces – is a condition where the fusion is putting out enough energy to cause additional fusions without the need for additional energy input…the point at which the hydrogen “burns” on its own. We’re not there yet – but we’re getting close.
We have ignition
A recent article in The Science Times reports an announcement from the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory and the publication of three papers in the peer-reviewed scientific literature describing experiments over the last year that NIF claims meet the criteria for having briefly achieved “ignition” in hydrogen fusion.
On August 8, 2021, NIF focused 192 laser beams with a total energy of 1.92 MJ (about a 100-watt light would use in 5 hours) on a small gold-lined cylinder made of depleted uranium. Inside this cylinder was a tiny capsule filled with frozen deuterium and tritium (two heavy isotopes of hydrogen, abbreviated DT). When the beams slammed into the capsule, it heated the fuel enough to initiate fusion, which then continued for nearly ten billionths of a second, heating some of the remaining DT mixture to the point of fusing and producing more than 1.3 megaJoules. While this shot still consumed more energy than it produced (they’re still at the “damp wood” stage), it’s the closest anyone has yet come to breaking even, energy-wise, and the first to achieve ignition.
While this experiment is an encouraging advance, Livermore has not yet been able to replicate it – subsequent shots have come close to ignition but have fallen short. That’s not necessarily a bad thing – each shot is a learning experience, and Livermore is looking at the variables associated with every shot (the exact performance of each laser beam, the amount of deuterium and tritium in the capsule that’s frozen versus gaseous, the shape of the DT ice, and so forth) – learning how each of these, individually and in combination, affects the output of the shots is helping researchers to hone in on the factors that are most important so they can work on perfecting the technique and on matching, and then to exceed, the results from last year.
Fusion power, if or when it finally comes to fruition, is a big deal – it gives a huge power density, it should produce little in the way of waste, it produces no carbon dioxide, and its fuel is the most common element in the universe. Here’s hoping we can finally retire that standard joke and that fusion power is now less than a quarter century in our future.
