Don’t get excited. It’s always nice to see incremental progress being made with the various fusion experiments happening around the world, but we are still a long way off from commercial fusion power, and this experiment doesn’t really bring us any close, despite the headlines. Before I get into the “maths”, here is some quick background.

Fusion is the process of combining light elements into heavier elements. This is the process the fuels stars. We have been dreaming about a future powered by clean abundant fusion energy for at least 80 years. The problem is – it’s really hard. In order to get atoms to smash into each other with sufficient energy to fuse, you need high temperatures and pressures, like those at the core of our sun. We can’t replicate the density and pressure at a star’s core, so we have to compensate here on Earth with even higher temperatures.

There are a few basic fusion reactor designs. The tokamak design (like the JET rector) is a torus, with a plasma of hydrogen isotopes (usually deuterium and tritium) inside the torus contained by powerful magnetic fields. The plasma is heated and squeezed by brute magnetic force until fusion happens. Another method, the pinch method, also uses magnetic fields, but they use a stream of plasma that gets pinched at one point to high density and temperature. Then there is kinetic confinement which essentially uses an implosion created by powerful lasers to create a brief moment of high density and temperature. More recently a group has used sonic cavitation to create an instance of fusion (rather than sustained fusion). These methods are essentially in a race to create commercial fusion. It’s an exciting (if very slow motion) race.

There are essentially three thresholds to keep an eye out for. The first is fusion – does the setup create any measurable fusion. You might think that this is the ultimate milestone, but it isn’t. Remember, the goal for commercial fusion is to create net energy. Fusion creates energy through heat, which can then be used to run a convention turbine. So just achieving fusion, while super nice, is not even close to where we need to get. If you are putting thousands of times the energy into the process as you get out, that is not a commercial power plant. The next threshold is “ignition”, or sustained fusion in which the heat energy created by fusion is sufficient to sustain the fusion process. (This is not relevant to the cavitation method which does not even try to sustain fusion.) A couple of labs have recently achieve this milestone.

But wait, there’s more. Even though they achieved ignition, and (as was widely reported) produced net fusion energy, they are still far from a commercial plant. The fusion created more energy than when into the fusion itself. But the entire process still used about 100 times the total energy output. So we are only about 1% of the way toward the ultimate goal of total net energy. When framed that way, it doesn’t sound like we are close at all. We need lasers or powerful magnets that are more than 100 times as efficient as the ones we are using now, or the entire method needs to pick up an order of magnitude or two of greater efficiency. That is no small task. It’s quite possible that we simply can’t do it with existing materials and technology. Fusion power may have to wait for some future unknown technology.

In the meantime we are learning an awful lot about plasmas and how to create and control fusion. It’s all good. It’s just not on a direct path to commercial fusion. It’s not just a matter of “scaling up”. We need to make some fundamental changes to the whole process.

So what record did the JET fusion experiment break? Using the tokamak torus constrained by magnetic fields design, they were able to create fusion and generate “69 megajoules of fusion energy for five seconds.” Although the BBC reports it produced, “69 megajoules of energy over five seconds.” That is not a subtle difference. Was it 69 megajoules per second for five seconds, or was it 13.8 megajoules per second for five seconds for a total of 69 megajoules? More to the point – what percentage of energy input was this. I could not find anyone reporting it (and ChatGPT didn’t know). But I did find this – “In total, when JET runs, it consumes 700 – 800 MW of electrical power.” A joule is one watt of power for one second.

It’s easy to get the power vs energy units confused, and I’m trying not to do that here, but the sloppy reporting is no help. Watts are a measure of power. Watts over time are a measure of energy, so a watt second or watt hour is a unit of energy. From here:

1 Joule (J) is the MKS unit of energy, equal to the force of one Newton acting through one meter.
1 Watt is the power of a Joule of energy per second

So since joules are a measure of energy, it makes more sense that it would be a total amount of energy created over 5 seconds (so the BBC was more accurate). So 700 MW of power over 5 seconds is 3,500 megajoules of energy input, compared to 69 megajoules output. That is 1.97%, which is close to where the best fusion reactors are so I think I got that right. However, that’s only counting the energy to run the reactor for the 5 seconds it was fusing. What about all the energy for starting up the process and everything else soup to nuts?

This is not close to a working fusion power plant. Some reporting says the scientists hope to double the efficiency with better superconducting magnets. That would be nice – but double is still nowhere close. We need two orders of magnitude, at least, just to break even. We probably need closer to three orders of magnitude for the whole thing to be worth it, cradle to grave. We have to create all that tritium too, remember. Then there is inefficiency in converting the excess heat energy to electricity. That may be an order a magnitude right there.

I am not down on fusion. I think we should continue to research it. Once we can generate net energy through fusion reactors, that will likely be our best energy source forever – at least for the foreseeable future. It would take super advanced technology to eclipse it. So it’s worth doing the research. But just being realistic, I think we are looking at the energy of the 22nd century, and maybe the end of this one. Not the 2040s as some optimists predict. I hope to be proven wrong on this one. But either way, premature hype is likely to be counterproductive. This is a long term research and development project. It’s possible no one alive today will see a working fusion plant.

At least, for the existing fusion reactor concepts I think this is true. The exception is the cavitation method, which does not even try to sustain fusion. They are just looking for a “putt putt putt” of individual fusion events, each creating heat. Perhaps this, or some other radical new approach, will cross over the finish line much sooner than anticipated and make me look foolish (although happily so).