This week it was announced a great advance in the investigation of nuclear fusion that I already commented here in a previous one, for what was expected and what had been leaked to the media before the press conference last Tuesday at the headquarters of the US Department of Energy. Now, in the past, it’s time to update and comment on the information. A bit late, I know, but it is what it is…
Since this follows what was previously published, they are going to allow me to save myself from repeating the explanation about what nuclear fusion is and how it works. If you are not familiar with it you will need it to understand what follows, but you can find the details here.
It can be said that what was revealed at the press conference It has far exceeded expectations. but also that there are reasons to moderate that triumphalist enthusiasm that has spread. When the US Secretary of Energy Jennifer Granholm told the media that what was achieved is “one of the most impressive scientific achievements of the 21st century”, possibly it should be recognized, but with nuances. Those nuances can even be explained by the comparison that Granholm made with the first flight of the Wright brothers in 1903, as I will explain.
The Secretary of Energy of the US government, Jennifer Granholm, at the press conference on the net gain of energy obtained on December 5, 2022 in a nuclear fusion experiment at the National Ignition Facility. DOE image.
As I already briefly recounted, in 2021 the researchers at the National Ignition Facility of the Lawrence Livermore National Laboratory (LLNL) in California had achieved a milestone that was much less publicized than the current one, but for some physicists it was the really relevant step (remember that the general media do not necessarily report the most important science, since they do not follow scientific publications, but rather what is aired through press releases and conferences).
On that occasion, 1.3 megajoules (MJ, million joules, the unit of energy) were obtained from burning the fuel compared to the 1.9 that the lasers injected. Anyone who does the calculation will see that this represents an energy gain of 0.68, that is, negative. But when the researchers reviewed their data and measurements, They confirmed that they had gotten the ignition.
There is an explanation for this: in reality, the energy that reaches the fusion fuel is less than that injected by the lasers into the receptacle and the capsule that contain it. Therefore, the energy that actually ignited the fuel was not 1.9 MJ, but a smaller amount. Small enough that the gain was actually positive when comparing the actual energy absorbed by the fuel to the 1.9 MJ produced in fusion.
This was not in any newspaper, but it created quite a stir among physicists because it was the demonstration of the principle, the breakthrough: the NIF was able to achieve ignition. In previous years, and given the lack of great progress, a climate of pessimism had been created in which many experts doubted that it could achieve it. Until then, only fusion energies had been obtained that were about 8 times lower, if my memory serves me correctly.
This new result showed that it was possible, and that it would be a matter of time. All that remained was to tweak the design of the system according to the simulations to get that little extra that would formally exceed what fusion physicists call he «scientific break even»the formal net gain considering the total energy provided (which is what has been announced now).
A slightly more detailed explanation: as Steven Kivrit explained here, when talking about net energy gain in a nuclear fusion experiment by inertial confinement, in reality We must distinguish between five scales of gain, as five milestones to overcome:
First, energy obtained in fusion with respect to the one that receives the fuel.
Second, energy obtained in fusion with respect to the one that receives the fuel capsule.
Third, energy obtained in fusion regarding the one who receives the call cavity, which is a kind of cylindrical gold container that contains the capsule; In this so-called indirect method, the lasers do not point directly at the fuel capsule, but inside the cavity, and this converts the light into X-rays that are the ones that burn the fuel. With a direct system, where the lasers bombard the fuel capsule without using a cavitythe second and third milestones are the same.
A hohlraum like those used in the NIF. It is a small gold cylinder that contains the deuterium and tritium fuel capsule inside. The lasers enter through the open ends. Image of the National Laboratory’s National Ignition Facility, Lawrence Livermore National Laboratory.
Fourth, energy obtained in fusion regarding the energy of electricity that lasers spend.
And finally, the fifth is that the electricity that can be obtained from fusion energy exceeds the electricity expended by lasers.
The first three concepts are different because throughout the process there are energy losses. From the contribution to cavity in the form of photons, only a part of it is translated into the X-rays incident on the fuel capsule. And of this, only a part reaches the interior of the fuel. As for the fourth step, it is because lasers are very inefficient; of all the energy they consume, only a part is emitted in the form of jets of light. And finally, no matter how much energy is obtained from nuclear fusion, in order to harness it, it must be converted into electricity. Only when this fifth step is complete will we have nuclear fusion to microwave what was left of yesterday’s pizza.
The 2021 experiment passed steps one and two, and was on the verge of the third. It happened that, after that result, the same was not achieved again. Aside from system design and simulations, the process is extremely sensitive to any insignificant error or defect. For example, if the symmetry of the capsule or the pellet of fuel fails even in a magnitude of an infinitesimal of the diameter of a human hair, everything goes to waste. This necessary perfection, together with certain changes in the system, They have been key to finally reaching the new milestone: the third step.
As announced this week, in this last shot 2.05 MJ were injected into the cavity and 3.15 MJ were obtained in the merger, which supposes a profit of something more than 1.5; that is to say, they have gone too far with respect to the objective to be achieved, and this certainly deserves to be celebrated. In general, it can be said that fusion scientists have welcomed the news with great enthusiasm.
But now come the nuances. Regarding Granholm’s words, for all the above it should be understood that this is not an achievement of December 5, 2021, but an achievement of 60 years of research in which milestones have been progressively exceeded. The announcement now is an important one in a long process.
That is why the comparison with the flight of the Wright brothers is well brought: Although it is often believed that they were the ones who flew for the first time, this is not the case. Many others had already flown before them. The first milestone, going up in a heavier-than-air craft, had been passed before. Also the second (more or less), the propulsion. What they achieved was to reach a third step, to build a device that could be piloted by controlling movement in the three axes of space. But even having overcome this challenge, translating the Wright brothers’ success into the first practical commercial aircraft still took years of development.
The latter will take decades in the case of nuclear fusion. First, the fact that the barrier of scientific break even does not mean that it has already been left behind. As explained, it is not guaranteed that in the next experiments that energy gain of 1.5 will be repeated. Hopefully the NIF researchers have managed to control all the variables in the experiment with sufficient skill and perfection so that the result can be easily replicated in the future. But we will have to wait and see.
Second and above all, because That mantra repeated in the media that the holy grail of clean, inexhaustible energy has been found is an exaggeration of colossal proportions. As I explained in the previous article and I think it will now be better understood, to overcome the fourth milestone, a net gain in energy from fusion compared to that consumed by lasers, the result obtained now would have to be multiplied by 10, by 100 or even perhaps for 1,000 (what happens is that this is not the purpose of the NIF, as I will explain now).
As for the fifth step, let’s not even talk about it; It’s still science fiction. Let’s not forget that the NIF is a $3.5 billion facility, the size of a football stadium, used to bombard with 192 lasers—which after each shot must be left to cool for almost a full day—a tiny frozen pellet of deuterium and tritium that suddenly turns into a little sun for a few billionths of a second, and then goes out. To turn this into a practical form of energy, engineers would have to figure out how to build a reactor that can rapidly replenish the small and very expensive pellets of fuel and firing lasers in bursts like a machine gun.
And this is not all: unfortunately, even when/if one day all of the above is possible, the truth is that No one has yet found a practical way to convert the energy from inertial confinement fusion into electricity. At the moment, all we have is a miniature explosion (actually an implosion) with incredible power, for a few billionths of a second.
Lastly, there is something that needs to be underlined, which justifies the great enthusiasm of those responsible for the US Department of Energy (DOE) who participated in the press conference, and which has not been sufficiently explained here either. The NIF is an installation dependent on the National Nuclear Security Administration (NNSA) and that It was not designed to investigate nuclear fusion as a source of energy, but for a very different purpose: to simulate the testing of thermonuclear weapons without the need to test the bombs themselves in the field.
In 1992 the US abandoned nuclear tests, and since then has relied on an experimentation program to check the operability and safety of its nuclear arsenal without directly exploding the bombs. The new result is a big step forward for the US nuclear program, who supports those who have opted for this approach against those who pressed to revive nuclear tests.
It is true that now, according to some specialized media, the US government is facing a dilemma. At the press conference, a new injection of 624 million dollars was announced so that the NIF can progress in its investigations from the recently reached milestone. Of course, the US has never concealed the purpose of the NIF, nor is it going to abandon it. But given the expectations generated throughout the world, and with the growing and enormous public pressure for the search for alternatives to fossil fuels, the advance that this new result brings to the line of energy research cannot be ignored. as it counts to Nature physicist Stephen Bodner, former head of the laser fusion program at the US Naval Research Laboratory, “The big question now is what DOE will do next: double down on weapons research at the NIF or pivot to a laser program targeting fusion power research”.