The internal energies of laser-guided fusion do not match predictions

Enlarge / Where the action takes place at the National Ignition Facility.

A paper was released Monday detailing some confusing results from the National Ignition Facility, which uses many high-energy lasers aimed at a small target to trigger a fusion reaction. Over the past few years, the facility has achieved a number of significant milestones, including igniting fusion and creating what’s called a burning plasma.

Now researchers have analyzed the properties of the plasma as it experiences these high-energy states. And to their surprise, they found that burning plasmas seem to behave differently from plasmas that have caught fire. At the moment there is no clear explanation for the difference.

Inflammation vs. to burn

In the experiments at issue here, the material used for fusion is a mix of tritium and deuterium, two heavier isotopes of hydrogen. These combine to produce a helium atom, leaving a reserve neutron to be emitted; the energy of the fusion reaction is released in the form of gamma rays.

The fusion process is triggered by a short, extremely intense burst of laser light aimed at a small metal cylinder. The metal emits intense X-rays, which vaporize the surface of a nearby pellet, creating an intense wave of heat and pressure in the pellet’s interior, where the deuterium and tritium reside. These form a very high-energy plasma, which creates the conditions for fusion.

If all goes well, the supplied energy ignites the plasma, meaning that no additional energy is needed to continue the fusion reactions during the small fraction of a second that elapses before the whole thing falls apart. At even higher energies, the plasma reaches a state called burning, where the helium atoms that form carry enough energy to ignite the nearby plasma. This is considered crucial, as it means that the rest of the energy (in the form of neutrons and gamma rays) can potentially be harvested to produce usable energy.

While we have detailed models of the physics that take place under these extreme conditions, we need to compare those models to what happens in the plasma. Unfortunately, given that both the plasma and the materials that used to surround it are exploding, that’s a big challenge. To get a picture of what might be going on, researchers have focused on one of the products of the fusion reaction itself: the neutrons it emits, which can pass through the wreckage and be picked up by nearby detectors.

Record temperature

The physics of the fusion reaction produces neutrons with a specific energy. If fusion happened in a material where the atoms were stationary, all the neutrons would come out with that energy. But it is clear that the atomic nuclei in the plasma – the tritium and deuterium – are moving violently. Depending on how they move relative to the detector, these ions can give some extra energy to the neutrons, or subtract a little.

This means that, instead of coming out as a sharp line at a specific energy, the neutrons come out at a range of energies that form a broad curve. The peak of that curve is related to the movement of the ions in the plasma, and thus the temperature of the plasma. Further details can be deduced from the shape of the curve.

Between the point of ignition and the point of combustion, we seem to have a good understanding of how the temperature of the plasma relates to the speed of the atoms in the plasma. The neutron data agrees nicely with the curve calculated from our model predictions. However, once the plasma transitions to burning, things don’t line up. It’s like the neutron data finds a completely different curve and follows it instead.

So, what could explain that other curve? It’s not that we have no idea; we have a lot of them and we can’t tell them apart. The team that analyzed these results suggests four possible explanations, including unexpected kinetics of individual particles in the plasma or failure to account for details in the behavior of the bulk plasma. Alternatively, the burning plasma may extend over a different area, or last for a different amount of time than we predicted.

In any case, as the authors state, “Understanding the cause of this deviation from hydrodynamic behavior may be important for achieving robust and reproducible ignition.”

nature physics2022. DOI: 10.1038/s41567-022-01809-3 (About DOIs).

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