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An Anonymous Coward writes:

There's a change in behavior when the plasma starts burning, and nobody knows why:

[...] 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 appear to behave differently from those that have experienced ignition. At the moment, there's no obvious explanation for the difference.

In the experiments, the material being used for fusion is a mix of tritium and deuterium, two heavier isotopes of hydrogen. These combine to produce a helium atom, leaving a spare neutron that's emitted; the energy of the fusion reaction is released in the form of a gamma ray.

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

If everything goes well, the energy imparted ignites the plasma, meaning that no additional energy is needed for the fusion reactions to continue for the tiny fraction of a second that passes before the whole thing blows apart. At even higher energies, the plasma reaches a state called burning, where the helium atoms that are forming carry so much energy that they can ignite the nearby plasma. This is considered critical because it means the rest of the energy (in the form of neutrons and gamma rays) can potentially be harvested to produce useful power.

While we have detailed models of the physics that goes on under these extreme conditions, we need to compare those models to what's going on inside the plasma. Unfortunately, given that both the plasma and the materials that formerly surrounded it are in the process of exploding, that's a significant challenge. To get a picture of what might be going on, researchers have turned to one of the products of the fusion reaction: the neutrons it emits, which can pass through the wreckage and be picked up by nearby detectors.

Hartouni, E.P., Moore, A.S., Crilly, A.J. et al. Evidence for suprathermal ion distribution in burning plasmas. Nat. Phys. (2022). DOI: 10.1038/s41567-022-01809-3 (About DOIs).

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