Internal energies of laser fusion do not match predictions

An article was published Monday that describes some puzzling results from the National Ignition Facility, which uses many very high-energy lasers focused on a small target to start a fusion reaction. Over the past few years, the facility has gone through some key 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 the burning plasma seemed to behave differently from those that had experienced ignition. There is currently no obvious explanation for the difference.

Ignition vs. Combustion

In the experiments discussed here, the material used for fusion is a mixture of tritium and deuterium, two heavier isotopes of hydrogen. They combine to produce a helium atom, leaving a spare neutron that is 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 that is directed at a small metal cylinder. The metal emits intense X-rays that vaporize the surface of a nearby pellet, creating an intense heat and pressure wave on the interior of the pellet where the deuterium and tritium reside. They form a very high-energy plasma, creating the conditions for fusion.

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

Until we have detailed models of the physics going on under these extreme conditions, we need to compare those models to what happens inside the plasma. Unfortunately, given that both the plasma and the materials that previously surrounded it are in the process of exploding, this is a significant challenge. To get a sense of what might be going on, researchers have turned to one of the products of the fusion reaction itself: the neutrons it emits, which can pass through the debris and be picked up by nearby detectors.

Temperature measurement

The physics of a fusion reaction produces neutrons of a specific energy. If fusion occurs in a material where the atoms are stationary, all the neutrons will exit with this energy. But apparently the atomic nuclei in the plasma – tritium and deuterium – are moving violently. Depending on how they move relative to the detector, these ions may add extra energy to the neutrons or take some away.

This means that instead of coming out as a sharp line at a certain energy, the neutrons come out at a range of energies that form a broad curve. The peak of this curve is related to the motion of the ions in the plasma and therefore to the temperature of the plasma. Additional details can be gleaned from the shape of the curve.

Between the flash point and the burn point we seem to have an accurate understanding of how the temperature of the plasma is related to the speed of the atoms in the plasma. The neutron data fit well with the curve that was calculated from our model predictions. Once the plasma switches to combustion, however, things no longer match. It’s as if the neutron data finds a completely different curve and follows it instead.

So what could explain this different curve? It’s not that we have no idea; we have a bunch of them and no way to tell them apart. The team that analyzed these results offered four possible explanations, including unexpected kinetics of individual particles in the plasma or a failure to account for details in the behavior of the underlying plasma. Alternatively, it may be that the burning plasma extends over a different area or lasts a different time than we would predict.

In any case, as the authors state, “understanding the cause of this deviation from hydrodynamic behavior may be important to achieve stable and reproducible ignition.”

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