Internal energies of laser fusion do not match predictions

On Monday, a paper was published that describes some puzzling results from the National Ignition Facility, which uses many high-energy lasers focused on a small target to start a fusion reaction. Over the past few years, the facility has achieved key milestones, including igniting fusion and creating what is called a burning plasma.

Now, researchers have analyzed the properties of plasma when it experiences these high-energy states. And to their surprise, they discovered that burning plasmas seem to behave differently than those that have undergone ignition. At the moment, there is no obvious explanation for the difference.

Ignition vs Combustion

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

The fusion process is triggered by a brief burst of extremely intense laser light that targets 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 inside the pellet, where deuterium and tritium reside. These form a very high energy plasma, setting the conditions for fusion.

If all goes well, the transmitted energy ignites the plasma, meaning no additional energy is needed for the fusion reactions to continue for the tiny fraction of a second that elapses before it all goes down. collapses. At even higher energies, the plasma reaches a state called combustion, where the helium atoms that form carry so much energy that they can ignite nearby plasma. This is considered critical, as it means that the rest of the energy (in the form of neutrons and gamma rays) can potentially be harvested to produce useful energy.

Although we have detailed models of the physics that take place in these extreme conditions, we must compare these models to what is happening inside the plasma. Unfortunately, given that the plasma and the materials that previously surrounded it are exploding, this is a tall order. To get an idea of ​​what might be going on, the researchers turned to one of the products of the fusion reaction itself: the neutrons it emits, which can travel through the wreckage and be picked up by detectors. near.

take a temperature

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

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 wide curve. The peak of this curve is related to the movement of the ions in the plasma, and therefore to the temperature of the plasma. Other details can be extracted from the shape of the curve.

Between the ignition point and the combustion point, we seem to have a precise understanding of the relationship between the temperature of the plasma and the speed of the atoms in the plasma. The neutron data aligns well with the curve calculated from our model predictions. Once the plasma switches to etching, 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 suggests four possible explanations, including unexpected kinetics of individual particles in plasma or an inability to account for detail in bulk plasma behavior. Alternatively, it could be that the burning plasma spans a different area or lasts a different time than we anticipated.

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

Natural Physics2022. DOI: 10.1038/s41567-022-01809-3 (About DOIs).