On Monday, a paper was released describing some confounding results from the National Ignition Facility, which uses lots of highly energetic lasers focused on a small target to start a fusion reaction. Over the past few years, the facility has gone through some major milestones, including fusion ignition and the creation of so-called burning plasma.
Now, the researchers have analyzed the properties of the plasma as it experiences these high-energy states. To their surprise, they found that the burning plasma seemed to behave differently than the one that had been subjected to ignition. At the moment, there is no clear explanation for the difference.
Ignition versus burning
In the experiments presented here, the material used for fusion is a mixture of tritium and deuterium, two isotopes heavier than hydrogen. These combine to produce a helium atom, leaving a spare 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 nearby grains, creating an intense wave of heat and pressure on the inside of the grains, where deuterium and tritium are located. These form high-energy plasmas, which create the conditions for fusion.
If all goes well, the transmitted energy ignites the plasma, which means that no additional energy is needed to keep fusion reactions going for the fraction of a second that passes before everything explodes. At higher energies, the plasma reaches a state called combustion, where the helium atoms that form carry so much energy that they can ignite the adjacent plasma. This is critical, as it means that the rest of the energy (in the form of neutrons and gamma rays) can be harvested to produce useful 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 inside the plasma. Unfortunately, since both the plasma and the material that previously surrounded it are in the process of exploding, this presents quite a challenge. To get a picture 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 pass through the debris and be picked up by nearby detectors.
Fusion reaction physics produces neutrons of a certain energy. If fusion occurred in a material where the atoms were fixed, all neutrons would exit with that energy. But it is clear that the atomic nuclei in the plasma – tritium and deuterium – are moving violently. Depending on how they move relative to the detector, these ions may transfer some extra energy to the neutrons, or subtract a little.
This means that instead of appearing as a sharp line at a certain energy, neutrons exit in a range of energies that form a broad curve. The peak of this curve is related to the motion of ions in the plasma, and thus the temperature of the plasma. More details can be extracted from the shape of the curve.
Between the point of ignition and the point of combustion, we seem to have a precise understanding of how the temperature of the plasma relates to the velocity of the atoms in the plasma. The data from the neutrons align well with the curve calculated from our model predictions. However, once plasma switches to combustion, things are no longer identical. It’s as if the neutron data finds a completely different curve and follows that instead.
So, what might explain this different curve? It’s not that we have no idea. We have a bunch of them and there is no way to tell them apart. The team analyzing these results suggested four possible explanations, including unexpected kinetics of individual particles in the plasma or a failure to account for details in the behavior of mass plasmas. Alternatively, the burning plasma could extend over a different area, or last for a different length of time than we would expect.
In any case, say the authors, “Understanding the reason for this departure from hydrodynamic behavior can be important for achieving robust and repeatable ignition.”