In a lot of ways, stars are our model for creating nuclear fusion here on Earth, with fusion power often promoted as "harnessing the power of the Sun." For all that, however, we have some surprising gaps in our understanding of what's going on inside stars. That's partly because we must infer what's going on there based on the elements and particles that reach the solar surface, and partly because finding ways to test our theoretical models of fusion reactions is so difficult.

So there's a certain appealing symmetry about a paper that was released by Nature Physics today. In it, researchers describe using the National Ignition Facility, built to study fusion using a giant laser, as a model for the interior of heavy stars. The results show that, despite their limitations, our earlier efforts to understand stellar fusion were on the right track.

Cross checking the cross-section

On a simple level, most stars fuse hydrogen to form helium. But things are obviously more complex than that. Most of the hydrogen in our Sun is the lightest form, with just a single proton as its nucleus. The helium produced in stars has two protons and two neutrons. Obviously, making helium from only protons requires a series of nuclear reactions, each with distinct probabilities of occurring that depend in part on the conditions inside the star. Complicating matters further, there are some other possible reactions that don't lead directly to helium but can still occur inside a star, producing things like heavier isotopes of hydrogen.

Physicists understand these and other particle interactions through something called a "cross-section." The value of a cross-section is related to the probability that two objects will undergo a specific interaction. So, for something like hydrogen, there's a cross-section that describes the possibility that two hydrogen atoms will fuse to form deuterium, a heavier isotope of hydrogen.

Figuring out the cross-section of various fusion reactions has largely been the domain of theorists who can use what we understand about the conditions in stars and the interactions of particles to calculate the probabilities of various nuclear reactions.

Confirming these calculations empirically, however, has been a bit of a challenge. Generally, calculations are confirmed using particle accelerators, which can smash various hydrogen and helium isotopes together to get a sense of their cross-section. But the work is challenging in particle accelerators, in part because they have trouble reaching the relatively low energy of fusion environments. In addition, the significant numbers of electrons floating around in the interior of stars can influence the cross-section; it's difficult to include them in accelerator experiments.

For these reasons, a team of physicists decided to study fusion under something closer to star-like conditions. To create those, they turned to the National Ignition Facility, which creates tiny nuclear fireballs using some really big lasers.

Check ignition

The National Ignition Facility is capable of directing 192 individual laser beams at an extremely small target. Within a handful of nanoseconds, they can deliver more than a megajoule into an area only a few millimeters across. The lasers hit the inside of a shell, striking a gold surface that emits intense X-rays. The X-rays vaporize the outer layers of the shell, creating an implosion that crushes the interior. Said interior contains a capsule filled with hydrogen isotopes that are briefly subjected to intense temperatures and pressures that are sufficient to set off fusion reactions.

The wreckage can then be searched for newly formed isotopes of hydrogen and helium, allowing the researchers to reconstruct the nuclear reactions that took place, along with the frequency.

This system has two good things going for it. To begin with, it can model different stellar interiors by varying the energy of the lasers and the target shape. For these experiments, the researchers created conditions similar to those expected in the core of a star with between 10 and 40 times the mass of the Sun—big stars, destined to end their lives in supernovae. By varying the composition of the target, the researchers were also able to calculate cross sections for a variety of hydrogen isotopes: deuterium-deuterium, deuterium-tritium, and tritium-tritium.

The big downside is that none of these reactions is especially relevant for the Sun and most other stars. Deuterium-deuterium fusion is important astronomically, though, as it's the only type of fusion that brown dwarf stars are able to initiate and requires lower energies than hydrogen fusion. It's also the first type of fusion to start as stars condense during their formation. So, it's interesting, but not as interesting as it might be.

The good news is that the results indicate our past work on estimating cross-sections was pretty good. Within the statistical uncertainty, the new measurements agree with those from particle accelerator experiments. And the researchers say that developments at the National Ignition Facility should allow them to test conditions similar to those in the Sun before too long.

Nature Physics, 2017. DOI: 10.1038/NPHYS4220 (About DOIs).