Physics + Math

13
Feb

Fusion Experiment Achieves Key ‘Bootstrapping’ Effect Not One But Four Times

We’ve come yet another step closer to harnessing the power of the Sun.

Researchers at Lawrence Livermore National Laboratory report that they’ve successfully produced significant amounts of fusion by zapping a target with their high-powered laser. And they haven’t done it just once. Lead paper author Omar Hurricane says they’ve done it at least four times since September 2013.

capsule-fusion
A metallic case called a hohlraum holds the fuel capsule that NIF researchers use during their fusion experiments.

Fusion is a process in which hydrogen atoms — in this case, a deuterium-tritium (two isotopes of hydrogen) mixture — are cooked and pressurized to make helium atoms. Previous experiments have successfully fused hydrogen isotopes to helium, but it hasn’t been a sustainable or efficient source of energy; NIF scientists weren’t getting enough of the power used to energize the laser into the fusion fuel itself. The equipment is also pricey; the laser at the National Ignition Facility (or NIF) costs more than $3 billion. But it pays off, since it can focus 500 trillion watts of power onto a tiny fuel pellet, kickstarting the fusion process. It’s just that until now, scientists haven’t been able to channel enough of the laser’s energy into the pellet such that the fusion fuel output is greater than the fusion fuel input.

What’s helped the team is a new laser technique. Here’s David Biello, writing for Scientific American:

One shot at Livermore’s National Ignition Facility (NIF) on November 19, 2013, that lasted less than 2 X 10^–8 seconds—less time than the blink of an eye—produced nearly twice as much energy as was applied, according to the new paper. Changing the timing of how the lasers put energy into the hohlraum, a tiny can that holds the fusion fuel pellet, proved key. The scientists concentrated more energy earlier in the shot to make conditions hotter earlier in the process, which seems to help hold the fuel pellet together longer as it implodes.

The researchers remain a long way from ignition — the “critical point” at which the fusion fuel feeds on itself to make more of itself — but they’re beginning to get the hang of how to create the conditions necessary for it to occur:

[...] the discovery team has also seen for the first time the early stages of the kind of physical processes needed to create such fusion. Specifically, the fuel showed evidence of what fusion physicists like to call “bootstrapping.” Essentially, the helium nuclei (otherwise known as alpha particles) thrown off by the fusing hydrogen isotopes left their energy behind, maintaining the conditions needed for yet more fusion. That helped more than double the superheating of the fusing fuel and suggests the team is halfway to the kinds of energies needed to achieve ignition. “As we pushed it in experiments, the bootstrapping kicks in more and more,” Hurricane says. “Seeing that kick is quite exciting and does show there is progress.”

To create the kind of pressure required for ignition, they’ll need to crush the capsule with the laser even faster, while still keeping the fuel containerized in a perfectly spherical shape (which keeps the energy focused enough so that it doesn’t scatter). They could also increase the laser’s power, too. But even if they were to reach ignition, scientists would need a fresh source of rare tritium to keep it going and to create an actual fusion power plant. The quest surges on, though, and when NIF comes up for review again in 2015, perhaps we will have a better sense of what’s possible.

Check out NOVA’s Sun Lab to learn more about fusion and the physical processes that keep it — and us — alive.