Giant X-ray lasers do a lot for physics. These powerful instruments can probe the interiors of tiny molecules, recreate the extreme conditions in stellar cores, and now, describe the plasmas inside fusion reactors in stunning detail.
In a recent Nature Communications paper, researchers at SLAC National Accelerator Laboratory present the first-ever images of instability in high-density plasma—that is, superheated, ionized gas that drives fusion reactors. Fusion reactions create unstable structures in the plasma—instabilities—that reduce the efficiency of those reactions. However, given the extremes of fusion experiments, researchers had struggled to find a way to properly study these instabilities.
“Our understanding of instabilities—when they grow, how they grow—is important to making fusion work,” said Siegfried Glenzer, a co-author of the study and a SLAC scientist, in a statement.
The challenges of fusion energy
Nuclear fusion combines two lightweight particles—most often hydrogen isotopes—to produce immense amounts of energy. In contrast, nuclear fission splits heavyweight particles to generate power.
Fusion doesn’t leave behind as much harmful, radioactive waste as fission, so researchers have been hard at work trying to bring fusion closer to practical use. But that’s easier said than done, and progress has been steady yet slow—leading some to joke that fusion is “always ten years away.”
One reason is that during experiments, reactors can become seriously chaotic as they heat the plasma to more than 100 million degrees. That should be enough to coax particles into a fusion reaction, but it often isn’t. The extreme temperatures and pressures typically generate unexpected turbulence or quirks inside the plasma that get in the way of smooth reactions.
Picturing the funky plasma
In that sense, the new study—which developed a platform for imaging plasma—offers a real solution to a serious problem. This technique uses powerful X-ray lasers to accelerate the electrons in plasma to very high energies, producing a stream of hot, feisty electrons similar to those found in fusion plasmas.
At the same time, a current of cold electrons travels toward the heated plasma from the opposite direction. When the two meet, the plasma develops filament-shaped instabilities that SLAC’s facilities captured at intervals of 500 femtoseconds (quadrillionths of a second).
By adjusting the timings of the X-ray pulses, the researchers were able to sketch out how filament structures developed inside plasmas over extremely short periods of time.
“This is the most detailed description of this instability yet,” Christopher Schoenwaelder, the study’s lead author and a SLAC scientist, said in the release.
The team then compared the images with theory-based computer simulations, testing the validity of existing models. As a result, they identified potential physical mechanisms that could explain how and why these instabilities form.
The giant X-rays deliver
What’s more, the team flagged that the instability also produced an astoundingly powerful magnetic field of 1,000 teslas—roughly 100,000 times stronger than fridge magnets. This is comparable to the magnetic field amplifications observed in exploding stars or high-energy cosmic rays, giving the new findings additional implications in astrophysics, according to the researchers.
That said, the team notes that this technique represents the start of investigations to come. Although physicists are now equipped with a tool to image the plasma, it’s still unknown whether similar dynamics apply to other forms of plasma instabilities—including types that researchers might not have observed yet. But as with many things in fusion research, this is at least a good start.