Plasma, ionized gas and the fourth state of matter, makes up over 99 percent of the ordinary matter in the universe. Understanding its properties is critical for developing fusion energy sources, modeling astrophysical objects like stars and improving manufacturing techniques for semiconductors in modern cell phones.
But watching and determining what happens inside high-density plasmas is difficult. Events can unfold in trillionths of a second and behave in complex, unpredictable ways.
In a study published in Optica, researchers at Lawrence Livermore National Laboratory (LLNL) developed a new diagnostic that captures plasma evolution in time and space with a single laser shot. This breakthrough creates plasma movies with 100 billion frames per second, illuminating ultrafast dynamics that were previously impossible to observe.
"In most high-energy, high-intensity laser experiments currently, we take a single image per laser shot," said LLNL scientist and lead author Liz Grace. "However, these plasmas are unstable and unpredictable, and small changes can have butterfly effects that impact the subsequent evolution. It's important to capture as much information at once as possible."
Each laser shot through a plasma is slightly different, so stitching that information together across different discrete shots can be a large source of error. In contrast, the new diagnostic, called Single-shot Advanced Plasma Probe HolographIc Reconstruction, or SAPPHIRE, captures everything in one go.
To accomplish this, the team uses a special laser pulse with what is called a "chirp." This means that the laser pulse and the colors contained within it are stretched out in time. For example, in the negative chirp used in this work, bluer light with shorter wavelengths races through first, followed later by redder light with longer wavelengths.
The upper half of the laser beam passes through the plasma, where it refracts and warps, while the lower half does not. On the other end of the plasma, the SAPPHIRE diagnostic separates those two beam halves, then recombines them to create a unique interference pattern for each wavelength of light - and therefore each timestamp.
With a bit of math, that interference pattern can be transformed into a map of electron density in the plasma, providing the researchers with an exquisitely detailed movie of how the plasma changes with time.
The authors tested SAPPHIRE on helium-nitrogen gas jets, but Grace said the diagnostic can be applied to measure time dependent underdense (translucent to the laser) plasma profiles created in pulsed power, waveguides, plasma optics, laser-based particle accelerators and more.
"I personally would love to see this diagnostic applied to fusion energy environments, including Z-pinch plasmas," she said. "In the paper, we provided a very thorough instruction manual of how to build your own, and I'm looking forward to seeing what people can come up with."
