Ultrafast Movie Reveals Unexpected Plasma Behavior
With an astonishing 500 billion frames per second, a new movie captures the evolution of a laser-induced plasma, revealing that its atoms have lost more electrons—and thus have stronger interactions within the plasma—than models predict [1]. The movie relies on a ten-year-old technology, called compressed ultrafast photography (CUP), that packs all the information for hundreds of movie frames into a single image. The results suggest that models of plasma formation may need revising, which could have implications for inertial-confinement-fusion experiments, such as those at the National Ignition Facility in California.
Dense plasmas occur in many astrophysical settings and laboratory experiments. Their behavior is difficult to predict, as they often change on picosecond (10−12 s) timescales. A traditional method for probing this behavior is to use a streak camera, which collects a movie on a single image by capturing a small slice of each movie frame. “It’s one picture, but every line occurs at a different time,” explains John Koulakis from UCLA. He and his colleagues have used streak cameras to study anomalous behavior in plasmas [2], but the small region of plasma visible with this technique left doubts about what they were seeing, he says.
To get a fuller picture, the researchers enlisted the help of Lihong Wang and his team at Caltech, who developed CUP. The technique uses a so-called digital micromirror, which has an array of small mirrors that can be positioned to either reflect or not reflect light to a camera. The mirror positions are chosen to form a random 2D pattern (resembling a QR code), which captures a sparsely sampled image of the target object.
Rather than taking separate snapshots for each movie frame, the CUP method overlays all the movie frames in a single image. Caltech team member Peng Wang compares it to taking a photo while someone is moving, which produces a blurry photo. Because of the sparse sampling, the blurring in the CUP image can be reversed using postprocessing algorithms that extract the individual frames and recover the movie. The CUP technique can film phenomena, such as the reflection of a light pulse off a mirror, at speeds as high as 70 trillion frames per second, equivalent to a shutter speed of 14 femtoseconds [3]. Faster techniques exist, but they require repeating the experiment many times and averaging the results. “CUP is the fastest single-shot imaging method,” Peng Wang says.
For the plasma movie, the researchers installed the CUP equipment around a chamber containing a noble gas, either argon or xenon. They fired an intense laser pulse into the gas, ionizing nearly all the atoms and forming a hot, glowing plasma. The CUP system collected a single-shot image of this emission, and from those data, the team recovered a 400-frame movie with 2 picoseconds between frames. The researchers chose this frame rate because it was not too slow or too fast to catch changes in the plasma while also allowing the capture of the full plasma lifetime (roughly 10 nanoseconds). “This capability provides a unique opportunity to analyze plasma dynamics in a single shot—an achievement unattainable with existing imaging technologies,” says Hari Harilal, a plasma expert from Pacific Northwest National Laboratory in Washington, who was not involved in the study.
By analyzing the emission at different visible wavelengths , the researchers could estimate the temperature and the ionization fraction—the average number of stripped electrons per atom. They found that the amount of ionization in the plasma was 4 times what theory predicts. This unexpected ionization in a plasma was previously reported in sonoluminescence experiments by UCLA team leader Seth Putterman and colleagues (see Focus: Plasma Extremes Seen through Gas Bubble). Putterman says that their new CUP results imply that the plasma is in a strongly coupled regime, in which the ions interact like molecules in a liquid.
The strongly coupled plasma state has been studied in simulations, but it was not expected to occur at the relatively low temperatures (around 20,000 K) that the team is exploring. This behavior could affect how researchers model the early stages of laser-generated plasmas in inertial-confinement-fusion experiments, Koulakis says.
Alexander Bataller, a nuclear-energy researcher from North Carolina State University, thinks that this work documents a significant advance in the study of dense plasmas. He says that the team has provided compelling evidence of a strongly coupled plasma in the early-time evolution of the laser-ionized gas. “With this new capability, the field can continue to advance our understanding of what appears to be a rather unusual and interesting state of matter,” he says.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- P. Wang et al., “Compressed ultrafast photography of plasmas formed from laser breakdown of dense gases reveals that internal processes dominate evolution at early times,” Phys. Rev. E 113, 015209 (2026).
- A. Bataller et al., “Observation of shell structure, electronic screening, and energetic limiting in sparks,” Phys. Rev. Lett. 117, 085001 (2016).
- P. Wang et al., “Single-shot ultrafast imaging attaining 70 trillion frames per second,” Nat. Commun. 11, 2091 (2020).