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Lightning in the water: Ultrafast X-ray provides new look at plasma discharge breakdown in water

Lightning is fast, but how fast? A Laboratory collaboration recently turned to synchrotron X-rays for an answer. The story of the discovery of the connection between electricity and lightning is well-known—in the 1700s, Benjamin Franklin, unaware of the danger of the electrical forces he was corralling, sent a key on a kite into the churn of a storm cloud. In the subsequent centuries, through experimentation and calculation, much has been learned about lightning, or plasma.

But unanswered questions about the science of ultrafast processes such as plasma remain, the search for answers hindered in part by the limited technological capacity to capture images. In a setting more controlled than Franklin’s thunderstorm, a research team led by laboratory scientist Zhehui (Jeph) Wang of the Thermonuclear Plasma Physics group at Los Alamos National Laboratory has captured the first sub-nanosecond image of electrical discharge in water. The research was described in a recent paper published in Physical Review Research.

Measured in nanoseconds or even sub-nanoseconds, ultrafast plasma processes are still not well-understood at a basic level. (Beyond those fundamental lines of study, plasma is important in applied contexts such as water treatment or in medicine, to treat disease, or to conduct surgery.) The speed of plasma reactions in water has simply outpaced the ability to capture them by conventional imaging. And, if captured, the optical emission of the plasma burst obscures the details of the process from view. The multidisciplinary team from Los Alamos leveraged a novel electrode-in-water setup built by graduate students Chris Campbell and Xin Tang of Texas A&M University, under the direction of Associate Professor David Staack. Joined by scientists at Argonne National Laboratory, the Los Alamos team used a state-of-the-art synchrotron X-ray Advanced Photon Source (APS) to resolve the imaging problems.

“Most people have never looked at this process with X-rays,” said Wang. “They just use visible light cameras. But you’ll be overwhelmed by the light form the plasma itself. Using X-rays, you can get rid of the background and pinpoint what you really want to see—the plasma.”

The fast-imaging X-rays were able to capture images at the rate above 200,000 images per second, or approximately 153 nanoseconds between images. That rate of speed was just enough to capture one single image of the plasma discharge in water. The exposure time of a single x-ray image is only about 50 picoseconds (a picosecond is one one-thousandth of a nanosecond), allowing for unblurred imaging of the ultrafast phenomena.

“Everybody was surprisingly excited,” Wang said. “Our original goal was just to see how we could find a fast process to test our cameras. The new observation and the underlying science completely surprised us. We had a room full of people using the Argonne beamline at the time—this was before the pandemic—and quite a few people just said, ‘wow.’”

Plasma, also known as the fourth state of matter, is ubiquitous in the universe. In addition to offering a way to study the open questions related to the speed of lightning, plasma-water interactions are expected to produce new matter phases, providing an excellent platform to examine phase transitions as energetic electrons mingle with molecules of water. The ionized particles quickly multiply, which can turn into a run-away process at a yet unfathomed speed. Where do the first electrons emerge from the electrode? What are the roles of the surface condition of the electrode? With the answers too complex to model or calculate using even the most modern computers, ultrafast imaging provides a shortcut to reveal one of the long-standing secrets of the natural world.

The simple experimental platform of the electrode-in-water setup also offers ample opportunities for multi-disciplinary materials science research. One potential area of research is the very high energy density produced by plasma-water interactions in the experiment, despite less than 1 joule (J) of energy being involved. The sizes of the lightning in water are only fractions of a millimeter. Another example of an unresolved question potentially bearing on materials science is how the new phases return to equilibrium, and at what rate.

Several graduate students and postdoctoral researchers played key roles in the initial success of the project. Chris Campbell recently joined the Laboratory as a senior graduate research assistant. Xin Tang recently was awarded a doctoral degree. Another postdoctoral fellow on the project, Yancey Sechrest, is now a staff scientist at the Laboratory. For Wang, the exploration of plasma and fast-imaging has additional benefits in creating a talent pipeline that can shape the Laboratory’s discovery science for years to come.

With the fast-imaging capability demonstrated, Wang is looking ahead to further refining the experimentation and widening the scope of the research. Controlling the experimental environment is a primary consideration—for instance, improving the construction of the electrode in materials and geometry, and trying different chemical solutions. In addition, improved fast-imaging capabilities can be employed through advances in imaging hardware and software. Given fast-imaging’s importance in the Lab’s mission, including mesoscale science, shockwave experiments, and laser-driven experiments such as inertial confinement fusion research, Wang hopes that researchers in predictive science, simulation, and even artificial intelligence may also benefit from future iterations of the research.

Through the novel combination of the X-ray synchrotron, ultrafast imaging, and the compact plasma device, the structures and properties of lightning in water may now be better understood, in addition to a broad range of scientific possibilities that researchers can truly visualize for the first time.”

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