In extreme conditions, heat does not flow between materials. It bounces off.

A new study supported by the U.S. National Science Foundation shows, for the first time, how heat moves — or rather, doesn't — between materials in a high-energy-density plasma state. The work is expected to provide a better understanding of inertial confinement fusion experiments, which aim to reliably achieve fusion ignition on Earth using lasers. How heat flows between a hot plasma and a material's surface is also important in other technologies, including semiconductor etching and vehicles that fly at hypersonic speeds.

High-energy-density plasmas are produced only at extreme pressures and temperatures. The study shows that interfacial thermal resistance, a phenomenon known to impede heat transfer in less extreme conditions, also prevents heat flow between different materials in a dense, super-hot plasma state. The research is published in Nature Communications and was led by Thomas White, a physicist at the University of Nevada, Reno, and his former doctoral student, Cameron Allen. White is a recipient of an NSF Faculty Early Career Development grant.

"Understanding how energy flows across a boundary is a fundamental question, and this work provides us with new insights into how this happens in the exceptionally energy-dense environments that one finds inside of stars and planetary cores," says Jeremiah Williams, a program director for the NSF Plasma Physics program.

White and Allen's experiment focused on how heat moves between metal and plastic heated to extreme temperatures and pressures. To do this, they used the high-powered Omega-60 laser at the University of Rochester in New York to heat copper foils and emit X-rays, which uniformly heat a metal tungsten wire next to a plastic coating. In their experiment, the tungsten wire was heated to about 180,000 degrees Fahrenheit while its plastic coating remained relatively cool at "only" 20,000 degrees Fahrenheit. Using a series of laser shots with progressively delayed timing, the researchers were able to see if the heat was moving between the tungsten and plastic.

"When we looked at the data, we were totally shocked because the heat was not flowing between these materials," White said. "It was getting stuck at the interface between the materials, and we spent a long time trying to work out why."

The reason was interfacial thermal resistance. The electrons in the hotter material arrive at the interface between the materials carrying thermal energy but then scatter off and move back into the hotter material, explains White.

"High energy laser labs provide an essential tool for developing a precise understanding of these extreme environments — and this has implications for a wide variety of important technologies, from medical diagnostics to national security applications," adds Williams.