Over 200 years ago, the famous mathematician Joseph Fourier described a phenomenon related to how heat is transferred between materials. That phenomenon has now been documented for the first time in high-energy-density (HED) plasmas, where matter exists at extreme pressures and temperatures – conditions found in fusion experiments, materials processing, and even deep inside planets.
Two weeks ago, Thomas White, the Clemons-Magee Endowed Professor in Physics at the ÍƼöÐÓ°ÉÔ´´, and his former doctoral student Cameron Allen published their article .
It is fairly common knowledge that heat flows from warmer to cooler objects, and physicists have operated with the assumption that would be the case for dense plasmas. However, in engineering fields, a well-known phenomenon impedes heat transfer between two objects: interfacial thermal resistance (ITR). For example, in computer chip manufacturing, efficient cooling is critical to prevent overheating and maintain performance, making ITR a major challenge. Whether similar effects occur in dense plasmas became a key focus of Allen’s research.
White and Allen wanted to study how heat moved between metal and plastic heated to extreme temperatures and pressures. To do this, they set up an experiment where an energetic laser was used to heat up copper foils and emit X-rays, which uniformly heat up a metal tungsten wire next to a plastic coating. In their setup, the tungsten wire was heated to about 180,000 degrees Fahrenheit when struck by the X-rays, while its plastic coating remained relatively cool at “only” 20,000 degrees Fahrenheit. By imaging a series of laser shots with progressively delayed timing, the physicists could track how heat moved (spoiler alert: it didn’t) between the tungsten and the 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 heat was not moving between the two materials was ITR, and White explained that 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.
“Heat does not very easily move between the two materials,” White said. “And to our surprise, this phenomenon was first suggested 200 years ago by Fourier.”
Allen, who now works at Los Alamos National Laboratory, and White conducted this experiment on the Omega-60 laser, located at the Laboratory for Laser Energetics in Rochester, NY. It is the second highest-energy laser in the United States and houses 60 high-powered laser beams within a facility spanning over 60,000 square feet. Use of the laser is extremely competitive, so researchers typically get only a single day to conduct their research project. Researchers try to complete as many laser shots as possible in that day, which includes setting up the experiment by mounting and positioning the target, pointing the laser beams, calibrating diagnostics, and ultimately firing the laser. This process takes about an hour. The facility is open for 12 hours, so the researchers can conduct 11 shots.
“It’s incredibly nerve-wracking day, especially for my graduate students,” White said.
White said this discovery has broad applications, particularly in inertial confinement fusion experiments, which aim to achieve fusion reactions on Earth using laser-driven compression. These experiments are critical for both national security applications and rely on complex targets composed of multiple layers of different materials.
“All I see there is an interface, next to an interface, next to an interface,” White said. “If heat isn’t flowing through these boundaries as expected, this may be the reason for current discrepancies between simulations and the experiments.”
"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," National Science Foundation Plasma Physics Program Director Jeremiah Williams said. "High energy laser labs provide an essential tool for developing a precise understanding of these extreme environments — and this has implications for wide variety of important technologies, from medical diagnostics to national security applications."
The research was funded by grants from the National Science Foundation (NSF) and the Department of Energy. White’s work was supported by an NSF CAREER award, which provides funding for early-career scientists conducting transformative research. The experiment was conducted at the Omega Laser Facility with the facility time through the National Laser Users’ Facility program.
