A next-generation X-ray imager for fusion ignition experiments

Inertial confinement fusion experiments create nuclear fusion by rapidly heating and compressing tiny fuel pellets until they implode under extreme pressure and temperature. Successful ignition, or the sustainment of fusion without additional pellet heating, was achieved for the first time in 2022 at Lawrence Livermore National Laboratory’s National Ignition Facility, the most powerful facility of its kind. Given the importance of this recent breakthrough, mapping how the temperature evolves across space and time in these experiments could accelerate progress in both basic research and application for industry. Ravinet et al. introduced an X-ray–based diagnostic method that measures fusion-plasma temperatures with improved spatial resolution.

The authors’ device, the Toroidal X-ray Imager, works by using multilayer interference coatings on mirrors used in its detectors. These nanoscale layers are optimized to capture three distinct X-ray energy bands for determining the temperature. The device is the first diagnostic capable of measuring all three energy bands at a single location.

“Our aperiodic stacks are made of up to 80 layers with different thicknesses,” author Franck Delmotte said. “We improved the spatial resolution of the imager with two toroidal mirrors for each channel and extended the instrument’s field of view by designing two different coatings for the two mirrors of each channel.”

The multilayer coatings were deposited by magnetron sputtering under conditions optimized to produce exceptionally smooth interfaces with roughness below 0.4 nanometer, minimizing X-ray scattering. Layer thicknesses were calibrated using laboratory X-ray reflectometry with a copper source. Synchrotron-based X-ray reflectometry then provided high-precision measurements of each channel’s spectral response, confirming that the coatings performed as designed.

After final alignment and mounting, the Toroidal X-ray Imager will be transported to the National Ignition Facility for inertial confinement fusion experiments.

Source: “Multilayer coatings for a multi-channel Wolter-like x-ray imager with large field of view and high resolution: Experimental development and study of the instrument performance,” by N. Ravinet, E. Meltchakov, A. Lejars, L. Cibik, C. Gollwitzer, T. N. Tran Thi, M. Vallet, and F. Delmotte, Review of Scientific Instruments (2026). The article can be accessed at https://doi.org/10.1063/5.0316988 .