Coherent interactions of free electrons and matter: toward tunable compact x-ray sources

Abstract

Compact laboratory-scale x-ray sources still rely on the same fundamental principles as did the first x-ray tubes developed more than a century ago. In recent years, significant research and development has focused on large-scale x-ray sources such as synchrotrons and free-electron lasers, leading to the generation of high-brightness coherent x-rays. However, the large size and high costs of such sources prevent their widespread use. The quest for a compact and coherent x-ray source has long been a critical objective in modern physics, gaining further importance in recent years for industrial applications and fundamental scientific research. Here, we review the physical mechanisms governing compact coherent x-ray generation. Of current interest are coherent periodic interactions of free electrons in crystalline materials, creating hard x-rays via a mechanism known as parametric x-ray radiation (PXR). Over the past decade, x-ray sources leveraging this mechanism have demonstrated state-of-the-art tunability, directionality, and broad spatial coherence, enabling x-ray phase-contrast imaging on a compact scale. The coming years are expected to show substantial miniaturization of compact x-ray sources, facilitated by progress in electron beam technologies. This review compares the most promising mechanisms used for hard x-ray generation, contrasting parametric x-ray radiation with inverse Compton scattering and characteristic radiation from a liquid-jet anode. We cover the most recent dvancements, including the development of new materials, innovative geometrical designs, and specialized optimization techniques, aiming toward x-ray flux levels suitable for medical imaging and x-ray spectroscopy at compact scales