Using time-integrated Kα images to study refluxing and the extent of pre-plasmas in intense laser-plasma experiment

Abstract

We report the results of an experimental and numerical modeling study of the formation of time-integrated Kα images by electrons excited during an intense laser-plasma interaction. We report the use of the spatial structure of time-integrated Kα images to quantitatively characterize the pre-plasma profile near the critical surface and to verify the near elimination of back-surface refluxing from targets when a thick layer of a low-Z material is attached to the back. The time integrated Kα images are found to be sensitive to the relative separation between the critical surface and the bulk target, permitting a single parameter exponential pre-plasma scale length to be determined by fitting to experimental results. The refluxed electrons affect different parts of the Kα images in a manner that varies depending on the location of the refluxing. We use these properties to characterize refluxing also by fitting to experimental results. Experiments were performed using the Titan laser at the Lawrence Livermore National Laboratory and the simulations used a customized version of the hybrid-PIC code, LSP. We find good quantitative match between experiment and simulation.