Towards direct spatial and intensity characterization of ultra-high intensity laser pulses using ponderomotive scattering of free electrons

TL;DR

This paper introduces a novel method using ponderomotive scattering of free electrons to directly measure the intensity and spatial structure of ultra-high intensity laser pulses, validated through experiments and simulations.

Contribution

It develops analytic models relating electron scattering energies to laser intensity, demonstrates experimental validation, and proposes neural networks for analyzing spatial aberrations.

Findings

Good agreement between direct and indirect intensity measurements

Electron scattering energies depend on gas species

Neural networks can analyze spatial laser structure

Abstract

Spatial distributions of electrons ionized and scattered from ultra-low pressure gases are proposed and experimentally demonstrated as a method to directly measure the intensity of an ultra-high intensity laser pulse. Analytic models relating the peak scattered electron energy to the peak laser intensity are derived and compared to paraxial Runge-Kutta simulations highlighting two models suitable for describing electrons scattered from weakly paraxial beams ($f_{\#}>5$) for intensities in the range of $10^{18}-10^{21}$Wcm$^{-2}$. Scattering energies are shown to be dependant on gas species emphasizing the need for specific gases for given intensity ranges. Direct measurements of the laser intensity at full power of two laser systems is demonstrated both showing a good agreement between indirect methods of intensity measurement and the proposed method. One experiment exhibited the role of spatial aberrations in the scattered electron distribution motivating a qualitative study on the effect. We propose the use of convolutional neural networks as a method for extracting quantitative information of the spatial structure of the laser at full power. We believe the presented technique to be a powerful tool that can be immediately implemented in many high-power laser facilities worldwide.