Backward-propagating MeV electrons from $10^{18}$ W/cm$^2$ laser interactions with water

TL;DR

This study experimentally demonstrates the generation of backward-propagating MeV electrons and high-energy x-rays from water targets using high-intensity laser pulses, supported by simulations showing beam-like electron behavior.

Contribution

It provides new insights into backward electron acceleration and x-ray production at relativistic laser intensities with water targets, highlighting the role of pre-pulse suppression.

Findings

Backward MeV electrons produce high-energy x-rays exceeding 800 keV.

High-energy electrons are generated in the backward direction during laser-water interactions.

Pre-pulse attenuation reduces high-energy x-ray emission.

Abstract

We present an experimental study of the generation of $\sim$MeV electrons opposite to the direction of laser propagation following the relativistic interaction at normal incidence of a $\sim$3 mJ, $10^{18}$ W/cm$^2$ short pulse laser with a flowing 30 $\mu$m diameter water column target. Faraday cup measurements record hundreds of pC charge accelerated to energies exceeding 120 keV, and energy-resolved measurements of secondary x-ray emissions reveal an x-ray spectrum peaking above 800 keV, which is significantly higher energy than previous studies with similar experimental conditions and more than five times the $\sim$110 keV ponderomotive energy scale for the laser. We show that the energetic x-rays generated in the experiment result from backward-going, high-energy electrons interacting with the focusing optic and vacuum chamber walls with only a small component of x-ray emission emerging from the target itself. We also demonstrate that the high energy radiation can be suppressed through the attenuation of the nanosecond-scale pre-pulse. These results are supported by 2D Particle-in-Cell (PIC) simulations of the laser-plasma interaction that exhibit beam-like backward-propagating MeV electrons.