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

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.