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

John T. Morrison; Enam A. Chowdhury; Kyle D. Frische; Scott Feister; Vladimir M. Ovchinnikov; John A. Nees; Chris Orban; Richard R. Freeman; W. Melvyn Roquemore

doi:10.1063/1.4916493

arXiv · 1501.02261

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

John T. Morrison·Enam A. Chowdhury·Kyle D. Frische·Scott Feister·Vladimir M. Ovchinnikov·John A. Nees·Chris Orban·Richard R. Freeman·W. Melvyn Roquemore

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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 $μ$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.

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John T. Morrison, Enam A. Chowdhury, Kyle D. Frische, Scott Feister, Vladimir M. Ovchinnikov, John A. Nees, Chris Orban, Richard R. Freeman, W. Melvyn Roquemore. 2015-03-15. Backward-propagating MeV electrons from $10^{18}$ W/cm$^2$ laser interactions with water. https://doi.org/10.1063/1.4916493

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