Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

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The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-ex
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The categories of this paper are: funct-an
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The categories of this paper are: patt-sol
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The categories of this paper are: comp-gas
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The categories of this paper are: quant-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-th
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The categories of this paper are: hep-lat
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The categories of this paper are: cs
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The categories of this paper are: solv-int
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The categories of this paper are: ao-sci
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The categories of this paper are: cmp-lg