Positron Production Using a Laser-Wakefield Electron Source*

Positron generation using wakefield-accelerated electrons driven into a second mm-scale target was investigated using the Callisto Laser at the Jupiter Laser Facility at Lawrence Livermore National Laboratory. This technique\footnote{G. Sarri et al., Phys. Rev. Lett., 110:255002, Jun 2013. } is in contrast to previous experiments that use direct laser-target interactions to create positron-electron pairs,\footnote{H. Chen et al., Phys. Rev. Lett., 105:015003, Jul 2010} and has the potential to make laser-produced positron sources widely available to smaller scale laboratories. Monte Carlo simulations show a near-collimated ($<$10 mrad) wakefield electron beam produces a positron beam with a significantly larger divergence angle ($>$100 mrad) due to multiple small angle coulomb scattering, resulting in an emitted pair density of $10^{13}$ particles/cm$^3$. At the Callisto Laser, we did not observe a signal consistent with positrons using two different charged particle spectrometers. This could be due to a high noise environment and a large detection threshold.