Electron-Positron Jets Created by Ultra-Intense Lasers

For decades, positrons could only be produced in the laboratory using either radioactive sources or electron accelerators. Recently, it has been shown that ultra-intense laser pulses (I$\lambda ^{2} >$ 10$^{19}$ W$\cdot \mu $m$^{2}$/cm$^{2})$ incident on solid targets can also provide copious positrons ($\sim $ 10$^{10})$ via pair production.\footnote{ H. Chen, S.C. Wilks, J.D. Bonlie, E.P. Liang, J. Myatt, D.F. Price, D.D. Meyerhofer, and P. Beiersdorfer, Phys. Rev. Lett., \textbf{102}, 105001(2009).} While time-integrated energy spectra of the positrons have been experimentally observed, the exact character of the relativistic positron-electron (e$^{+}$-e$^{-}$) beam is only now becoming clear. The detailed physics behind the generation and acceleration of the positrons will be presented. Particle-in-cell simulations using the hybrid code Lsp\footnote{D.R. Welch, et. al, Phys. Plasmas \textbf{13}, 063105 (2006).} of the entire process, from laser-generated electrons to positrons ejected from the solid target, compare favorably to observed energy distributions. Using this benchmark as a base, the actual spatial-temporal energy and density profiles of the ejected positron (and electron) beam are investigated, where space charge is found to be an important effect in determining the properties of this ``jet.'' In particular, this jet is found to consist of a leading, dense electron bunch that is immediately followed by a nearly charge-neutral e$^{+}$-e$^{-}$ beam. In addition to exploring the exciting possibility of using this source to create e$^{+}$-e$^{-}$ plasmas, injection of this jet into a low-density plasma behind the target reveals a strong plasma wakefield effect that dominates the beam-plasma interaction. This, in turn, suggests ways to use this jet as a source of positrons for small-scale laser wakefield accelerator research. This work was performed in collaboration with H. Chen, A.J. Link, and D.R. Welch under the auspices of the U.S. DOE under Contract DE-AC52-07NA27344 and LDRD 10-ERD-044.