Narrow Energy-Spread Proton Beams Generated in a Gas Jet by High-Power CO$_{2}$ Laser Pulses

At the UCLA Neptune Laboratory, we have investigated laser driven ion acceleration using a high-power CO$_{2}$ laser pulse in a H$_{2}$ gas jet tuned around the critical plasma density of 10$^{19}$cm$^{-3}$ for 10$\mu $m light. The CO$_{2}$ laser pulses consist of a train of 3ps pulses separated by 18ps with a peak power of up to 4TW and total energy of 50J [1]. Protons have been accelerated from this interaction to energies up to 22MeV, which far exceeds that predicted by ponderomotive force scaling for our vacuum a$_{o}\sim $2. Furthermore, these high energy protons are contained within an energy spread of $\Delta $E/E$_{FWHM} \quad \sim $ 1{\%}, and have an estimated transverse emittance of down to $\sim $1mm$\cdot $mrad. The evolution of the plasma density profile was probed with 532nm interferometry revealing a steep rise ($<$ 10 $\lambda )$ to overcritical densities followed by long exponential fall on the back side of the plasma. 2D OSIRIS simulations run with the experimentally measured plasma density profile have uncovered a multistage process for the production of monoenergetic protons based on the shock acceleration mechanism which will be discussed.\\[4pt][1] D. Haberberger et. al., Opt. Exp. 18, 17865 (2010)