Narrow energy spread, 25MeV protons from the interaction of a time-structured CO$_{2}$ laser pulse with a gas target

Experimental results and 2D OSIRIS simulations of laser-driven proton acceleration from the interaction of a time-structured 10$\mu $m CO$_{2}$ laser pulse train and a gaseous target are presented. The wavelength of a CO$_{2}$ laser provides a unique opportunity to change the target density from 0.5 to 5n$_{cr}$ in a controlled manner by changing the H$_{2}$ gas jet pressure. The CO$_{2}$ laser pulses consist of a train of 3ps pulses separated by 18ps with a peak power of $\sim $4TW and a total energy of $\sim $50J. The initial results show the production of proton energies of up to 25MeV, which far exceeds that predicted by ponderomotive force scaling for an a$_{o}\sim $2. Furthermore, in the density range around 2n$_{cr}$, these high energy protons are contained within a narrow energy spread of $\Delta $E/E $\sim $ 10{\%}. These results are attributed to the unique time structure of the CO$_{2}$ laser pulses, underdense LPI's such as self-focusing due to large P/P$_{cr}$ values, and profile steepening/hole boring.