Manipulation of laser-generated energetic proton spectra in near critical density plasma

Abstract

We present simulations that demonstrate the production of quasi-monoenergetic proton bunches from the interaction of a CO2 laser pulse train with a near-critical density hydrogen plasma. The multi-pulse structure of the laser leads to a steepening of the plasma density gradient, which the simulations show is necessary for the formation of narrow-energy spread proton bunches. Laser interactions with a long, front surface, scale-length (≫ c/ωp) plasma, with linear density gradient, were observed to generate proton beams with a higher maximum energy, but a much broader spectrum compared to step-like density profiles. In the step-like cases, a peak in the proton energy spectra was formed and seen to scale linearly with the ratio of laser intensity to plasma density.