PROTON ACCELERATION IN CO2 LASER-PLASMA INTERACTIONS AT CRITICAL DENSITY∗

Over the last several years, the Target Normal Sheath Acceleration (TNSA) mechanism in solid density plasmas produced by a laser pulse has achieved proton energies up to 10’s of MeV and quasi-monoenergetic beams at lower energies. Although solid-target experiments have demon-strated high-charge and low-emittance proton beams, little work has been done with gaseous targets which in princi-ple can be operated at a very high repetition frequency. At the Neptune Laboratory, there is an ongoing experiment on CO2 laser driven proton acceleration using a rectangular (0.5x1.2mm) H2 gas jet as a target. The main goal is to study the coupling of the laser pulse into a plasma with a well defined density in the range of 0.5 to 2 times critical density and characterize the corresponding spectra of ac-celerated protons. Towards this end, the Neptune TW CO2 laser system is being upgraded to produce shorter 1-3ps pulses. These high-power pulses will allow us to investi-gate acceleration of protons via the TNSA and Radiation Pressure Acceleration mechanisms as well as their combi-nation. The current status of the proton source experiment will be presented