Laser Energy Conversion to Solitons and Monoenergetic Protons in Near-critical Hydrogen Plasma*

Recent theoretical and experimental studies point to better efficiency of laser-driven ion acceleration when approaching the critical plasma density regime. Simultaneously, this is the condition for observing solitons: "bubble"-like quasi-stationary plasma formations with laser radiation trapped inside. Exploring this regime with ultra-intense solid state lasers is problematic due to the lack of plasma sources and imaging methods at ~10 cm electron density. The terawatt picosecond CO2 laser operated at Brookhaven's Accelerator Test Facility offers a solution to this problem. At the 10 μm laser wavelength, the critical plasma density is 10 cm, which is attainable with gas jets and can be optically probed with visible light. Capitalizing on this approach, we focused a circular-polarized CO2 laser beam with a0=0.5 onto a hydrogen gas jet and observed monoenergetic proton beams in the 1 MeV range. Simultaneously, the laser/plasma interaction region has been optically probed with a 2 harmonic picosecond Nd:YAG laser to reveal hole boring and stationary soliton-like plasma formations. 2D PIC simulations agree with experimental results and aid in their interpretation.