Laser-Induced Cavities and Solitons in Overcritical Hydrogen Plasma

A picosecond CO2 laser was used successfully in a number of experiments exploring advanced methods of particle acceleration. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam's energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼1021 cm-3, corresponding to laser wavelength λ = 1 μm. CO2 laser offers a solution. The CO2 laser's 10 μm wavelength shifts the critical plasma density to 1019 cm-3, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO2 laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1-2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.