• I. Pogorelsky, M. Babzien, M.N. Polyanskiy, V. Yakimenko
  BNL, Upton, Long Island, New York
• N. Dover, Z. Najmudin, C.A.J. Palmer, J. Schreiber
  Imperial College of Science and Technology, Department of Physics, London
• G. Dudnikova
  UMD, College Park, Maryland
• M. Ispiryan, P. Shkolnikov
  Stony Brook University, StonyBrook

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 ~10²¹/cc electron density. The terawatt picosecond CO₂ laser operated at Brookhaven's Accelerator Test Facility offers a solution to this problem. At 10 μm laser wavelength, the CO₂ laser shifts the critical plasma density to 10¹⁹/cc 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 2nd harmonic picosecond Nd:YAG laser to reveal stationary soliton-like plasma formations. 2D PIC simulations agree with experimental results and aid in their interpretation.