Intense underwater laser propagation is fundamental for many applications, e.g. laser surgery and laser acoustic generation. Meter-scale nonlinear propagation of picosecond ultraviolet pulses in water was measured and modeled. The SNOPROP code incorporates a 2D azimuthally-symmetric model of nonlinear laser pulse propagation in liquid water, solves coupled nonlinear Schrödinger propagation equations, and includes: stimulated Raman scattering, Kerr self-focusing, group-velocity dispersion, and both optical field and collisional cascade ionization. The SNOPROP liquid water chemistry model was recently augmented to include a set of energy-dependent cross sections, which it uses to calculate collisional rates and both conduction band electron density and temperature. Recent implementation of radially-varying computational grids is expected to improve SNOPROP efficiency, and comparison with published experimental data can provide validation. SNOPROP modeling fidelity will be enhanced by planned incorporation of ionization rates from ongoing experimental characterization of the nonlinear response of water.
Long-wave infrared (LWIR) lasers based on high-voltage pulsed discharges in the high-pressure CO2 gas have reached multi-TW peak power. As optical pumping appears to be a more viable pathway toward ultrahigh power and high repetition rate, we investigated multiple apparatus including direct and indirect optical pumping. Indirect optical pumping through stimulated Raman scattering in N2 gas could be efficient, and is relatively insensitive to the pump wavelength. On the other hand, laser technologies for direct optical pumping have higher maturity levels at wavelengths of ~ 1.4 μm and ~ 2.0 μm among multiple excitation bands in the mid-IR wavelength.
We present our recent experimental results of monoenergetic protons accelerated from the interaction of an intense terawatt CO2 laser pulse with a near-critical hydrogen gas target, with its density profile tailored by a hydrodynamic shock. A 5-ns Nd:YAG laser pulse is focused onto a piece of stainless steel foil mounted at the front edge of the gas jet nozzle orifice. The ablation launches a spherical shock into the near-critical gas column, which creates a sharp density gradient at the front edge of the target, with ~ 6X local density enhancement up to several times of critical density within ~<100 microns. With such density profile, we have obtained monoenergetic proton beams with good shot-to-shot reproducibility and energies up to 1.2 MeV.
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