Journal
PHYSICS OF PLASMAS
Volume 30, Issue 5, Pages -Publisher
AIP Publishing
DOI: 10.1063/5.0142239
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We present the results of proton acceleration from hydrogen plasmas irradiated by intense CO2 laser, with the density gradient modified by Nd:YAG laser ablation-driven hydrodynamic shocks. The experimental results agree well with the simulations, but certain laser shots produced unexpectedly high proton energies. This might be attributed to nonlinear propagation effects and focusing of laser pulses through the plasma channel formed by preceding pulses.
We report on proton acceleration from intense CO2 laser-irradiated hydrogen plasmas at near-critical densities, with the density gradient steepened by Nd:YAG laser ablation-driven hydrodynamic shocks. While the experimental results, such as the quasi-monoenergetic proton spectra and their scaling with respect to the laser energy, are generally in agreement with the simulations, certain laser shots produced significantly higher proton energies than anticipated during the experiment. The increased proton energy may be linked to nonlinear propagation effects in the steepened plasma density ramp before the critical surface, including relativistic self-focusing and, for the case of temporally-structured laser pulses observed in the experiment, focusing of the trailing pulse through the plasma channel formed by the leading pulse 25 ps ahead. The occurrence of channel focusing in the underdense hydrogen plasma is supported by a subsequent pump-probe experiment with a dark-field imaging technique, where the formation of ion channels was observed after the passage of an intense CO2 laser pulse.
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