Off-harmonic optical probing of high intensity laser interaction with hydrogen targets using a stand-alone probe laser system

The development of high-intensity short-pulse lasers in the Petawatt regime offers the possibility to design new compact accelerator schemes by utilizing high-density targets for the generation of ion beams with multiple 10 MeV energy per nucleon. The optimization of the acceleration process demands comprehensive exploration of the plasma dynamics involved, for example via spatially and temporally resolved optical probing. Experimental results can then be compared to numerical particle-in-cell simulations, which is particularly sensible in the case of cryogenic hydrogen jet targets. However, strong plasma self-emission and conversion of the plasma’s drive laser wavelength into its harmonics often masks the interaction region and interferes with the data analysis. Recently, the development of a stand-alone and synchronized probe laser system for off-harmonic probing at the DRACO laser operated at the Helmholtz-Zentrum Dresden–Rossendorf showed promising performance.
Here, we present an updated stand-alone probe laser system applying a compact CPA system based on a synchronized fs mode-locked oscillator operating at 1030 nm, far off the plasma’s drive laser wavelength of 800 nm. A chirped volume Bragg grating (Optigrate Corp) is used as a hybrid stretcher and compressor unit. The system delivers 160 fs pulses with a maximum energy of 0.9 mJ. By deploying the upgraded probe laser system in the laser-proton acceleration experiment with the renewable cryogenic hydrogen jet target, the plasma self-emission could be significantly suppressed while studying the temporal evolution of the expanding plasma jet. Recorded probe images resemble those of z-pinch experiments with metal wires and indicate a sausage-like instability along the jet axis, which will be discussed.