Off-harmonic optical probing of high intensity laser-matter interaction with 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 to generate proton beams with multiple 10 MeV energy per nucleon. The optimization of the acceleration process calls for a 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 well suited in the case of cryogenic hydrogen jet targets [1]. 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 synchronizable probe laser system for off-harmonic probing at the DRACO laser at the Helmholtz-Zentrum Dresden – Rossendorf showed promising performance [2]. Here, we present an updated stand-alone probe laser system applying a compact CPA system based on a synchronized fs mode-locked oscillator (Light Conversion) 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 subsequent diode pumped regenerative Yb:CaF2 laser amplifier includes a spectral shaping element and chirped mirrors for GDD compensation. The system delivers 160 fs pulses with a maximum energy of 0.9 mJ and thus extends the recent developments [3] into the sub 200 fs region. Additionally, we present recent experimental results deploying the upgraded probe laser system and its harmonics in an experiment dedicated to laser-proton acceleration from a renewable cryogenic hydrogen jet at the DRACO laser.