Stand-alone laser system for off-harmonic optical probing of high intensity laser interaction with cryogenic hydrogen jet targets

The availability of high-intensity short-pulse lasers in the Peta-Watt regime drives the development of new and compact accelerator schemes like for example the generation of multiple 10 MeV proton beams from high-density targets. To optimize the acceleration performance and to pave the way towards medical applications of these particle beams both target and diagnostic development are of great importance. Particularly cryogenic hydrogen jet-targets offer the benefit of being debris-free and capable of high-repetition rate applications. Together with spatially and temporally resolved optical probing techniques this target is most suitable for a comparison to numerical particle-in-cell simulations. However, the strong plasma self-emission often masks the laser-target interaction point and thus complicates the data analysis. Recently, the development of a synchronized stand-alone probe-laser-system operating off the harmonics of the driver laser showed promising performance.
Here we show the performance of an upgraded probe-laser-system operating at a central wavelength of 1030nm far off the fundamental wavelength of the drive laser at 800nm. It consists of a synchronized fs-oscillator and a novel and robust CPA system based on a chirped volume Bragg grating as a hybrid stretcher and compressor unit, chirped mirrors for GDD compensation and a regenerative amplifier with Yb.CaF2 as laser medium. The system delivers 160fs pulse duration together with 0.9mJ energy at a repetition rate of 200Hz.
The application of the upgraded probe-laser-system in an experimental campaign dedicated to laser-proton acceleration together with cryogenic hydrogen jet-targets showed a significant improvement of imaging quality for the laser-target interaction concerning the plasmas self-emission. The recorded probe-images resemble those of z-pinch experiments with metal wires and indicate a sausage-like instability along the hydrogen jet axis.