Fig. 1. Experimental setup.

DRACO PW laser pulses (red) are focused onto a cylindrical cryogenic hydrogen jet target (blue) pre-expanded using pre-pulses of variable time delay τ_pp. A synchronized optical probe laser pulse with 515 nm wavelength (green) was used as a backlighter for shadowgraphy imaging with a spatial resolution of 1.5 μm. Thomson parabola spectrometers (TPS), as well as optional radiocromic film stacks (RCF) were positioned along the laser propagation direction and along the 45^∘ axis for proton energy spectra measurement. A ceramic screen (CS) captured laser light behind the target. Image credit: Greg Stewart, SLAC National Accelerator Laboratory.