Electron probing of electromagnetic fields generated by intense laser pulses interacting with a liquid jet target*

Recent results have demonstrated the feasibility of using electron beams from a Laser Wakefield Accelerator (LWFA) to measure electric and magnetic fields on ultrafast timescales. In this work, we use the ultrafast LWFA electron beam from the BELLA HTW system to probe the interaction of a longer-pulse laser (t ~ 200 ps, E ~ 1 J, I ~ 10¹⁴ W/cm²) with a cylindrical water jet at 1 Hz repetition rate. We also investigate the implementation of beam optics to manipulate the probe divergence while preserving good imaging properties. By varying the relative delay, the electron probe images reveal the dynamics of electromagnetic fields generated during the laser-water interaction. In particular, the images initially capture an ionization channel through the water vapor along the laser trajectory, followed by evidence of rapid heating and ablation of the liquid target. The electron probing results help explain some of the discrepancies observed between hydrodynamics simulations and experimental measurements. Additionally, these results show that electron probing can be used to help diagnose kinetic effects and non-equilibrium conditions in high-energy-density plasmas.