Ultrafast X-ray imaging of the current filamentation instability in high-intensity laser-solid interactions*

The interaction of high-intensity lasers with solid-density targets is actively being explored for applications in compact ion acceleration, fusion energy and astrophysics. Of particular interest is the current filamentation instability due to its role in fundamental plasma processes, such as plasma heating, electron transport and magnetogenesis. However, the intricate microphysics responsible for magnetic field seeding and amplification in laboratory environments remain poorly understood due to the difficulty of experimental measurements, which have been challenged by small spatio-temporal scales and high plasma density.

We present the results of an experiment that combined a high-intensity optical laser with an X-ray laser in a pump-probe configuration. Achieving a spatial resolution of <200nm we succesfully probed in detail the plasma evolution from 0.5 to 800 ps. In particular, we imaged the evolution of the current filamentation instability driven by counter-streaming electrons within the solid density plasma. The measured filament density perturbations show an asymmetry in their density distributions, revealing the importance of ion motion to facilitate the instability growth. Additionally, the filament wavelength is observed to increase in time due to the nonlinear evolution and subsequent filament merging. The results provide a new level of characterization of the current filamentation instability in solid-density plasmas to aid the developement of future theoretical models.