Ultrafast X-ray imaging of filamentation instabilities in high-intensity laser-solid interactions*

The interaction of ultraintense lasers with solid-density targets is actively being explored for a wide range of applications, from compact ion acceleration to fusion energy. Current instabilities driven by the laser accelerated relativistic electrons in the dense plasma are thought to play an important role in electron transport and plasma heating. The same instabilities play an important role in magnetic field amplification in astrophysical plasmas. Despite their interest, so far, the direct observation of these instabilities has been challenged by the associated ultrafast temporal and small spatial scales and by the high density of the plasma for which conventional optical diagnostics are not appropriate. We present the results of experiments conducted at SLAC's Matter in Extreme Conditions (MEC) end station that combine a high-intensity optical laser with the Linac Coherent Light Source (LCLS) X-ray laser. Using the new MEC X-ray Imager, we have successfully imaged for the first time the solid-density region of the laser-target interaction, achieving a spatial resolution of <200 nm and following in detail the plasma evolution from sub-ps to >100 ps. In particular, we resolve the development and evolution of the current filamentation instability caused by the momentum anisotropy of laser-accelerated relativistic electrons and the subsequent background return current. The wavelength and density modulation of the filaments increases in time due to the nonlinear evolution and filament merging. The results will be discussed and compared with theory and simulations providing an unprecedented characterization of high-intensity laser-solid interactions and associated relativistic plasma phenomena.