Kinetic Simulations of Target Heating, Ionization and Micro-Explosion with High Intensity XFEL Beams

High intensity X-ray Free Electron Lasers (XFEL) are an ideal tool to heat materials directly and isochorically, which can cause them to be modified and damaged irreversibly. During XFEL-matter interactions, the energy of an XFEL beam will be mainly absorbed by photoionization, creating numerous high-energy photo- and Auger electrons. Modelling this process is quite complex since both atomic physics and plasma physics are involved. Atomic collisional-radiative (CR) codes such as FLYCHK are widely used to simulate such processes. However, the CR codes typically assume local thermodynamic equilibrium (LTE) and are limited to zero dimension.
In order to understand the sample damaging mechanisms, we performed two-dimensional kinetic particle-in-cell simulations with radiation transport (PIC-RT) to retrieve the temporal processes of XFEL-matter interactions. The dynamics of XFEL-Matter interactions can roughly divide into three different time scales: 1) electron heating and photoionization by XFEL in ~ 10 fs ; 2) collisional heating and ionization by high-energy photo- and Auger electrons with several keV energy in tens of fs to sub-ps; and 3) collective hydrodynamic explosion driven by ~ TPa thermal pressure from ~100 fs to nanoseconds.
The simulation results are compared to our recent experiment that a variety of samples were irradiated by Japanese XFEL SACLA with intensities on the order of 10^20 W/cm^2. The post-analysis of the irradiated samples showed that large holes with radius sizes more than one order of magnitude higher than the XFEL spot were created for metallic samples. The hole size is also much larger than the stopping range of high energy electrons. According to our PIC-RT simulations, we attribute the generation of such large holes to the micro-explosion process. Kinect simulation of the hole generation with multiple time scales is also useful and complementary to understand the change of X-Ray diffraction pattern in the experiment that infers significant material structural change on femtosecond timescales.