Ionization dynamics of cluster targets irradiated by x-ray free-electron-laser light

Interactions of x-ray free-electron laser (XFEL) light with a single cluster target are numerically investigated by using a three-dimensional particle-in-cell code. The plasma dynamics as well as relevant atomic processes are taken into account, such as photoionization, the Auger effect, collisional ionization and relaxation, and field ionization. It is found that as the XFEL intensity increases to as high as ∼10²¹ (photons/pulse)/mm², the field ionization, which is the dominant ionization process over the other atomic processes, leads to rapid target ionization. The target damage due to the irradiation by XFEL light is numerically evaluated, which gives an estimation of the XFEL intensity so as to suppress the target damage within a tolerable range for imaging.