Intensity-dependent resonant transmission of x-rays in solid-density aluminum plasma

X-ray free-electron lasers(XFELs) provide unique opportunities to generate and investigate dense plasmas. The absorptionand transmission properties of x-rayphotons in dense plasmas are important in characterizing the state of the plasmas. Experimental evidence shows that the transmission of x-rayphotons through dense plasmas depends greatly on the incident XFEL intensity. Here, we present a detailed analysis of intensity-dependent x-raytransmission in solid-density aluminum using collisional-radiative population kinetics calculations. Reverse saturable absorption(RSA), i.e., an increase in x-ray absorptionwith intensityhas been observed for photon energiesbelow the K- absorptionedge and in the intensityrange of 1016–1017 W/cm2 for XFEL photons with 1487 eV. At higher intensities, a transition from RSA to saturable absorption(SA) is predicted; thus, the x-ray absorptiondecreases with intensityabove a threshold value. For XFEL photon energiesof 1501 eV and 1515 eV, the transition from RSA to S...