Simulations of collisional effects in an inner-shell solid-density Mg X-ray laser

Abstract

Inner-shell K α X-ray lasers have been created by pumping gaseous, solid, and liquid targets with the intense X-ray output of free-electron lasers (FELs). For gaseous targets lasing relies on the creation of K -shell core holes on a time-scale short compared with filling via Auger decay. In the case of solid and liquid density systems, collisional effects will also be important, affecting not only populations but also line-widths, both of which impact the degree of overall gain, and its duration. However, to date, such collisional effects have not been extensively studied. We present here initial simulations using the CCFLY code of inner-shell lasing in solid-density Mg, where we self-consistently treat the effects of the incoming FEL radiation and the atomic kinetics of the Mg system, including radiative, Auger and collisional effects. We find that the combination of collisional population of the lower states of the lasing transitions and broadening of the lines precludes lasing on all but the K α of the initially cold system. Even assuming instantaneous turning on of the FEL pump, we find the duration of the gain in the solid system to be sub-femtosecond. This article is part of the theme issue ‘Dynamic and transient processes in warm dense matter’.

Dynamic and transient processes in warm dense matter

“…More recently, and of particular significance to the work in this issue, the advent of X-ray free-electron lasers has made it possible to achieve volumetric heating on sub-picosecond timescales [ 45 , 47 ]. Using this property, Ren et al [ 48 ] discuss the application of inner shell photoionization to creation of a solid density X-ray laser using volumetric pumping with a free electron laser, with particular attention paid to collisional effects of photo-ejected electrons, an issue of relevance to volumetric X-ray heating of WDM. In another paper, Hu et al [ 49 ] discuss electron-lattice equilibration within the context of ultra-fast X-ray diagnostics, but again, with physics encompassing processes relevant to WDM production.…”