Kinetic Modeling of Ultraintense X-ray Laser-Matter Interactions

Abstract

Although hard-x-ray free-electron lasers (XFELs) have only existed since 2009 when the Linac Coherent Light Source (LCLS) at Stanford created its first laser pulse, their unique capabilities have already had a profound impact on the physical, chemical, and biological sciences. The LCLS can produce ultrashort ($< 100$ fs), mJ x-ray laser pulses with more than $10^{12}$ photons each, making it the brightest x-ray source ever produced in a laboratory by several orders of magnitude, and more than a billion times brighter than synchrotron sources. These properties enable XFELs to create and probe well-characterized warm and hot dense plasmas of relevance to high energy density science, planetary science, laboratory astrophysics, relativistic laser plasmas, and fusion research. An x-ray pulse produced by the LCLS or SACLA (Japan) can be intensified to $10^{20}$ W/cm$^2$ when focused to submicron spot sizes, making it possible to isochorically heat solid matter well beyond a million degrees ($>100$ eV) by sequential single-photon inner-shell photoionization and subsequent Auger decay. Several newly developed atomic interaction models including photoionization, K-shell vacancy decay, KLL Auger ionization, and continuum-lowering have been implemented in a particle-in-cell plasma simulation code, PICLS---which self-consistently solves the x-ray transport---to enable the simulation of non-thermal, solid density, x-ray laser-driven plasmas, offering unique insight into experimental regimes of interest in which the plasma dynamics have a significant effect on the thermodynamic properties of the system. The code is validated against the results of two recent experiments and is used to simulate the ultraintense x-ray heating of solid iron targets in anticipation of an upcoming experimental campaign at the LCLS.