  The extremely high power densities and short durations of single pulses of
x-ray free electron lasers (XFELs) have opened new opportunities in atomic
physics, where complex excitation-relaxation chains allow for high ionization
states in atomic and molecular systems, and in dense plasma physics, where XFEL
heating of solid-density targets can create unique dense states of matter
having temperatures on the order of the Fermi energy. We focus here on the
latter phenomena, with special emphasis on the problem of optimum target design
to achieve high x-ray heating into the warm dense matter (WDM) state. We report
fully three-dimensional simulations of the incident x-ray pulse and the
resulting multielectron relaxation cascade to model the spatial energy density
deposition in multicomponent targets, with particular focus on the effects of
nonlocal heat transport due to the motion of high energy photoelectrons and
Auger electrons. We find that nanoscale high-Z/low-Z multicomponent targets can
give much improved energy density deposition in lower-Z materials, with
enhancements reaching a factor of 100. This has three important benefits.
First, it greatly enlarges the thermodynamic parameter space in XFEL x-ray
heating studies of lower-Z materials. Second, it allows the use of higher probe
photon energies, enabling higher-information content X-ray diffraction (XRD)
measurements such as in two-color XFEL operations. Third, while this is merely
one step toward optimization of x-ray heating target design, the demonstration
of the importance of nonlocal heat transport establishes important common
ground between XFEL-based x-ray heating studies and more traditional laser
plasma methods.
