  Here we propose to exploit the low energy bandwidth, small wavelength and
penetration power of ultrashort pulses from XFELs for resonant Small Angle
Scattering (SAXS) on plasma structures in laser excited plasmas. Small angle
scattering allows to detect nanoscale density fluctuations in forward
scattering direction. Typically, the SAXS signal from laser excited plasmas is
expected to be dominated by the free electron distribution. We propose that the
ionic scattering signal becomes visible when the X-ray energy is in resonance
with an electron transition between two bound states (Resonant coherent X-ray
diffraction, RCXD). In this case the scattering cross-section dramatically
increases so that the signal of X-ray scattering from ions silhouettes against
the free electron scattering background which allows to measure the opacity and
derived quantities with high spatial and temporal resolution, being
fundamentally limited only by the X-ray wavelength and timing. Deriving
quantities such as ion spatial distribution, charge state distribution and
plasma temperature with such high spatial and temporal resolution will make a
vast number of processes in shortpulse laser-solid interaction accessible for
direct experimental observation e.g. hole-boring and shock propagation,
filamentation and instability dynamics, electron transport, heating and
ultrafast ionization dynamics.
