Understanding the properties and behaviour of matter at extreme, Mbar pressures is essential to describe a wide range of physical phenomena.  During the past decade, following outstanding progress in high-power laser engineering, several physical fundamental problems have been pointed out in this field. In particular, significant interest arises in shock physics, which is of prime importance for various domains such as astrophysics, inertial confinement fusion, planetology and technology of advanced material creation. However, to examine the matter under extremely high energy concentration in space and time requires the implementation of diagnostics with the capability to resolve the processes lasting not longer than picoseconds, with micron scale spatial gradients and in a strongly vibrant environment. As recent experiments testify, a significant part of diagnostic challenges can be overcome with the use of newly developed high-resolution X-ray radiography with X-ray free-electron lasers (XFEL) as a probe beam.
The X-ray radiography method is often non-alternative in studies of various hydrodynamic phenomena in laser plasma, including inertial fusion implosions and plasma instabilities, shock waves and equation-of-state measurements, or astrophysically relevant supersonic plasma flows. They are dynamic, short-lived, low-contrast, containing micron features of interest, while fully opaque for visible light. Taking into account the demand to provide quantitative measurements on plasma density or compression, the XFEL is an ideal source of ultra-narrow spectral bandwidth; high coherence and extreme brightness pulses that enable diffraction-enhanced imaging of objects with low-density gradients. The femtosecond duration of XFEL pulses provides a high temporal resolution far exceeding a common nanosecond timescale of plasma hydrodynamics.
XFEL’s capabilities in plasma radiography became even more remarkable when accompanied by an appropriate high-resolution fluorescent crystal detector. At SCALA  (SPring-8 Angstrom Compact free electron Laser) XFEL for  the HED platform, it was developed the new phase-contrast imaging scheme consists on the coupling of the sub-micron resolution LiF crystal detector [1,2] with the unique sub-micron resolution scintillator-based x-ray imaging detector (CCD) [3]. Without using x-ray focusing optics the scheme is very compact and allows 0.4 µm spatial resolution in the field of view defined by the entire size of the XFEL beam.
As a particular results, a Rayleigh-Taylor unstable system can be now studied and tracked with ~ 1 mm resolution down to dissipation phase. The unexpexted features in the turbulent power spectrum had been already observed [4]. The laser induced shock wave propagation can be mapped in such detail like a paired plastic-elastic structure directly observed. That allowed the fitting and validation of a failure model for diamond in the range of several Mbar [5].
At nowadays X-ray radiography diagnostics significantly progressed in parallel with development of new imaging schemes and HED physics successfully addressing all the demands and sometimes inspiring the progress in studies of matter under extreme conditions.



 

X-ray radiography, phase-contrast imaging, XFEL, hydrodynamic phenomena, HEDP