Luca Antonelli (York): X-ray Phase-Contrast Imaging for Implosion Physics
Abstract: Recent and enormous investment in laser facilities across the world is enabling the exploration of the material properties under extreme conditions. This includes the study of the materials underhigh pressure relevant to planetary physics, the generation of ultra-high magnetic fields, particle-pair production in high intensity laser fields, Weibel-instability-mediated collisionless shocks and inertial confinement fusion (ICF). Validating theoretical models and numerical simulations requires high quality data and this often demands new diagnostic capabilities. Techniques that combine good spatial and temporal resolution such as X-ray absorption radiography can yield a detailed picture of an experiment, though they are compromised by contrast which limits the range of observable densities. It is possible to extend the density resolution of X-ray imaging by monitoring the phase-shift of the X-ray photons as they pass through the target. Absorption contrast imaging is dependent on the imaginary part of a materials refractive index whilst phase contrast imaging uses the real part of the refractive index. To access the real part it is necessary to ensure the X-ray source has some degree of transverse spatial coherence. This spatial coherence leads to a diagnostic that is particularly sensitive to density gradients perpendicular to the line of sight. It is an ideal technique for studying shock-waves, density filaments and material interfaces, as demonstrated on X-ray free electron lasers where monochromatic intense coherent X-ray beams are available.We show that phase contrast enhanced imaging is possible on high energy lasers using broadband bremsstrahlung radiation sources. We discuss propagation-based imaging and the key elements for successfully deploying phase contrast imaging on multiple systems such as Phelix-GSI in Germany,Vulcan in the UK and Omega EP in the USA. As an application, we compare absorption and phase contrast imaging with a hybrid technique of phase-contrast enhanced radiography to study a Shock Ignition Implosion.