Inverse-collimated proton radiography for imaging thin materials.

Relativistic, magnetically focused proton radiography was invented at Los Alamos National Laboratory using the 800 MeV LANSCE beam and is inherently well-suited to imaging dense objects, at areal densities >20 g cm-2. However, if the unscattered portion of the transmitted beam is removed at the Fourier plane through inverse-collimation, this system becomes highly sensitive to very thin media, of areal densities <100 mg cm-2. Here, this inverse-collimation scheme is described in detail and demonstrated by imaging Xe gas with a shockwave generated by an aluminum plate compressing the gas at Mach 8.8. With a 5-mrad inverse collimator, an areal density change of just 49 mg cm-2 across the shock front is discernible with a contrast-to-noise ratio of 3. Geant4 modeling of idealized and realistic proton transports can guide the design of inverse-collimators optimized for specific experimental conditions and show that this technique performs better for thin targets with reduced incident proton beam emittance. This work increases the range of areal densities to which the system is sensitive to span from ∼25 mg cm-2 to 100 g cm-2, exceeding three orders of magnitude. This enables the simultaneous imaging of a dense system as well as thin jets and ejecta material that are otherwise difficult to characterize with high-energy proton radiography.

Experiments of the Richtmyer-Meshkov instability.

The Richtmyer-Meshkov instability is caused by a shock interacting with a density-stratified interface. The mixing of the fluids is driven by vorticity created by the interaction of the density and pressure gradients. Because the flow is shock driven, the ensuing mixing occurs rapidly, making experimental measurements difficult. Over the past 10 years, there have been significant improvements in the experimental techniques used in shock-driven mixing flows. Many of these improvements have been driven by modelling and simulation efforts, and others have been driven by technology. High-resolution measurements of turbulence quantities are needed to advance our understanding of shock-driven flows, and this paper reviews the current state of experimental diagnostics, as well as paths forward in studying shock-driven mixing and turbulence.