Multiple light scattering in metallic ejecta produced under intense shockwave compression

Abstract

A roughened metallic plate, subjected to intense shock wave compression, gives rise to an expanding ejecta particle cloud. Photonic Doppler velocimetry (PDV), a fiber-based heterodyne velocimeter, is often used to track ejecta velocities in dynamic compression experiments and on nanosecond time scales. Shortly after shock breakout at the metal-vacuum interface, a particular feature observed in many experiments in the velocity spectrograms is what appear to be slow-moving ejecta, below the free-surface velocity. Using Doppler Monte Carlo simulations incorporating the transport of polarization in the ejecta, we show that this feature is likely to be explained by the multiple scattering of light, rather than by possible collisions among particles, slowing down the ejecta. As the cloud expands in a vacuum, the contribution of multiple scattering decreases due to the limited field of view of the pigtailed collimator used to probe the ejecta, showing that the whole geometry of the system must be taken into account in the calculations to interpret and predict PDV measurements.

Reconstruction and interpretation of photon Doppler velocimetry spectrum for ejecta particles from shock-loaded sample in vacuum

“…For m 0 = 2 mg/cm 2 , the original peak disappears and the spectrum again exhibits a single peak. The doublepeak spectrum has been observed in previous experiments 19,37 and simulations, 25,38 and is the result of the direct exposure of incident light at the free surface.…”

“…Although the spectrum is nonzero at the position of the free surface in this model, neglecting the multiple-scattering results prevents the second peak from appearing. 25,38 In this paper, we mainly consider the information supplied by the original peak of the PDV spectrum. Thus, this defect has only a very slight influence on the accuracy of the present model.…”

Reconstruction and interpretation of photon Doppler velocimetry spectrum for ejecta particles from shock-loaded sample in vacuum*

“…For m 0 = 2 mg/cm 2 , the original peak disappears and the spectrum again exhibits a single peak. The double-peak spectrum has been observed in previous experiments [18,34] and simulations, [24,35] and is the result of the direct exposure of incident light at the free surface. The detail explanation is shown in Subsection 3.2.…”