Using Silica Aerogel to Characterize Hypervelocity Shrapnel Produced in High Power Laser Experiments

Abstract

We present a technique using silica aerogel to characterize the number, velocity, size, and spatial distribution of shrapnel. Measuring shrapnel produced in a large laser facility can aid in improving target designs to reduce shrapnel, thereby extending optics lifetime. We tested the use of aerogel as a diagnostic by creating shrapnel on the Helen Laser at AWE in England and capturing it in prepositioned 20 mg/cc silica aerogel-filled holders placed from 10 to 50 cm away. Synchrotron radiation at the Stanford Linear Accelerator was used to produce radiographic images of the embedded shrapnel at three different angles. The contrast for 19 keV x-rays for Ta and aerogel allowed very fine resolution of the Ta particles, down to ∼ 5 microns diameter. Reconstruction of the data resolved each particle's depth and size. We developed an expression that relates particle size and depth of penetration to incident velocity assuming the particle acts like a 'snowplow' into the aerogel where the dominant energy loss is dissipation into viscous drag. The aerogel data is being used to validate and tune hydrodynamic modeling of the target break-up. We also tested a more complex technique of the use of aerogel to characterize shrapnel that uses a thin sheet of aerogel over a fused silica substrate. We conclude that these techniques may be viable for measuring solid shrapnel, droplets, and even condensed vapor with sufficient resolution to quantify their asymmetric deposition within the target chamber.