PDV Velocimetry Features as a Glue Layer Diagnostic in Windowed Shock Experiments

Abstract

A series of experiments were conducted using samples with different glue layer thicknesses between a LiF window and HE driven, shocked sample coupons. We completed a total of 13 shots (4 with integrating sphere, 8 with glue and one in vacuum), all with PBX 9501 driving a Sn coupon with a LiF window on the metal sample. Four shots were done with 3.5 mm Sn coupons, 5 mm LiF, and integrating spheres. Two shots of those 4 were done with the glower on to look for changes in emissivity and two were done in the same geometry to measure the radiance of the shock heated sample to measure the correction to the emissivity (if needed). The other 9 shots had LiF windows (6 or 10 mm thick) bonded with Loctite 326 glue of varying thicknesses (of these one was "glueless"-with an attempt at a "glueless" Sn/LiF interface with a rough vacuum). Three shots had thin Cr metal coatings (100 or 200 nm thick) on the Sn coupon. We observe splitting of the velocity spectrogram trace at shock breakout that appears to be correlated with the glue layer thickness. This resolves a puzzling feature that appeared in several gas gun shots where the glue layer was measured (mechanically using a stack-up method) to be a few microns or less (sometimes negative values resulted). If the data here is interpreted correctly, then those layers were likely thicker-perhaps 2 or 3 times the reported thicknesses measured with mechanical methods. Shots BB100525 through BB100527-2: Integrating Sphere Shots (with PDV and Cold IR Pyrometer) These 4 shots were conducted using a single Lightpath collimated probe in the integrating sphere, constructed with a pulsed glower and optical coupling to allow simultaneous radiance measurements in the 1-5 µm wavelength range. Two shots were conducted with the glower on (for dynamic emissivity measurements) and two were conducted with the glower off to measure the self-light of the shock heated target to allow us to correct the emissivity measurement for this effect. The PDV data were taken with a 40 GS/s digitizer so we could analyze the single PDV channel with higher time resolution that we might normally be able to do. See the appendix for details and figures of these shots.