Michael A. Shinas

Measurement of an explosively driven hemispherical shell using 96 points of optical velocimetry

We report the measurement of the surface motion of a hemispherical copper shell driven by high explosives. This measurement was made using three 32 channel multiplexed photonic Doppler velocimetry (PDV) systems, in combination with a novel compound optical probe. Clearly visible are detailed features of the motion of the shell over time, enhanced by spatial correlation. Significant non-normal motion is apparent, and challenges in measuring such a geometry are discussed.

Fundamental experiments in velocimetry

One can understand what velocimetry does and does not measure by understanding a few fundamental experiments. Photon Doppler Velocimetry (PDV) is an interferometer that will produce fringe shifts when the length of one of the legs changes, so we might expect the fringes to change whenever the distance from the probe to the target changes. However, by making PDV measurements of tilted moving surfaces, we have shown that fringe shifts from diffuse surfaces are actually measured only from the changes caused by the component of velocity along the beam. This is an important simplification in the interpretation of PDV results, arising because surface roughness randomizes the scattered phases.

Optical distance measurements to recover the material approach missed by optical velocimetry

Optical velocimetry is limited to measuring the component of the target velocity along the axis of the optical beam, thereby allowing a laterally moving tilted surface to approach a probe undetected. We are not discussing the detection of the lateral motion, but rather the detection of material approaching the probe due to lateral motion of a surface that is not perpendicular to the beam. This motion is not measured in optical velocimetry, and consequentially, integrating the velocity will in general give an incorrect position. We will present three approaches to overcome this limitation: Tilted wave-front interferometry, which maps time of flight into fringe displacement; pulse bursts for which we measure the change in the average arrival time of a burst, and amplitude modulation interferometry, in which a change in path length shows up as a change in the phase of the modulation. All three of these have the potential to be integrated with existing velocimetry probes for simultaneous velocity and displacement measurements. We will also report on initial tests of these approaches.

Design, assembly, and testing of a photon Doppler velocimetry probe

A novel fiber-optic probe measures the velocity distribution of an imploding surface along many lines of sight. Reflected light from each spot on the moving surface is Doppler shifted with a small portion of this light propagating backwards through the launching fiber. The reflected light is mixed with a reference laser in a technique called photon Doppler velocimetry, providing continuous time records. Within the probe, a matrix array of 56 single-mode fibers sends light through an optical relay consisting of three types of lenses. Seven sets of these relay lenses are grouped into a close-packed array allowing the interrogation of seven regions of interest. A six-faceted prism with a hole drilled into its center directs the light beams to the different regions. Several types of relay lens systems have been evaluated, including doublets and molded aspheric singlets. The optical design minimizes beam diameters and also provides excellent imaging capabilities. One of the fiber matrix arrays can be replaced by an imaging coherent bundle. This close-packed array of seven relay systems provides up to 476 beam trajectories. The pyramid prism has its six facets polished at two different angles that will vary the density of surface point coverage. Fibers in the matrix arrays are angle polished at 8°to minimize back reflections. This causes the minimum beam waist to vary along different trajectories. Precision metrology on the direction cosine trajectories is measured to satisfy environmental requirements for vibration and temperature.

Velocity spectra from explosively driven powders and balls

The capability to measure velocity distributions using Photon Doppler Velocimetry (PDV) has given rise to much data that were not measurable with previous velocimetry techniques. In our PDV measurements on explosively driven metals, we have often seen a single velocity disappear in a wide distribution of velocities. We have attributed this to HE gases, metal pieces, or a mix emerging from cracks in the metal after it fails. However, we are unaware of any experiments that demonstrate this interpretation. We have applied X-rays, cameras and PDV to explosively driven powders, balls and brass rings and found PDV spectra similar to what we observed in our experiments in which the metal fails. We present these spectra to help workers interpret their velocity spectra.

Improved probe and analysis for VISAR

We have designed and used for several years now a ¼ inch O.D., 11.5 inch length optical probe for imaging light from a surface area inside a confined space. The design is based on a commercial SelFoc gradient index objective and relay rod combination with acceptance angle +-30 degrees. We have used our probe both for framing camera images and for imaging spots on a surface onto a fiber array for interferometry. Probe efficiency is 1x10-6 at an object distance of 10 centimeters, where, for imaging onto the array, the probe has a depth of field from 2 cm to infinity. If a spot size of 1 mm is acceptable, the object can be brought within a few mm. For interferometry, the probe images enough of the surface to require automation from the analysis software. Below we report our probe construction and performance calculations, and software automation and analysis improvements.