Session B3: Velocimetry I: Velocimetry, Ranging and Position Measurement

Down-Bore Two-Laser Heterodyne Velocimetry of an Implosion-Driven Hypervelocity Launcher

The implosion-driven launcher uses explosives to shock-compress helium, driving well-characterized projectiles to velocities exceeding 10 km/s. The masses of projectiles range between 0.1 -- 10 g, and the design shows excellent scalability, reaching similar velocities across different projectile sizes. In the past, velocity measurements have been limited to muzzle velocity obtained via a high-speed videography upon the projectile exiting the launch tube. Recently, Photonic Doppler Velocimetry (PDV) has demonstrated the ability to continuously measure in-bore velocity, even in the presence of significant blow-by of high temperature helium propellant past the projectile. While a single-laser PDV is limited to approximately 8 km/s, a two-laser PDV system is developed that uses two lasers operating near 1550 nm to provide velocity measurement capabilities up to 16 km/s. The two laser PDV system is used to obtain a continuous velocity history of the projectile throughout the entire launch cycle. These continuous velocity data are used to validate models of the launcher cycle and compare different advanced concepts aimed at increasing the projectile velocity to well beyond 10 km/s.   

Surface Registration Using a Multi-beam Optical Probe and Backscatter Reflectometer

We introduce a technique for locating the position and orientation of an object surrounding a compound optical probe. The probe allows for simultaneous measurement of $\sim$ 150 points of laser velocimetry, and prior to a dynamic experiment it is important to know the initial position of the surface. Using an optical switch and a Luna OBR$^{TM}$ rangefinder, we measure the surface distance at hundreds of locations. These measurements are then combined with a dimensional inspection of the object and analysis routines to report the orientation and location of the inner surface relative to the probe. Location resolutions of \textless 10 microns are demonstrated.   

Reconstruction and Modelling of Cylinder Test Wall Expansion from Heterodyne Velocimetry Data

The `cylinder test' is comprised of a cylinder of explosive encased in a copper tube and detonated at one end. Analysis of the copper wall expansion can be used to generate a JWL equation of state for the explosive. The wall arrival times are traditionally measured using angled probe boards. These times are converted to radial expansion times using the measured steady state detonation velocity. This expansion represents the intersection of the wall with a radial line, hence its differential is the radial intersection velocity. The true radial wall velocity is different due to the small component of particle velocity along the axis. Wall velocities can be directly measured using a Heterodyne Velocimetry (HetV) diagnostic, to a high degree of temporal resolution. However, the wall profile cannot be reconstructed from a standard HetV probe due to a lack of spatial information. This work describes how velocity traces from two HetV probes at different angles can be combined to evaluate the path of a particle on the copper wall, and how the wall profile may then be reconstructed. The method is applied to data from cylinder test experiments on a conventional high explosive. Results are validated using hydrocode modelling coupled with Detonation Shock Dynamics theory.   

Use of a moving diffraction grating to challenge our understanding of velocimetry data

It has now been well established that optical-interference velocimetry will not detect the approach of material arising from the lateral motion of a tilted surface. The usual explanation is that in order to receive light from a tilted surface, the light must travel in a non-specular direction, and so the surface must be rough compared to the wavelength of light. The lateral motion of this rough surface scrambles the phase of the return signal, preventing the interferometry from reporting a signal. We report on experiments that use level and tilted diffraction gratings moving laterally to the velocimetry beam to try to circumvent this: the surface is smooth compared to the wavelength on short scales, but the diffraction maxima allow us receive light in a non-specular direction. Initial results show that this will present a challenge to our understanding of velocimetry measurements. for unlimited release LA-UR-15-20636.   

Simultaneous broadband laser ranging and PDV: A diagnostic for non-planar dynamic experiments

In non-planar experiments, the distance from the target to the sensing probe cannot usually be determined by integrating the measured velocity. We summarize several optical ranging techniques that have measured the target distance directly and present our fiber-optic system, which simultaneously measures the distance and velocity of rapidly moving surfaces. The distance measurement is based on the technique described by Xia and Zhang [Optics Express, \textbf{18}, 4118 (2010)] which determines the target distance independent of the target speed at sampling rates near 50 MHz, and is ideally suited for dynamic applications. We have extended the full range of the diagnostic to many cm and multiplexed it with a photonic Doppler velocimetry system so that distance and velocity can be tracked using a single fiber-optic probe. The diagnostic was demonstrated on a spinning target as an example of how integration of the target velocity sometimes gives the incorrect surface position, and how this system obtains it directly. The diagnostic was also tested on an explosive experiment in which surface ejecta and copper fragments were identified in both the position and velocity signals. We show how the position measurement compliments the velocity data. Potential applications are discussed. \textit{This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946 with the U.S. Department of Energy, and supported by the Site-Directed Research and Development Program.}