Material scientists have developed computational modeling to predict the dynamic response of materials undergoing stress, but there is still a need to make precision measurements of surfaces undergoing shock compression. Miniature photonic Doppler velocimetry (PDV) probes have been developed to measure the velocity distribution from a moving surface traveling tens of kilometers per second. These probes use hundreds of optical fibers imaged by optical relays onto different regions of this moving surface. While previous work examined large surface areas, we have now developed a PDV microscope that can interrogate 37 different spots within a field of view of <1 mm, with a standoff distance of 17 mm, to analyze the motion differences across grain boundaries of the material undergoing dynamic stress. Each PDV fiber interrogates a 10 μm spot size on the moving surface. A separate imaging system using a coherent bundle records the location of the PDV spots relative to the grain boundaries prior to the dynamic event. Designing the mounting structures for the lenses, fibers, and coherent bundle was a challenge. To minimize back reflections, the fibers are index matched onto the first relay lens, which is made of fused silica. The PDV fibers are aligned normal to the moving surface. The imaging probe views the surface at an 18° angle. The coherent bundle is tilted 11° to its optical relay. All components are assembled into a single probe head assembly. The coherent bundle is removed from the probe head to be used for the next dynamic event. Alignment issues will also be discussed.
High-Speed Multi-Frame Laser Schlieren is used for visualization of a range of explosive and non-explosive events.
Schlieren is a well-known technique for visualizing shock phenomena in transparent media. Laser backlighting and a
framing camera allow for Schlieren images with very short (down to 5 ns) exposure times, band pass filtering to block
out explosive self-light, and 14 frames of a single explosive event.
This diagnostic has been applied to several explosive initiation events, such as exploding bridgewires (EBW), Exploding
Foil Initiators (EFI) (or slappers), Direct Optical Initiation (DOI), and ElectroStatic Discharge (ESD). Additionally, a
series of tests have been performed on "cut-back" detonators with varying initial pressing (IP) heights. We have also
used this Diagnostic to visualize a range of EBW, EFI, and DOI full-up detonators. The setup has also been used to
visualize a range of other explosive events, such as explosively driven metal shock experiments and explosively driven
microjets. Future applications to other explosive events such as boosters and IHE booster evaluation will be discussed.
Finite element codes (EPIC, CTH) have been used to analyze the schlieren images to determine likely boundary or initial
conditions to determine the temporal-spatial pressure profile across the output face of the detonator. These experiments
are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to
detonation to full detonation to transition to booster and booster detonation.
We have developed a suite of optical diagnostics and analyses for
probing the velocity and spatial distribution of ablatively launched
metal with nano-scale precision. We utilize a nanosecond laser
pulse to launch a thin layer of metal and then use optical and
opto-electronic devices to diagnose the velocity and topography.
Our Photonic Doppler Velocimeter (PDV) utilizes the heterodyne
principle that allows us to track multiple velocity components. We
have investigated a number of different methods for analyzing this
data to provide increased velocity and temporal resolution. We also
discuss the possibilities to extend the sensitivity of the PDV
system to provide a compact diagnostic with a broad range of
capabilities. Our topographer is based on the Shack-Hartmann
interferometer that can resolve the changing shape of the ablated
metal surface as it is launched. We compare the experimental data
to hydrodynamic simulations to provide a feedback loop to improve
our theoretical models. The ultimate goal is to develop a
well-understood laser-based firing set for direct optical initiation
(DOI) of explosives.
Los Alamos National Laboratory is currently designing a series of direct optically initiated (DOI) detonators. The primary purpose of this series of detonators is to achieve a level of safety in the face of unintentional initiation from an electrical source. The purpose of these experiments is to determine the minimum spotsize that will initiate the low density initial pressing in these laser detonators. With this information it is expected that a more robust optically initiated detonator can be designed and manufactured. Results from a series of experiments will be discussed. First a range of small core diameter fiber optics with varying energy injection levels will be tested to find the minimum energy level necessary to achieve reliable initiation. Second, a range of apertures will be employed to trim the spotsize down to a minimum size that will still maintain reliable initiation. This information will help to understand whether the initiation criteria for the DOI Laser Detonator are dominated by energy density, total energy or a combination of these criteria.
We report on the development of novel high-speed techniques to measure the surface topography and instantaneous velocity of ablatively launched thin metal layers with sub-nanosecond temporal resolution. Applications for laser detonator technology require the understanding of laser fiber optical energy deposition and ablative launch of a thin metal layer into an explosive. Characterization of the ablation process requires a time-resolved diagnosis of the ejected material state (topography, velocity, density, pressure, etc.). A pulsed Nd:YAG fibercoupled laser is used to ablate a 250 nm layer of titanium deposited on a 500 μm thick fused silica substrate at fluences below 10 J/cm<sup>2</sup>. Time-resolved imaging of the free expansion of the metal surface is accomplished with Fourier plane imaging using a Shack-Hartmann lenticular array coupled to a fast framing camera. The imager performs topographical surface measurements by detecting changes in the optical wavefront of a reflected picosecond probe laser beam off the expanding surface. Consequently, single-event sub-nanosecond time-resolved "movies" of surface motion dynamics are captured. Crosscheck of the Shack-Hartmann imager is done using advanced velocimetry. A 1550 nm heterodyne laser-based Photonic Doppler Velocimeter is used to measure surface velocity. Using a 1550 nm single mode fiber laser, 10 GHz InGaAs detectors and telecom hardware, we directly record the resulting beat signal produced by the accelerated surface onto a fast digitizer. Free surface velocities as high as 6.5 μm/ns are recorded. Comparisons between the dynamic topography, surface velocimetry and laser hydrocode simulations are presented.
We report on the development of a suite of novel techniques to measure important characteristics in intense ultrashort laser solid target experiments such as critical surface displacement, ablation depth, and plasma characteristics. Measurement of these important characteristics on an ultrafast (~50 fs) time scale is important in understanding the primary event mechanisms in laser ablation of metal targets. Unlike traditional methods that infer these characteristics from spectral power shifts, phase shifts in frequency domain interferometry (FDI) or laser breakthrough studies of multiple shots on bulk materials, these techniques directly measure these characteristics from a single ultrafast heating pulse. These techniques are based on absolute displacement interferometry and nanotopographic applications of wavefront sensors. By applying all these femtosecond time-resolved techniques to a range of materials (Al, Au, and Au on plastic) over a range of pulse energies (10<sup>11</sup> to 10<sup>16</sup> W/cm<sup>2</sup>) and pulse durations (50 to 700 fs), greater insight into the ablation mechanism and its pulse parameter dependencies can be determined. Comparison of these results with hydrocode software programs also reveals the applicability of hydrocode models.