Session U4: ED-3a: Other Diagnostics

Impact Stress Measurement Using Piezoelectric Probes with PZT and LN Elements

Previous gas gun experiments using low density foam flyers examined the dynamic response of Dynasen CA-1136 piezoelectric probes having lead zirconate titanate (PZT) elements for stresses in the range 0.07 to 0.3 GPa. Recent experiments have extended the dataset down to 0.01 GPa, compared PZT based probes with lithium niobate (LN) based probes and compared the measured stress from manganin gauges with the stress from the piezoelectric probes. For 0.1 g/cc polystyrene foam impacting probes with APC 850 PZT elements and generating stresses between 0.1 and 0.2 GPa, the effective piezoelectric charge coefficient was 3 to 4 times the quoted value of 400 pC/N. The coefficient decreased to around 1 to 1.5 times the quoted value as the impact stress was reduced to 0.01 GPa. Differences were observed between 0.3 g/cc polyurethane and 0.1 g/cc polystyrene foams suggesting that the probe response is dependent on both the stress and the material properties of the impactor. The measured stresses from the LN probes were significantly closer to the stresses obtained from the manganin gauges.   

Using the Thermoelectric Effect to Measure Hugoniot of Metals

Accurate determination of shock velocity is a key to measurements of the Hugoniot curve. When a strong shock passes through a bi-metal junction, it heats the junction and generates a thermoelectric voltage which depends on the final temperature, and the relative Seaback coefficients of the metals. The thermoelectric measurement has the advantage that it does not require free surface or an insulating layer between the metals. If the metals junction is not shocked simultaneously, the rise time of the thermoelectric signal is proportional to the transverse time of the shock. Therefore the thermoelectric method is relatively insensitive to tilt and bow of the shock. The measurement of shock arrival using the thermoelectric effect was reported in the literature, but the conditions for clean and reliable measurements were never explored. Through many experiments using high velocity impactors and HE shocks, we demonstrate that the thermoelectric method has some complications, but can produce shock velocity data with accuracy better than 2{\%}.   

Coherent THz electromagnetic radiation emission as a diagnostic of ultrafast phase transformations in shocked CdSe

We review our experimental observations of THz radiation from shocked piezoelectric materials. Using molecular dynamics simulations coupled to Maxwell's equations, we show that the ultrafast transformation of wurtzite CdSe to high pressure phases under shock compression is accompanied by detectable electromagnetic radiation emission. The radiation is in the 100 GHz frequency range, corresponding to the timescale of the onset of the phase transformation. The sign of the electric field contains information about the atomic transformation pathway. This work was performed in part under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Pyrometry temperature studies of shocked tin including investigations exploring surface defects, anvil diameter and the integration with emissivity diagnostics

Accurate temperature measurement of shock-loaded systems continues to present experimental challenges. With short measurable time durations diagnostic methods are almost exclusively restricted to optical techniques. By preventing full sample pressure unloading, through the use of an anvil, partial release temperature measurements can be deduced from multiple wavelength optical pyrometry. This paper presents our recent studies of tin shocked to 28GPa including investigations exploring surface defects, anvil dimensions and the integration with emissivity diagnostics. The results indicate that a ring groove, 5mm across and with a nominal machined depth of 50 microns, acts to enhance the measured temperature by approximately 150K. Additionally on reducing the LiF anvil diameter from 20mm to 15mm, comparable partial release temperatures were observed. With the anticipated development of multiple anvil target designs, the smaller anvil diameter is desirable. British Crown Copyright 2009/MOD.   

Powder X-ray Diffraction Using the Omega Laser

The past several years have seen dramatic improvements in dynamic ramp-compression experiments to measure stress-density using laser and pulsed-power drivers. Goals for future experiments center on achieving pressures over 1 TPa (10 Mbar), while keeping the samples in a solid phase and applying additional diagnostics to probe the nature of these states. X-ray scattering is a natural probe for such studies due to the copious x-ray energy produced by laser sources. Such experiments allow studies of the crystal structure, texture, strength, and possibly temperature of ramp-compressed solids at unprecedented density. With this in mind we have developed a powder x-ray diffraction diagnostic fielded at the Omega laser. We will report our results on ramp-driven iron, tin and copper. This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.   

Characterizing Detonator Output Using Dynamic Witness Plates

A sub-microsecond, time-resolved micro-particle-image velocimetry (PIV) system is developed to investigate the output of explosive detonators. Detonator output is directed into a transparent solid that serves as a dynamic witness plate and instantaneous shock and material velocities are measured in a two-dimensional plane cutting through the shock wave as it propagates through the solid. For the case of unloaded initiators (e.g. exploding bridge wires, exploding foil initiators, etc.) the witness plate serves as a surrogate for the explosive material that would normally be detonated. The velocity-field measurements quantify the velocity of the shocked material and visualize the geometry of the shocked region. Furthermore, the time-evolution of the velocity-field can be measured at intervals as small as 10 ns using the PIV system. Current experimental results of unloaded exploding bridge wire output in polydimethylsiloxane (PDMS) witness plates demonstrate 20 MHz velocity-field sampling just 300 ns after initiation of the wire. Successful application of the PIV system to full-up explosive detonator output is also demonstrated.