Near-MHz temperature and H2O measurements in post-detonation fireballs of 25 g hemispherical explosives using scanned-wavelength-modulation spectroscopy.

A laser absorption spectroscopy diagnostic integrated within a hardened optical probe was used to measure temperature and water mole fraction at 500 kHz in post-detonation fireballs of explosives. In the experiments, an exploding-bridgewire detonator initiated a 25 g hemisphere of explosive (N5 or PETN). This produced a hemispherical fireball that traveled radially towards a hardened measurement probe. The probe contained a pressure transducer and optical equipment to pitch fiber-coupled laser light across a 12.6 cm gap onto a detector. Tunable diode lasers emitting near 7185.6 and 6806c m -1 were used to measure the absorbance spectrum of H 2 O utilizing peak-picking scanned-wavelength-modulation spectroscopy with a scan frequency of 500 kHz and modulation frequencies of 35 and 45.5 MHz, respectively. This enabled measurements of temperature and X H 2 O in the shock-heated air and trailing fireball at 500 kHz. Time histories of pressure, temperature, and H 2 O mole fraction were acquired at different standoff distances to quantify how the fireball evolved in space and time as well as to compare measured quantities between PETN and N5 fireballs. The standard deviation of temperature and X H 2 O during one representative test were found to be 17 K (1.3%) and 0.011 (5%), respectively. These measurements demonstrate this diagnostic's ability to provide rapid and reliable measurements in harsh, highly transient post-detonation environments produced by solid explosives.

H2O and temperature measurements in propagating hydrogen/oxygen flames using a broadband swept-wavelength ECQCL.

We present experimental results using a swept-wavelength external cavity quantum cascade laser (swept-ECQCL) diagnostic to measure broadband absorption spectra over a range of 920-1180c m -1 (8.47-10.87 µm) with 2 ms temporal resolution in premixed hydrogen/oxygen flames propagating inside an enclosed chamber. Broadband spectral fits are used to determine time-resolved temperatures and column densities of H 2 O produced during combustion. Modeling of the flowfield within the test chamber under both equilibrium conditions and using a 1D freely propagating flame model is compared with the experiment in terms of temporal dynamics, temperatures, and H 2 O column density. Outputs from the numerical models were used to simulate radiative transport through an inhomogeneous combustion region and evaluate the performance of the spectral fitting model. Simulations show that probing hot-band H 2 O transitions in the high-temperature combustion regions minimizes errors due to spatial inhomogeneity. Good agreement is found between the experimental and modeling results considering experimental uncertainties and model assumptions.

Elucidating uranium monoxide spectral features from a laser-produced plasma.

Uranium, because of its pyrophoricity, oxidizes rapidly in an oxygen-containing high-temperature environment. However, so far, the identification of uranium oxide (UO) emission from a laser-produced plasma system is limited to a spectral feature around 593.55 nm. The aim of this study is to elucidate UO emission features in the visible spectral regime from uranium plasmas generated in an environment with varying oxygen concentrations. The plasmas are produced by focusing nanosecond laser pulses on a uranium metal target in a controlled ambient environment. Space- and time-resolved optical emission spectroscopic investigations are used for isolating UO molecular emission structures from crowded U atomic line emission. Our studies highlight that the emission from a U plasma, even in the presence of trace oxygen is accompanied by a strong background-like emission with partially resolved bands from uranium monoxide and higher oxides. We also report several UO spectral emission bands in the visible spectral region.

Emission Spectra of Uranium Particulates at High Temperature.

The emission spectrum of micron-scale uranium particulates at high temperatures in the ultraviolet, visible, and near-infrared spectral regions is investigated using a heterogeneous shock tube. Temperatures from 3000 to 9000 K are characterized in an inert argon environment and with incremental amounts of added oxygen. Atomic line spectra do not emerge above the continuum emission spectrum until between 4500 and 5000 K in pure argon, and 6100 and 6600 K in 1% oxygen. For 5% oxygen, however, the threshold for atomic emission drops below 3800 K. Uranium monoxide molecular emission in the strongest visible band at 595.4 nm is not observed at any condition. Uncertainties in particle temperature determination in high-temperature shock tube environments are discussed, and limitations to such measurements are presented, such as those from experimental factors such as the powder loading method and expected detection limits of uranium species in relevant conditions.

Ultraviolet Emissions from Explosive Detonation Breakout.

Detailed spectroscopic measurements of high explosive detonation breakout in the ultraviolet region are presented. Molecular features associated with CN, NH, OH, and N2 are observed and analyzed. Spectra indicate extreme temperatures well in excess of 5000 K in the first few microseconds after breakout. Molecular bands are found to originate from the detonation products, as opposed to the ambient air, and are strongly attenuated in the presence of oxygen. Implications for forensic analysis of source explosive are discussed.

Spectral Emission Signatures from Cased High-Explosive Charges.

Spectroscopic signatures of cased high-explosive charge denotations are examined using emission spectroscopy with sub nanometer resolution. Eleven distinct case materials are investigated for atomic features of their major alloying elements. Molecular features of case material combustion products are also investigated for five case materials. Emission is monitored within the 275-425 nm range for atomic features and in the 310-755 nm range for molecular features. Major alloying elements with concentrations greater than 5% are generally detected through atomic emission. Al, Cu, Fe, Mg, Cr, Mn, Pb, and Ni are all detected in concentrations less than 5%. Undetected elements include Zn, Nb, Ta, and V. Molecular emission from aluminum monoxide, titanium monoxide, and CN is measured for aluminum alloy, titanium alloy, and carbon fiber cases, respectively.

Simultaneous Imaging and Spectroscopy of Detonation Interaction in Reactive and Energetic Materials.

A dual framing camera system was coupled with custom-designed ultrafast imaging spectrometer optics to yield simultaneous imaging and imaging spectroscopy of extremely short detonation interaction events in reactive materials. For short exposures of 100 ns or less, spectral resolutions of 2.4 Å are achievable, allowing for time-resolved identification of key intermediate species evolving from prompt reaction. Under some circumstances, emission can be fit to a local emission temperature, assuming the optically thin limit. Applications to reactive metal systems involving aluminum, magnesium, titanium, boron, and silicon are demonstrated.

Ultraviolet absorption spectroscopy in optically dense fireballs using broadband second-harmonic generation of a pulsed modeless dye laser.

Broadband frequency doubling of a modeless dye laser pulse is used to enable single-shot absorption spectroscopy in the ultraviolet for optically dense, energetic-materials fireball applications. Band widths of approximately 1-3 nm are generated in the 226 and 268 nm regions using a doubling crystal. Strong focusing of the fundamental beam onto the crystal is found to be sufficient to achieve 1-5% conversion efficiency with a pulse intensity sufficient to saturate the array detector even after 75% attenuation through the fireball. The technique is demonstrated with nitric oxide (NO) absorption in a gas cell and is then used to perform the first detection and temperature fitting of aluminum monofluoride (AlF) and magnesium monofluoride (MgF) in a fireball environment.

Absorption spectroscopy measurements in optically dense explosive fireballs using a modeless broadband dye laser.

A modeless broadband dye laser is applied to probe inside optically dense fireballs generated by high explosives using single-shot, high resolution absorption spectroscopy. Despite attenuation of the main beam by 98%, high signal-to-noise ratio absorption spectra of Al, Ti, and AlO are readily obtained at resolutions of 0.007 nm, and luminosity from the fireball is strongly rejected. Detection limits for atomic species are less than 200 ppb. The method offers good time resolution of chemistry within the fireball, and scaling laws suggest that this technique should be valid in explosives tests at least up to the gram scale.