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

Abstract

A laser absorption spectroscopy diagnostic integrated within a hardened optical probe was used to measure temperature and water mole fraction at 500kHz in post-detonation fireballs of explosives. In the experiments, an exploding-bridgewire detonator initiated a 25g 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 12.6cm 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 500kHz and modulation frequencies of 35 and 45.5MHz, respectively. This enabled measurements of temperature and X H 2 O in the shock-heated air and trailing fireball at 500kHz. 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 17K (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.