Temperature and H₂O Measurements at 500 kHz in Hemispherical Post-Detonation Fireballs Using Scanned-Wavelength-Modulation Spectroscopy

Laser absorption spectroscopy (LAS) was used to measure temperature and X_H2O at a rate of 500 kHz in post-detonation fireballs of solid explosives. A 25 g hemisphere of pentaerythritol tetranitrate (PETN) was initiated with an exploding-bridgewire detonator to produce a post-detonation fireball that traveled radially toward a hardened optical probe. The probe contained a pressure transducer and the near-infrared optics needed to measure H₂O absorption transitions near 7185.6 cm^-1 and 6806 cm^-1 using peak-picking scanned-wavelength modulation-spectroscopy with first-harmonic-normalized second-harmonic detection (scanned-WMS-2f/1f). The two lasers were scanned across the peak of an absorption line at 500 kHz and modulated at either 35 MHz for the laser near 7185.6 cm^-1 or 45.5 MHz for the laser near 6806 cm^-1. This enabled measurements of temperature and X_H2O at 500 kHz in the shock-heated air and trailing post-detonation fireball. Time histories of pressure, temperature, and XH₂O were acquired at multiple standoff distances in order to quantify the temporal evolution of these quantities in the post-detonation environment produced by PETN.