Near-MHz temperature and H<sub>2</sub>O measurements in post-detonation fireballs of 25 g hemispherical explosives using scanned-wavelength-modulation spectroscopy
Charles J. Schwartz, Joshua W. Stiborek, Austin Butler, Damon Chen, Daniel R. Guildenbecher, Marc Welliver, Nick Glumac, Christopher S. Goldenstein
Abstract
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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>6806</mml:mn> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">c</mml:mi> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> were used to measure the absorbance spectrum of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">X</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> in the shock-heated air and trailing fireball at 500 kHz. Time histories of pressure, temperature, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">X</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> 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.