MHz-rate laser absorption spectroscopy for temperature and N2O concentration measurements in laser-induced ADN propellant decomposition

In this work, we employed laser absorption spectroscopy (LAS), capable of operating at MHz scan rates, together with high-speed photography to investigate the thermal decomposition of ammonium dinitramide (ADN). Rapid decomposition was initiated using a 10.6 μm CO 2 laser that delivered an incident heat flux of 100 W/cm 2 . An interband cascade laser (ICL) was utilized to quantify N 2 O concentration and temperature during the thermal conversion. However, the insufficient ICL scan depth as compared to the bandwidth required for LAS diagnostics, especially when operating at MHz frequencies or under elevated pressures, caused inaccurate baseline extrapolation and reduced spectral fitting accuracy. To address this challenge, a bias-tee circuitry was integrated with the ICL to increase the scan depth from 0.136 cm -1 to 0.220 cm -1 at 1 MHz scan rate. In addition, an iterative baseline retrieval method was adopted to further improve the fitting precision. The method’s performance and noise sensitivity were validated via numerical simulations and gas cell experiments prior to its application in practical measurements. At 1 atm, ADN decomposition proceeded violently at initial before gradually stabilizing, yielding gas temperatures up to 500 K and a peak N 2 O concentration of 6%. At 0.1 atm, lower oxygen levels facilitate higher N 2 O accumulation as temperatures climb to 550 K. This diagnostic platform, combining high-speed LAS with a laser-heating apparatus, enables real-time, quantitative characterization of energetic material decomposition, providing critical insights into the chemical kinetics of ADN.