Thermal and radiative transport analysis of laser ignition of energetic materials

Laser ignition of energetic materials is an attractive technology for replacement of low energy electro-explosive devices which pose a safety hazard. The development of this technology has historically been based on go/no-go threshold testing using off-the-shelf laser diodes and solid state lasers. Here we seek to build a more fundamental understanding of the laser ignition process by analyzing the interactions and response of the energetic material to the incident laser beam. We begin with a radiative heat transfer model of the laser-beam-assisted heating of a homogeneous energetic material with given optical properties. An analytical solution of the 2-flux model equations is developed and this expression for the volumetric absorption of laser energy in an absorbing and isotropically scattering medium is coupled to the conservation of energy equation. Two limiting cases-minimum power and minimum energy thresholds for ignition - are discussed, and the minimum energy threshold is calculated directly from the energy equation in the limit of zero dissipative losses. The effects of power density and beam shape are of particular interest and two common configurations are analyzed. Although the applicability of thermal models is limited by large uncertainties in the optical properties of energetic materials, the analysis provides a qualitative understanding of the ignition process and a correlation between ignition thresholds and the various material properties and design parameters.