Estimates on the ignition of high-explosives by laser pulses

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Abstract
The response of high-explosive materials to laser pulses is examined using a one-dimensional theory. Two distinct regimes of laser flux density I and pulse duration τp are identified, one concerned with a thermal initiation process and the other with an impulse or shock initiation process. For the thermal problem the competition between laser-induced ablation and thermal runaway is explored, and analytical formulas are found for an ignition time t* and maximum useful laser flux density I*. When I is below I* thermal initiation by the laser pulse is predicted to occur after an exposure time t* (I), but the rapid removal of material by laser-induced ablation can preclude this type of initiation when I exceeds I*. Values of I* are found to vary from 20 kW/cm2 for PETN to 0.05 kW/cm2 for TNT. The corresponding t* values are about 4 μs for PETN and 0.3 s for TNT. For decreasing flux densities I0 below I*, the ignition time t* increases approximately as I−1.80. For the impulse problem a self-regulating-ablation model is used to predict the pressure P exerted on the target surface as a function of the I and τp values of the pulse. This information is coupled with a P2τ ignition criterion for an HE to derive an inequality to be satisfied for impulse ignition: [τp(ns)]3/4[I0 (GW/cm2)]3/2≳17[P2 (kbar) τ (μs)]min. For pulse durations in the hundreds of ns region, this condition requires flux densities of tens of GW/cm2 for HE’s of interest.

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