Effect of variations in the gas-dynamic unloading on the laser initiation of the PETN–aluminum composite

Thresholds of explosive decomposition $H_{cr}$ under irradiation using the first harmonic of a pulsed neodymium laser (14 ns) are experimentally determined for pentaerythritol tetranitrate (PETN) with aluminum nanoparticles (100 nm) at concentrations ranging from 0.05 to 1 wt %. Amplitudes of optoacoustic signals are measured versus concentrations of impurities in the sample at a fixed laser fluence. The experiments are performed in two regimes. In the first one, the irradiated surface of the sample is covered with a finite-weight glass plate that impedes the gas-dynamic unloading. In the second regime, an external pressure of no less than 10$^7$ Pa is applied to the glass plate to prevent the gas-dynamic unloading. The interpretation of the experimental effects is proven by the results of theoretical simulation.