Abstract

We have investigated laser-material coupling, laser-plasma interaction, and material removing mechanisms with high- power copper vapor lasers (CVL). It is found that material coupling with high-radiance visible laser light improves quickly as material undergoes rapid temperature and phase changes during the 50-70-ns laser pulse. The measurements of vapor expansion on aluminum and steel reveal that strong plasma-absorption-induced ignition starts at a laser intensity of 1-2 GW/cm². This plasma ignition induces a detonation wave that travels with a speed of a few texts of km/s. Hydrodynamic relations indicates that the vapor density at the end of laser pulse is 3(10²⁰)−1(10²¹) cm⁻³ with a pressure of a few thousand atmosphere near the target surface. This high-vapor density with a temperture exceeding 10,000 K leads to strong plasma absorption via photoionization. For a CVL-produced aluminum plasma, its absorption depth decreases from approximately 1 mm to less than 0.1 mm as laser intensity increases from 1 GW/cm² to 10 GW/cm². The increased plasma shielding at high laser intensity prevents further material removal by direct laser heating. Plasma absorption via inverse bremsstrahlung is determined to be negligible at this laser wavelength because of a relatively low electron density, measured to be peaked at ~5(10¹⁸) cm⁻³. The vapor expansion starts to slow down at ~1 µs after laser pulse and a shock front detaches from the vapor front with compressed air in between. The shock moves away with nearly sonic speed. The vapor cloud stays attached on the target surface for a few ms.

© 1996 Optical Society of America