Pulsed-CO2-laser interaction with aluminum in air: Thermal response and plasma characteristics

Abstract

Simultaneous target heating, plasma spectral emission, and time-resolved plasma interferometry data were taken for the interaction of CO2 (λ=10.6 μm) laser pulses with aluminum targets in air. The laser spot area on the planar targets was 1.4 cm2, the pulse duration was 1.8 μs, and the maximum peak flux on target was 8×107 W/cm2. The target thermal response is analyzed to provide both the total energy deposited on target and the spatial distribution of the energy. The total energy deposited increases in direct proportion to the laser-pulse energy, with a coupling coefficient of 18% above the plasma ignition threshold. The spatial maximum value of the energy-deposition density, in contrast to the total energy, remains constant as the laser-pulse energy increases, demonstrating that target heating takes place over an area increasing with the laser-pulse energy. Even at the plasma ignition threshold, the area of energy deposition exceeds substantially the laser spot area. The spectroscopic measurements suggest that continuum radiation occurs earlier than and dominates line radiation at wavelengths near 4000 Å. At shorter wavelengths many lines from target elements are observed. Plasma electron temperatures of 12 000 K for aluminum and 16 000 K for nitrogen were derived from LTE calculations. Below the plasma ignition threshold, open-shutter photography shows weak emission at the target surface, while above threshold a bright plasma propagating away from the target is observed. Time-resolved interferometry of the plasma was done using a Q-switched ruby laser in a Mach-Zehnder interferometer. The blast wave propagates with distance nearly proportional to (time)2/3 after the laser pulse, in agreement with theory for one-dimensional propagation. The initial velocity is 1.46×105 cm/s for a pulse energy of 50 J. The plasma refractivity shows that free electrons are an important constituent of the plasma; at the shock front, the electron density is about 1018 cm−3. The target-heating observations, together with the plasma temperature and propagation measurements, are consistent with models of target heating by thermal radiation from the laser-heated plasma. Target heating by thermal radiation alone appears to be insufficient. Enhanced absorption of the laser flux directly, as by a phase transition of the target material, is not consistent with the observation of the increasing area of energy deposition.