Mechanical response and thermal coupling of metallic targets to high-intensity 1.06-μ laser radiation

Abstract

Mechanical response and thermal coupling measurements are reported for aluminum and titanium targets exposed to high‐intensity 1.06‐μ laser radiation. Measurements are made in air and vacuum for pulse lengths from 1 to 100 μsec, providing incident fluences of between 10⁶ and 10⁸ W/cm². Total momentum delivered to the target and time‐resolved pressure developed over the target surface were measured at irradiances spanning the threshold for laser‐supported detonation (LSD) wave ignition. The slope of the impulse/energy ratio shows a marked discontinuity at LSD threshold intensity. Peak target surface pressure is found to increase as the 2/3 power of the beam intensity in agreement with the hydrodynamic model of LSD wave propagation. Thermal coupling coefficients α for Al and Ti drop continuously from ∼0.3 to 0.07 over the intensity range examined. This behavior is consistent with the presence of an optically absorbing plasma at the target surface. The decrease in α is attributed to an increase in the plasma propagation velocity with intensity.