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.