The role of beam homogeneity in mechanical coupling evaluation of laser ablation-generated impulse

The material emitted from the target surface during laser ablation generates a net thrust (propulsion) in the opposite direction. The energetic efficiency of this laser-driven propulsion is given by the mechanical coupling coefficient (Cm). In this work we considered nanosecond UV laser ablation of the aluminum 6061 alloy to study the Cm behaviour with different irradiating conditions. This is done by systematically changing: fluence, uniform/nonuniform intensity, and incident angle of the laser beam. In particular we found that, when dealing with nonuniform laser intensity, characterizing Cm exclusively in terms of fluence is not fully satisfactory because the energy distribution over the irradiated area plays a key role in the way material is removed -- interplay between vaporization and phase-explosion -- and thrust is generated.