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It is well-known that continuous-wave lasers can accelerate small particles to high velocities, and, more recently, it has been observed that laser damage of optical materials occurs at laser intensities that are orders of magnitude lower in the presence of accelerated absorbing particles. At these high powers the primary interactions are ablative, and a complete set of force balance equations in this regime have only recently been described. In our experiments, small groups of 35-41m diameter stainless steel particles are accelerated using a high-power continuous wave (CW) Yb-doped fiber laser illuminating at intensities between 1MW/cm2 and 2MW/cm2. The trajectories of the particles are tracked using a high-speed camera until they leave the camera field of view or evaporate, a process typically occurring on timescales of a few milliseconds. The lifetimes and trajectories of the superheated particles are calculated using a system of coupled equations to track particle velocity, mass, momentum, radius, vapor opacity, temperature, and temperature distribution. Upon illumination, the particles heat to their vaporization temperatures, with evaporating atoms transferring momentum to the remaining particle. Significant laser attenuation occurs due to an opaque glow region around the particle consisting of dense evaporated atoms and ions. Since the particle is not uniform in temperature, the number of evaporating atoms varies with position and imparts a net acceleration to the particle until a terminal velocity on the order of a few tens of meters per second is reached where drag forces offset further acceleration. Based on the calculated interplay between temperature gradient and acceleration, heat transfer within the evaporating particles must be dominated by radiation diffusion, a process that usually only dominates in astrophysical objects, for example in the photospheres of stars. It is further observed that absorbing particles accelerated by continuous wave (CW) lasers initiate catastrophic failure in the form of micromachined drill holes. This process was tested using stainless steel, PMMA, and silica particles with fused silica, sapphire, and spinel substrates. Hole drilling occurred at laser power densities as low as 250kW/cm2. A potential dependence of accelerated particle breakdown on substrate bandgap may suggest that the underlying physical process is similar to that seen for CW laser breakdown of contaminated optical coatings.
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