Laser plume evolution in the process of nanopowder preparation using an ytterbium fibre laser

We study the laser plume dynamics by means of highspeed shooting in the intrinsic light, as well as using the shadow method with a laser monitor. It is found that the laser plume arising under the impact of a radiation pulse from an ytterbium fibre laser with a power of 670 W on a Nd : Y₂O₃ target with an Nd concentration of 1 mol % is first a plasma consisting of the target material vapour, and then becomes a mixture of vapour and droplets. The first droplets in this plasma appear in ~200 μs after the formation of the laser plume, and in 400 – 500 μs the major part of the substance is removed in the form of liquid droplets. We have also found that the depth of the laser-produced crater linearly depends on the laser pulse duration, thus confirming the absence of essential shielding of laser radiation by melt droplets. The higher the target transparency, the longer the delay time of the formation of the laser plume and the greater its spread. Sometimes, instead of the laser plume formation, one can observe a light flash inside a semitransparent target. The explanation of these results is presented.