Effect of counter shock wave on the expanding plume

Abstract

Effects of head on collision on two laser ablation plumes in background gas are revealed by observation of plume expansion dynamics. Nb₂O₅ and Ni targets were set parallel to each other and irradiated simultaneously by two YAG lasers in He gas under various gas pressures. The plume expansion dynamics was monitored by a gated ICCD camera. The plumes stagnate at 500 Pa after collision of two plumes. The backward movements of the plumes are observed above 2000 Pa, even though the plumes do not collide with each other. We estimated propagation of shock front by point blast model and the results indicate that the backward movement at high background gas pressure is due to the collision between plume and shock front. These results indicate that collision with counter shock wave is very important for plume expansion dynamics.