Thermal self-interaction of vertical laser beams in the presence of photoabsorption convection

An experimental investigation was made of thermal self-interaction of high-power laser beams of circular and elliptic cross section under conditions of induced convection in the case of vertical (downward) propagation of radiation in a cell containing a silicone liquid. The velocity of the induced convective flow was measured and its dependence on the power of the heating radiation was determined. In the case of axisymmetric beams it was found that the angular divergence increased as a result of thermal self-interaction by an amount which was an order of magnitude less than that predicted by a theory postulating a thick lens in a medium at rest. Changes in the shape of an elliptic beam, which became more symmetric, were observed. The characteristic times of the transient processes were measured. These processes were found to be determined mainly by convective heat transfer.