Cut off of a laser beam by an expanding supersonic plasma

The process by which energy transfer from a laser beam to a target is cut off by breakdown far from the target is analyzed numerically for the two-dimensional problem of the propagation of a supersonic radiation wave in air with a constant density. The wave is caused by the absorption of a laser pulse with an intensity that is constant over the beam cross section and in time. Results are reported for the case where the air density is one-tenth the standard value and the radiation intensity is 1000 MW/sq cm (from a Nd-glass laser). The nature of the spatial distribution of the temperature in the plasma is illustrated by plotting 10,000-K isotherms that correspond to the actual boundary of the plasma region.