Observed velocity of slow motion of an optical discharge

An analysis is made of the difference between the measured velocity of an optical discharge front moving toward the laser source and the theoretical velocity of thermoconductive or radiative propagation of a discharge in a cold gas adjoining directly the front. The difference is explained using a model of flow of a dense cold gas around a hot low-density gaseous sphere. The effect is entirely due to the two-dimensional nature of the motion of gas past the discharge. Only a small fraction of the cold gas enters the discharge, whereas the bulk flows around the hot plasma region as if it were a solid. An optical discharge is modeled numerically for a parallel laser beam in a two-dimensional gas stream. These numerical calculations confirm the results predicted by the simple model. The calculated velocities are compared with the published experimental results.