Mechanism for the void formation in the bright spot of a fiber fuse

A simple mechanism for the formation of a chain of voids (cavities) behind the spot of the laser optical discharge in an optical fiber is proposed. This mechanism is related to the motion of liquid in the opposite direction with respect to the propagation direction of the laser radiation between the isotherms that bound the charge-separation region and low-viscosity region. The motion is caused by the extrusion of the low-viscosity layer owing to the excessive pressure induced by the charge repulsion. The void shape (a bullet moving along the laser radiation) is determined by the isotherm sharpness. The bridge formation temperature (about 5000 K) is estimated based on the comparison of the extrusion velocity of the low-viscosity layer and the velocity of the bright spot.