Powerful femtosecond laser pulses that propagate through narrow ablatingcapillaries cause modification of capillary walls, which is studiedexperimentally and theoretically. At low intensities, laser-induced periodicsurface structures (LIPSS) and porous coating composed of sub-micron particlesappear on the walls. At higher intensities, the surface is covered by depositeddroplets of the size up to 10 $\mu$m. In both cases, the ablated material formsa solid plug that completely blocks the capillary after several hundreds orthousands of pulses. The suggested theoretical model indicates that plugformation is a universal effect. It must take place in any narrow tube subjectto ablation under the action of short laser pulses.
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