With long-pulse-duration (4 nsec), moderate-intensity (≃${10}^{13}$ W/${\mathrm{cm}}^2$) Nd-laser irradiation of flat targets, time-integrated images of $\frac{3}{2}{\omega }_0$ and $2{\omega }_0$ emission reveal filaments along the laser axis. Combined with fine-scale structure found in x-ray images, these filaments suggest self-focusing of hot spots in the incident beam.

• Received 13 October 1980

DOI:https://doi.org/10.1103/PhysRevLett.46.328