Macroscopic self-focusing effects in a laser-produced low-density plasma

Large-scale self-focusing effects are evidenced by focusing a 300 MW Nd laser beam with a F/8 spherical lens in a fairly low-density gas (such that nsub(e) approx. nsub(c)/100). They include important beam recollimation and formation of an ''empty'' channel which was visualized by holographic interferometry. In these conditions plasma expansion and laser energy deposition were both larger ahead than behind the focal point