We demonstrate strong lasing on the Ni-like 4<i>d</i><sup>1</sup><i>S</i><sub>0</sub>→4<i>p</i><sup>1</sup><i>P</i><sub>1</sub> transition at 18.9, 20.3, 22.0, and 24.0 nm for Mo, Nb, Zr, and Y ions, respectively, using the transient collisional excitation scheme. Approximately 5 J of laser energy in a combination of a 600-ps pulse and a 1-ps pulse from the Compact Multipulse Terawatt (COMET) tabletop laser system is used to irradiate slab targets of these materials. Small-signal gains of 17–26cm <sup>-1</sup> are determined on the 4<i>d</i>→4<i>p</i> transition, with overall gain–length products <i>gL</i> of 11–12. Lasing is observed and gain is measured on the 4<i>f</i><sup>1</sup><i>P</i><sub>1</sub>→4<i>d</i><sup>1</sup><i>P</i><sub>1</sub> transition, which is pumped by collisional excitation combined with self-photopumping, for what is to our knowledge the first time.
© 1999 Optical Society of America
(140.7090) Lasers and laser optics : Ultrafast lasers
(140.7240) Lasers and laser optics : UV, EUV, and X-ray lasers
(320.5390) Ultrafast optics : Picosecond phenomena
(340.7480) X-ray optics : X-rays, soft x-rays, extreme ultraviolet (EUV)
(350.5400) Other areas of optics : Plasmas
James Dunn, Joseph Nilsen, Albert L. Osterheld, Yuelin Li, and Vyacheslav N. Shlyaptsev, "Demonstration of transient gain x-ray lasers near 20nm for nickellike yttrium, zirconium, niobium, and molybdenum," Opt. Lett. 24, 101-103 (1999)