Generation of a 45-ps-duration 15.5-nm x-ray laser

Using a series of 100-ps optical pump pulses separated by 300 ps, we have produced 45-ps FWHM output pulses in neonlike yttrium operating at a wavelength of 15.5 nm. This is to our knowledge the shortest-pulse collisionally pumped x-ray laser yet produced. The importance of using a traveling-wave pump to increase the output power is also discussed.