The x-ray laser is a seemingly logical but in practice quite difficult extension of conventional laser technology. Visible and nearultraviolet lasers operate on electronic transitions in the outer or valence shells of atoms. Electronic transitions from outer shells to inner shells involve much more energy, and thus produce x rays, which have much shorter wavelengths. In theory, a population inversion on such an inner-shell transition should, under the right conditions, produce an x-ray laser.

The problem is that the right conditions are extremely difficult to produce. X-ray transitions are so energetic that it takes extremely high peak powers to produce population inversions. Moreover, x-ray transitions have very short excited-state lifetimes, so the energy must be applied and extracted very quickly.

Two fundamentally different approaches were demonstrated successfully in the 1980s by the Lawrence Livermore National Laboratory. One used the most intense energy source ever produced by the human race a nuclear explosion and details remain highly classified. The other used short, intense pulses from high-energy lasers built for fusion research, and is unclassified.

X-ray laser research began in the early 1960s but soon foundered because of the difficulty of producing the conditions required for lasing. By the late 1970s many unsuccessful experiments had been tried (Waynant and Elton, 1976), and most government support had dried up. Ironically, the first sign of progress appeared soon afterward, when British researchers saw evidence of laser gain in highly ionized carbon (Jacoby et al., 1981).