Observation of strong soft-x-ray amplification at 8.8 nm in the transient collisional-excitation scheme

We have observed strong amplification with a gain of 14.5 cm⁻¹ for the 4d→4p transition of the Ni-like La ions at a wavelength of 8.8 nm pumped by a chirped pulse amplification Nd:glass laser with an energy of 18 J. In this experiment, the laser pulses consist of a prepulse and a main pulse with durations of 200 ps and 7 ps, separated by 250 ps. The keV x-ray spectroscopy and theoretical calculation indicate that the plasma temperature increased effectively by reducing the prepulse intensity, and that the substantial gain is generated in a small overcritical region heated by the electron heat transport.