Space- and time-resolved investigation of short wavelength x-ray laser in Li-like Ca ions

X-ray lasers in recombination pumped Li-like ions were initially demonstrated by Jamelot et al, in 1985 with Mg9+ and Al”+ ions.’ Because of its advantages in requiring lower drive energy, scaling faster to shorter wavelength, and hence being less expensive, the Li-like recombination scheme has attracted many scientists around the world.‘-” Although great progress has been achieved in shortening the lasing wavelength and raising the gain coefficient, some problems still remain unresolved for understanding the lasing mechanism. Recombination pumped Li-like x-ray lasers originate from mainly nf-3d transitions in the Li-like ions. The nf levels are populated through a direct collisional recombination from the ground state of He-like ions. The nf-3d population inversion is maintained by fast radiative decay from the 3d to the 2p levels while the radiative decays of nf levels to lower states are slower. Thus, a higher density of the plasma is needed to ensure the nf level populations, and a fast cooling is needed to increase the recombination of nf level and to avoid the collisional excitation of 3d levels. As the atomic number of the lasing ions increases, this condition becomes more restricted. If free expansion is the dominant cooling mechanism, for 2=20, the drive pulse was predicted to be less than 100 ps at 1.05 ym drive wavelength by a self-similar code coupling with a collisionradiation model. Recently in a new round of x-ray laser experiment; we have successfully demonstrated the soft x-ray amplification for 4f -3d (57.7 A) transition of the Li-like Ca ions in slab CaF, targets irradiated by 900 ps, 1.05 pm optical laser pulses. One of the reasons we use this long drive pulse is that shorter pulses are not available on the LF12 Facility at present. We have also obtained a set of timeand spaceresolved spectra, which may provide some valuable information for understanding the lasing action. A simplified numerical simulation was performed to meet the experimental results, showing the possible important role of other cooling mechanisms beside adiabatic expansion in long laser pulse drived experiments. The experiment was carried out at the two-beam LF12 laser system. Each beam delivers -600 J energy at 1.05 pm with a 900 ps-duration full width at half-maximum (FWHM) quasi-Gaussian pulse. In the experiment, the north beam was line-focused by a six-element cylindricallens array to form a 12.5 mm>i 120 ym uniform focus on the target surface, with a corresponding intensity of -4 x 1013 W cmM2. Length of the slab CaF, target with polished surface varied from 2 to 10 mm. A flat field grazing incidence grating spectrograph (FFGIGS) ” with a grazing incidence pre-optics consisted of a cylindrical mirror and a spherical mirror was aligned to the axis of the line focus. One dimensional spatially resolved spectra were recorded on x-ray lilm or by a soft x-ray streak camera.‘” Because only relative calibration of the film and no calibration of the spectrograph and the camera have been performed, we are not able to measure the absolute intensities of the output x-ray laser, hence the energy. But for gain demonstration, knowing the relative intensity is enough. Fofthe same reason, time resolved gain cannot be deduced. ‘When the streak camera (time resolution about 50 ps) was used, it was so adjusted that its scanning slit in front of the photocathode was set parallel to the dispersion axis of the spectrum and to cover the desired part of the spectrum from 40 to 90 A. By precisely controlling the position of the scanning slit or the target, time-resolved spectra at different distances from the target surface can be obtained. A typical on-axis time-integrated CaF, spectrum from the FFGIGS spectrograph is shown in Fig. 1. The spec-