@misc{oai:repo.qst.go.jp:00079352,
 author = {Wang, J. and Murakami, K. and R. Shinmori and Kishimoto, Maki and Namba, S. and Kishimoto, Maki},
 month = {Nov},
 note = {X-ray laser is applied to many scientific and engineering fields such as X-ray microscope, X-ray photoelectron spectroscopy, X-ray lithography, and X-ray holography. Because the plasma-excited X-ray laser is driven by the stimulated emission phenomenon of multiply-charged ions in laser generated plasmas, a driver laser system with an extremely high output energy was necessary. Therefore, the development of a compact and high-repetition X-ray laser, which can be operated and maintained even in a university laboratory, has been carried out. As major lasing scheme of the X-ray laser, there are two methods, that is, transient collisional excitation scheme and recombination plasma scheme. In the recombination plasma scheme that we focus on, a relatively light element is ionized by laser irradiation onto the metal or gas target and high-temperature and high-density plasma is generated. When the hot, dense plasma is rapidly cooled due to an adiabatic expansion, a non-equilibrium plasma is created, where a three-body recombination process dominate over the other processes. Subsequently, the electron captured into highly-excited states are subjected to collisional deexcitation, resulting in the transition into lower states. Consequently, a population inversion between lower levels is generated. 
The groups of RIKEN and Toyoda Institute of Technology have adopted the recombination plasma scheme and observed the soft X-ray amplification (Li-like Al 3d-4f, 15.5nm) in a high-density Al plasma generated by 16 pulse trains of Nd:YAG laser system. In this study, the similar recombination plasma scheme was employed and lasing experiments were conducted for optimization of laser oscillation on Li-like Al soft X-ray laser. The pumping laser system was a compact Nd:YAG laser (pulse width: 10 ps or 100 ps, pulse interval: 200 ps, total output energy: 2 J for 10 ps laser, 3 J for 100-ps laser, 16 pulse trains), line focusing onto the Al target by a prism array. The laser intensity was 3×10^(11)-1×10^(12) W/cm^2 (focusing size: 0.05 mm×11 mm). Soft X-rays from the laser plasma was measured by a grazing incident spectrometer with a flat field grating. In the experiments,the gain coefficient estimated by the 100-ps laser pulse trains (lasing transition: Li-like 3d-4f 15.5 nm). The gain coefficients obtained are g=8.6 cm-1 and 5.5 cm-1, which is much higher than the value obtained by the previous experiment (g~3.2). 
 Also, we measured the gain coefficient for 10-ps laser pulse train. The optimal laser pulse duration will be discussed in detail., 3rd Asia-Pacific Conference on Plasma Physics},
 title = {Lithium-like aluminum ion recombination plasma X-ray laser at 15.5 nm},
 year = {2019}
}