量研学術機関リポジトリ「QST-Repository」は、国立研究開発法人 量子科学技術研究開発機構に所属する職員等が生み出した学術成果（学会誌発表論文、学会発表、研究開発報告書、特許等）を集積しインターネット上で広く公開するサービスです。 Welcome to QST-Repository where we accumulates and discloses the academic research results(Journal Publications, Conference presentation, Research and Development Report, Patent, etc.) of the members of National Institutes for Quantum Science and Technology.
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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.
Lithium-like aluminum ion recombination plasma X-ray laser at 15.5 nm
