高速Zピンチ放電による水素様窒素軟X線レーザー実現に向けた研究 Study for realization of a H-like N soft X-ray laser excited by fast z-pinch discharge 高速Zピンチ放電による水素様窒素軟X線レーザー実現に向けた研究
Study for realization of a H-like N soft X-ray laser excited by fast z-pinch discharge
To realize an H-like N Balmer alpfa recombination soft X-ray laser by utilizing a z-pinch plasma, a pulsed power based experimental set-up has been developed, followed by the theoretical estimation, and possibility of the population inversion has been discussed.At the maximum pinch, electron temperature of Te ≈ 200 eV and electron number density of ne ≈ 1×1020 cm-3 are needed in order to generate NVIII ions. In addition, in the subsequent expansion phase where the population inversion between the principal quantum number n=2 and 3 is expected, generation of a rapidly cooled expanding plasma with Te < 50 eV and ne ≈ 1×1018-19 cm-3 in which the collisional de-excitation process from a higher excited level dominates is required. Furthermore, the cooling should be completed within about 10 ns before thermal equilibrium with respect to a ground state of the H-like N ion is established. As a consequence, triangular current with peak amplitude of over 50 kA and decay time of about 30 ns is required.Developed pulsed power system consists of a water capacitor charging section and a capillary discharge section. Owing to adoption of a 2 stage LC generator, a 2:54 step-up transformer and a water capacitor, highly energy-efficient and stable charging of the capacitor was realized. And, utilizing a water transmission line set in between a gap switch and a discharge load, the current wave form shaping by utilizing a voltage wave propagation delay has been proposed.Using the triangular current, existence of Lyman series and recombining continuum of H-like N ion in the decay phase of the current has been anticipated in the time evolution of XRD signals. Comparing the framing photographs taken by using a high speed camera and MHD simulation results, rapid axially uniform expansion within 20 ns has been anticipated due to the sufficient decreases in the magnetic pressure and additional Joule heating. Time integrated spectroscopic pinhole image clarified that the pinched plasma with Te ≈ 150 eV and ne ≈ 0.5×1020 cm-3 rapidly cooled down to that with Te ≈ 50 eV and ne ≈ 1×1018 cm-3 within 20 ns. As a result, relative enhancement of the Balmer alpfa line emission from the recombining plasma has been observed, which might be initiated by the stimulated emission from the recombining plasma. However, much higher electron and NVIII densities are required in the future study to confirm the lasing.