Abstract
We determined the optimum electron temperature for water-window soft x-ray emission by observing the spectral behavior in the 2.3–4.4 nm region of highly charged zirconium (Zr) ions in hot hydrogen plasmas produced in a magnetically confined fusion device. In the water-window soft x-ray spectral region, the main emission involved resonant ${3}d - {4}f$3d−4f ($\Delta n = {1}$Δn=1) transitions. According to the collisional-radiative (CR) model verified by the experimental results, the ionic population for the charge states from ${{\rm Zr}^{14 + }}$Zr14+ to ${{\rm Zr}^{20 + }}$Zr20+ required to produce the emission was maximized in the electron temperature range of 80–180 eV. For laboratory-scale water-window soft x-ray source application, it is therefore important to produce an electron temperature significantly lower than 200 eV for the production of the suitable ionic charge states of ${{\rm Zr}^{14 + }} - {{\rm Zr}^{20 + }}$Zr14+−Zr20+.
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