Gain predictions for nickel-like gadolinium from a 181-level multiconfigurational distorted-wave collisional-radiative model

We present small-signal gain coefficients for soft x-ray transitions in nickel-like gadolinium. The gains are predicted by a steady-state collisional-radiative model containing 181 detailed levels, including n=5 states. All possible electron collisional transitions among these levels are calculated in the distorted-wave approximation. Our calculations display features that may help to explain discrepancies between design predictions and the gains recently observed in a europium plasma. Also, we identify for the first time n=3 to n=3 inversions in the nickel-like ion that suggest that the ``water window'' at 43.7 A{} might be reached using such relatively low-Z elements as gadolinium.