Soft-x-ray amplification at 503 Å in nickellike ytterbium

Nickellike ions of ytterbium (Yb42+) have been produced in long exploding foil plasmas. The plasmas were formed by short-pulse laser irradiation of thin foils of ytterbium with line-focused high-intensity 0.53-μm light. Soft-x-ray line emissions at 50.26 and 56.09 Å are identified as 4d–4p, J = 0−1 transitions in the nickellike ionization stage of ytterbium. The energy emitted in the line at 50.26 Å along the axis of the plasma is observed to increase nonlinearly with plasma length, consistent with a small-signal gain of 1.2 ± 0.4 cm−1.