Population inversion and gain measurements for soft X-ray laser development in a magnetically confined plasma column

We present population inversion and gain measurements from an experimental investigation of possibilities to obtain high gain and lasing action in the soft X-ray region. Our approach to soft X-ray laser development is based on rapid plasma cooling after the laser pulse by radiation losses, leading to fast recombination and collisional cascade into upper excited levels of CVI, for example, while the lower excited levels depopulate rapidly by radiative transitions, thus creating population inversions and gain. A ≈ 0.5 kJ CO
2
laser was focused onto a target of solid carbon or teflon; or CO
2
, O
2
, Ne gas, and the resulting plasma confined in a 50-90 kG magnetic field. Spectroscopic diagnostics with absolute intensity calibration were used to measure level populations. Population inversions were observed between the
4d
and
3d
levels in the lithium sequence ions: CIV, OVI, FVII, and NeVIII, and a gain of 0.1 (10 percent) was estimated for the OVI
4f-3d
transition at 520 Å. In experiments with a solid carbon target, we observe relatively high CVI 182 Å emission in the axial direction compared to the transverse direction, which, if due to stimulated emission, would correspond to a gain of 4. Extension of these results to potential lasing transitions below 100 Å will be discussed.