Continuum Hard-Photon K-Shell Yields From Z-Pinch Implosions: Present Status and Scaling to Higher Currents

The use of the recombination continuum for X-ray radiation production in Z-pinches is discussed as an option for generating high yields in the warm photon energy range, $\hslash \omega \gt 10$ keV. The free-bound direct-recombination continuum emission is an inherently weaker radiation production mechanism than the resonant-line emission. However, it is challenging, if at all possible, to heat stagnated Z-pinch plasmas with atomic numbers $Z_{A} \gt 36$ to temperatures above ~10 keV needed for efficient K-shell line emission in the warm photon energy range. As the atomic number of the load material increases from $Z_{A} = 18$ (argon) to $Z_{A} = 26$ and 29 (iron and copper, respectively), the scaling parameter determining the ratio of the recombination continuum to hydrogen-like line K-shell yield increases by a factor of 2–3. This indicates a possibility of using wire-array Z-pinch implosions on next-generation pulsed-power facilities (NGPPs) to produce significant continuum yields in warm photons. Such an option is feasible, provided that stripping substantial fractions of iron and copper ions at stagnation to an H-like state can be demonstrated. We analyze K-shell continuum yields measured in recent Z experiments with argon double-shell gas-puff and stainless-steel nested wire-array loads, with a view of scaling present-day results to higher driver currents.