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 has been discussed as an option 1 for generating high yields in the warm photon energy range, ħω > 13 keV. The free-bound continuum emission is an inherently weaker radiation production mechanism than K-shell line emission. But it is challenging, if at all possible, to heat stagnated Z-pinch plasmas with atomic numbers Z A >36 to temperatures ~10 keV needed for efficient K-shell line emission. Experiments with argon gas-puffs 2,3 on Decade Quad and pre-refurbishment Z produced up to 10 to 20% of the total K-shell yield in high-energy continuum above the K-edge, 4.4 keV. 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 factor determining the ratio of the recombination continuum to line K-shell yield increases by a factor of 2 to 3. This indicates a possibility of using wire-array Z-pinch implosions on next-generation pulsed power facilities 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 gas-puff and stainless-steel wire-array loads, with a view of scaling present-day results to higher driver currents.