Z-pinch tests of nickel at 18 MegaAmps and atomic number scaling considerations

Summary form only given, as follows. In a z-pinch, the energy required to strip an atom to its hydrogen-like state and excite K-shell X-rays increases faster than the cube of the atomic number. The effect is evident in the best K-shell yields reported to date: about 270 kJ of argon (Z=18), 125 kJ of titanium (Z=22) and 50 kJ for stainless steel (Z=26). With the objective of further testing our understanding of z-pinches, we conducted the first nickel (Z=28) implosions on the Sandia Z accelerator. A 2 cm long nested array (7 cm outer, 3.5 cm inner diameters) load, driven with 18 MA in 105 ns, produced 16/spl plusmn/3 kJ of nickel K-shell x-rays (>7.9 keV) in a 7 ns pulse. We used nickel alloys (10% chromium in one shell, 5% manganese in the other) to diagnose the shells´ interaction. Streaked x-ray spectra with nanosecond resolution showed that the inner shell´s mass heats up more quickly to a higher intensity than the outer shell. The Lyman to helium line ratios imply that the inner shell´s mass also gets hotter than the outer material. A striking result was that the tracer K-lines were much brighter than their relative masses would imply. Chromium was 20 to 30 times stronger (relative to nickel) than its mass fraction would imply; for manganese, the factor was 10 to 15. While opacity in the nickel might explain some of this, the dominant effect is probably linked to atomic numbers: chromium (Z=24) and manganese (Z=25) are much easier to excite than nickel. Another consequence of the tracers was that the analysis of the broadband sensor data - PCDs, calorimeters and bolometers - was greatly complicated by the significant tracer emission and the L-shell continua above 5 keV.