K-shell radiation and bright spot characteristics of high-energy-density Fe-Cr-Ni plasmas influenced by X-pinch load geometry

K-shell radiation is diagnostically advantageous for the study of high-energy-density plasmas. The current work investigates the influence of load geometry on K-shell x-rays in Z-pinch plasmas produced on the 1-MA Zebra generator. Stainless steel (Fe: 69%, Cr: 20%, Ni: 9%) X-pinch wire loads are fielded with an interwire angle of 31 degrees (small-angle) or 62.5 degrees (large-angle), respectively and studied using various x-ray diode, imaging, and spectroscopic diagnostics. The large-angle geometry produced large, individual x-ray-emitting bright spots ( > 3 keV) at the wire cross-point averaging areas >= 1 mm2, with noteworthy soft x-ray ( > 0.75 keV) bursts and radiation yields <= 14.6 kJ. The small-angle geometry produced multiple x-ray-emitting bright spots extending the pinch axis with smaller average sizes ( <= 0.5 mm2), consistent with stronger current, increased radiation yield ( >= 15.5 kJ), and notable hard x-ray ( > 9 keV) bursts, which peak with largest cumulative bright spot size. Spectroscopic analysis is performed with non-LTE collisional-radiative modeling, indicating a cold, nonthermal plasma region radiating from neutral to Ne -like ions, a hot, thermal region radiating from Fe and Cr He-and Li-like ions, and an intermediate region radiating K alpha satellites from Li-like to O-like ions. Modeling of the nonthermal and satellite features imply a fractional hot electron abundance-0.1% and-0.5% for large-and small-angle geometries, respectively. Relative intensity analysis is performed on He alpha and K alpha lines, revealing intensity ratios that deviate from the original wire composition. Optical depth estimates allude to optically thick, thermal K-shell plasmas with densities consistent with non-LTE modeling. (c) 2023 Elsevier Ltd. All rights reserved.