High resolution spectra of the W L transitions in the energy range 8 keV to 12 keV from warm dense plasmas generated by the Naval Research Laboratorys Gamble III pulsed power machine were recorded by a transmission-crystal x-ray spectrometer with ± 2 eV accuracy. The discharges were up to 2 MV voltage, 0.5 MA current, and 2.4 MJ/cm-3 energy density. The plasma-filled rod pinch (PFRP) anode produces a plasma with Ne ≈ 1022 cm-3 and Te ≈ 50 eV. By analyzing the line shapes, it was determined that the Lβ2 inner-shell transition from the 4d5/2 level was shifted to higher energy by up to 23 eV relative to nearby transitions from n = 3 levels. In addition, the Lβ2 transition was significantly broader and asymmetric compared to the n = 3 transitions. The energy shift of the Lβ2 transition results from the ionization of electrons outside the 4d shell that perturbs the transition energies in the ions to higher values. The increased line width and asymmetry results from unresolved transitions from a range of ionization states up to +28. The ionization distribution was determined by comparison of the measured energy shifts and widths to calculated transition energies in W ions, and the ionization was correlated with Gamble discharge parameters such as the anode type and the high voltage delay time. This work demonstrates a new hard x-ray spectroscopic technique for the direct measurement of the ionization distribution in warm dense plasmas of the heavy elements W through U that is independent of the other plasma parameters and does not require interpretation by hydro, atomic kinetics, and radiative simulation codes.
Citation: High Energy Density Physics
Pub Type: Journals
ionization distribution, plasma diagnostics, warm dense matter, x-ray spectroscopy