[en] Recent results from x-pinches at the NTF provide experimental evidence for the existence of strong electron beams in x-pinch plasmas and motivate the development of a new diagnostic, x-ray spectropolarimetry, for investigating the anisotropy of such plasmas. This diagnostic is based on theoretical modeling of polarization-dependent spectra measured simultaneously by spectrometers with different sensitivity to polarization. Results of the first polarization-sensitive experiments at the NTF are presented. K-shell emission from Ti x-pinches is recorded simultaneously by two identical spectrometers with the dispersion plane perpendicular and parallel to the discharge axis. The spectroscopic analysis of more than eight Ti x-pinch shots show how spectropolarimetry complements the usual diagnostics of a z-pinch plasma. The polarization-sensitive spectra, generated by a Maxwellian electron beam at LLNL EBIT have been collected and analyzed. These data make an important contribution to the plasma polarization spectroscopy program at the NTF. In particular, the study of multiply-charged Ti ion spectra help in the interpretation of the polarization-sensitive spectra from Ti x-pinches at the NTF

[en] An analysis of the energy, spatial character, and temporal evolution of electron beams and hard x rays from 0.9 to 1.0 MA high-Z X pinches is presented. Experimental results from Ti, Fe, Mo, and W X pinches show that X pinches are an effective source of hard x rays with energies greater than 30 keV. Electron beams with energies up to 2 MeV higher than the applied anode-cathode voltage are generated along the pinch axis before the maximum current is reached. The beams have diameters of about 3 mm and generate bursts of hard x rays with sizes between 1 and 2 mm and total time durations of up to 150 ns. The measured 100-500 keV x-ray distribution is spectrally anisotropic. Hard x-ray synchrotron radiation in the side-on direction is proposed as a possible explanation of this anisotropy

[en] Plasma polarization spectroscopy work done by our group since the 3rd US-Japan PPS Workshop is overviewed. Theoretically, the polarization dependence on various electron distribution functions for He-like, Ne-like, and Ni-like x-ray transitions for a wide range of Z has been investigated. In particular, this study was focused on the polarization dependence for monoenergetic and steep electron distribution functions. The diagnostically important spectral lines and features of K-, L-, and M-shell ions were identified which can be used in x-ray spectropolarimetry of plasma. Importance of polarization-sensitive LLNL Electron Beam Ion Trap data is emphasized. The results of the UNR polarization-sensitive Ti and Mo x-pinch experiments are discussed

[en] University-scale Z-pinch devices are able to produce plasmas with a broad range of sizes, temperatures, densities, their gradients, and opacity properties. Radiative properties of such plasmas depend on material, mass, and configuration of the wire array loads. Experiments with two different types of loads, double planar wire arrays (DPWA) and X-pinches, performed on the 1 MA Zebra generator at UNR are analyzed. X-pinches are made from Stainless Steel (69% Fe, 20% Cr, and 9% Ni) wires. Combined DPWAs consist of one plane from SS wires and another plane from Alumel (95% Ni, 2% Al, 2% Si) wires. The main focus of this work is on the analysis of plasma jets at the early phase of plasma formation and the K-and L-shell radiation generation at the implosion and stagnation phases in experiments with the two aforementioned wire loads. The relevant theoretical tools that guide the data analysis include non-LTE collisional-radiative and wire ablation dynamics models. The astrophysical relevance of the plasma jets as well as of spectroscopic and imaging studies are demonstrated.

[en] Absorption features from K-shell aluminum z-pinch plasmas have recently been studied on Zebra, the 1.7 MA pulse power generator at the Nevada Terawatt Facility. In particular, tungsten plasma has been used as a semi-backlighter source in the generation of aluminum K-shell absorption spectra by placing a single Al wire at or near the end of a single planar W array. All spectroscopic experimental results were recorded using a time-integrated, spatially resolved convex potassium hydrogen phthalate (KAP) crystal spectrometer. Other diagnostics used to study these plasmas included x-ray detectors, optical imaging, laser shadowgraphy, and time-gated and time-integrated x-ray pinhole imagers. Through comparisons with previous publications, Al K-shell absorption lines are shown to be from much lower electron temperature (∼10–40 eV) plasmas than emission spectra (∼350–500 eV).

[en] The double planar wire array (DPWA) Z pinch is a highly efficient radiation source with unique implosion dynamics and precursor formations. The inductively divided current successively ablates the wires and injects the material to the interior of the array. Three uniquely imploding DPWA load types were identified and classified according to the critical load parameter: low, intermediate, or high aspect ratio. Radiation pulse shaping was obtained from secondary precursors: off-axis mass carrying high current densities during the implosion phase. Time-gated EUV spectroscopy of off-axis mass accumulations is used to assess a 60 eV electron temperature prior to the implosion phase. These structures are shown to form by a different mechanism than the secondary precursors. High yields, compact size, and shaped radiation pulses merit further consideration of the DPWA as a candidate for inertial confinement fusion research.

[en] Analysis is presented on scaling of radiated x-ray power, energy and implosion timing of single planar wire arrays (SPWA), double planar wire array (DPWA), and compact cylindrical wire array (CCWA) loads (diameter is 3 and 6 mm) of Mo and W with respect to current peak (0.8-1.4 MA), mass and array dimensions at 100 ns current pulse. Such scaling investigations are important for understanding the potential of these loads as an ICF radiation source⁷. These data are used to identify promising directions to pursue with regard to highest x-ray output, smallest load size, and most consistent shot-to-shot performance. It is shown that W SPWA and DPWA total energy yield and peak power increased near-quadratically with current. DPWA scans of inter-planar gaps from 1.5 mm to 9 mm show an output maximum at 1.5 mm with decreasing output for 6 mm and 9 mm. A DPWA width scan shows that radiation yields decrease slowly as the width is decreased, which may allow for more compact loads without significant sacrifice to the output radiation. A mass scan of several W loads show that the implosion timing increases with mass.

[en] This paper presents a detailed investigation of the temporal, spatial, and spectroscopic properties of L-shell radiation from 0.8 to 1.0 MA Mo x pinches. Time-resolved measurements of x-ray radiation and both time-gated and time-integrated spectra and pinhole images are presented and analyzed. High-current x pinches are found to have complex spatial and temporal structures. A collisional-radiative kinetic model has been developed and used to interpret L-shell Mo spectra. The model includes the ground state of every ionization stage of Mo and detailed structure for the O-, F-, Ne-, Na-, and Mg-like ionization stages. Hot electron beams generated by current-carrying electrons in the x pinch are modeled by a non-Maxwellian electron distribution function and have significant influence on L-shell spectra. The results of 20 Mo x-pinch shots with wire diameters from 24 to 62 μm have been modeled. Overall, the modeled spectra fit the experimental spectra well and indicate for time-integrated spectra electron densities between 2x10²¹ and 2x10²² cm^-3, electron temperatures between 700 and 850 eV, and hot electron fractions between 3% and 7%. Time-gated spectra exhibit wide variations in temperature and density of plasma hot spots during the same discharge

[en] Experiments with different stainless steel (SS) wire loads were performed on the 1 MA Zebra Z-pinch generator at University of Nevada, Reno. The wire array loads consisted of 7.6 μm SS wires and had 10 wires for the planar wire array with an interwire gap of 1 mm and 8 wires for the cylindrical wire array of a 16 mm diameter. In addition, a single-wire experiment with a 25 μm SS wire was carried out. The different wire loads were used to provide a broader spectrum of plasma conditions. Time-integrated and time-gated x-ray images, as well as time-integrated, spatially-resolved and spatially-integrated x-ray spectra, were collected and analyzed. Both K-shell and L-shell radiation were recorded using LiF and KAP crystal spectrometers, respectively. Non-LTE kinetic models of Fe and Ni are employed to derive plasma parameters. For axially resolved L-shell spectra, the resulting electron temperatures are between 230 and 300 eV (higher near the cathode) and electron densities vary from 10¹⁹ to 10²⁰ cm^-3 dependent on the load. The advantage of using Z-pinch plasmas for astrophysical applications is highlighted.

[en] The development of spectroscopic modeling of M-shell tungsten z-pinch plasma is presented. The spectral region from 3.5 to 6.5 A includes three distinct groups of transitions, and the best candidates for M-shell diagnostics are identified. Theoretical modeling is benchmarked with LLNL electron beam ion trap data produced at different energies of the electron beam and recorded by crystal spectrometers and a broadband microcalorimeter. A new high temperature plasma diagnostic tool, x-ray spectropolarimetry, is proposed to study polarization of W line emission and is illustrated using the results of x-pinch polarization-sensitive experiments. The x-ray line polarization of the prominent M-shell tungsten lines is calculated, and polarization markers are identified. The advantage of using x-pinch W wire experiments for the development of M-shell diagnostics is shown