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It has been observed over the years that the energy coupled to the load in many zpinch experiments is larger than can be accounted for by the sum of the jxB work and classical Ohmic heating. Moreover, this energy enhancement appears to be a function of the generator design, increasing as the risetime of the current is increased. In recent experiments on the Saturn generator, for example, which was operated at current risetimes in excess of 160 ns, observed energy enhancements were factors of 2 to 4 times the energy input expected from JxB work alone. When Saturn operates with risetimes of less than 90 ns, much smaller energy enhancements over the JxB energy are seen. In the past, it was conjectured that some form of anomalous resistivity was needed to account for the extra energy input, while recently, a new idea was proposed based on the buildup of internally generated tubes of magnetic flux energy. It was hypothesized that the growth of the Rayleigh-Taylor instability at the surface of the z-pinch plasma would generate bubbles of magnetic flux-tube energy that deposit their energy in the plasma at a current-to-the-third-power rate. While 0-D modeling of the Saturn experiments shows that an anomalously high toad resistance can input the required energy needed to match the x-ray data, an alternate mechanism than magnetic flux-tubes exists for anomalous heating that is based on the production of micro-instabilities at the pinch surface. Both this and the flux-tube model are phenomenological and require guidance from experiments to be implemented. Several issues that arise from these enhanced energy coupling mechanisms are discussed in this paper. [ABSTRACT FROM AUTHOR]

The K-shell yield performance of Titan double-shell argon gas puff loads imploded on the Z machine and Decade Quad (DQ) is compared to the performance of Double-Eagle loads employing the same nozzle configuration. Specifically, the K-shell yields obtained on the Z and DQ machines are compared with I[sup 4], 1², and I[sup 0] current scaling projections made from Double-Eagle yields, where I is the peak load current. This analysis allows initialization and multidimensional effects to be factored out of the scaling behavior. This projection analysis is useful for evaluating argon gas-puff load performance because it relates I[sup 4], I², and I[sup 0] K-shell yield scaling transitions that a load, of a given specific energy, is predicted to undergo as the total load mass increases. Our analysis shows that the maximum K-shell yields were near optimum for the nozzle configuration used and the energy coupled to the load in the Z and Decade Quad experiments. At present it is not understood why an additional 22 nH of inductance for DQ, 6 nH for the Z machine, and 8 nH for Double-Eagle is required in their respective equivalent circuit models to reasonably match measured peak currents and implosion times. Note, we do not think that there is extra inductance in the real circuit; it is included in the modeled equivalent circuit as a method to account for the possible anomalous effects that are reducing the current from its expected value. These effects may include: multidimensional effects, current losses, and anomalous resistivity. [ABSTRACT FROM AUTHOR]

Over the last three decades there has been a quantum jump in the production of x-rays from pulsed power driven Z-pinch plasmas. Total radiative yields have gone from a few kilojoules to almost two megajoules. This increase occurred as a result of higher current drivers coupled with improvements in our understanding of the issues most relevant to good load design. Critical analyses of experimental data have led to a better understanding of the load dynamics, which includes all phases of load evolution extending from the cold start to the final collapsed phase and the emission of the x-ray pulse. A Z pinch is a deceptively simple device that has a very complex plasma dynamics. It can be a platform for demonstrating a variety of textbook plasma instabilities. However, its primary application in the present context is as an intense source of x-ray radiation. Therefore it is attractive both as a direct source of x-rays and for creating hohlraum conditions for plasma fusion experiments. After a few historical comments are offered on how radiation has been treated in modeling Z pinches, some of the methodologies and models that are employed in this endeavor are discussed. These include both nonLTE and LTE ionization dynamic models and escape probability radiation transport and LTE radiation diffusion models. To illustrate their use, comparisons are made between experimental data from a stainless steel wire array pinch implosion and 1-D MHD calculations that employ these models. The consequences that stem from the compromises and trade-offs that result from the different approximations used in these models are addressed. We will explore the role that radiation plays in the dynamic evolution of a Z-pinch and demonstrate the need for as near a self-consistent radiation-hydrodynamics treatment as possible. [ABSTRACT FROM AUTHOR]

As J × B forces implode a Z-pinch load, a variety of phenomena that affect its radiative behavior may occur. These include snowplows and shocks, instabilities, amplification of initial load asymmetries, and high-density condensations (possibly caused by a local radiative collapse). On the axis, arrival of the main load mass occurs, but it may be preceded by a precursor plasma. An important aim of experimental diagnostics is to determine the degree and importance of such physical processes as a function of the properties of the load and the driving generator. X-ray spectroscopy and imaging are well-established components of most Z-pinch diagnostic suites. The purposes of the present work are to determine the x-ray signatures of some of these phenomena and to ascertain the x-ray energies that provide the clearest and least ambiguous indicators of these processes. To accomplish this, we have carried out radiation hydrodynamics calculations of argon gas puff and titanium wire array loads imploded on Sandia's Z generator. These calculations provide a large database of x-ray spectra and images. Some clear image signatures are seen in the calculations. These include intensity enhancement near the outer edge of the pinch, which is characteristic of the heating and compression that occurs in the snowplowed region of the pinch prior to its final on-axis assembly. Spatial image intensity profiles in the lower energy x-rays are generally indicative of the density profile of the pinch. The profile of the image at different photon energies within the K-shell spectral region is related to the presence of an electron temperature gradient. [ABSTRACT FROM AUTHOR]

Frequently, the most energetic radiation emitted in abundance from a Z pinch is photons from the H- and He-like stages of the load element(s), i.e., the K shell. Consequently, the K-shell spectroscopy of Z pinches is well established and often used for diagnostics. Four recent advances are described in this paper that have been made in the diagnostic use and interpretation of K-shell radiation emitted from Z pinches. First, use of a trace element in double shell gas puff loads on Double EAGLE (at Maxwell Physics International) has demonstrated that it is the subsequently heated inner puff gas that radiates most of the K-shell photons from the assembled pinch. This finding is consistent with previous data analyses and theory which showed that the hot pinch interior radiates most of the K-shell energy, and shows that little or no mixing of the outer and inner puffs occurs during the implosion and assembly. Second, the use of the intercombination to resonance line ratio to infer density has been reconciled with density determinations based on absolute power. Third, photoexcitations have been shown to substantially alter the interpretation of this line ratio when it is applied to diagnosing the density profile from spatially resolved spectroscopy. Lastly, some wire array experiments using Al:Mg alloy loads at Sandia National Laboratories have specifically demonstrated how more spectral data, properly analyzed, translates into more detailed information about the pinch conditions. [ABSTRACT FROM AUTHOR]

We have developed uniform-fill nozzles for long-implosion PRS experiments to produce argon K-shell x-rays (photon energy > 3.1 keV). With a 7 cm diameter nozzle, which is appropriate for a 200 ns driver, we have achieved a stable implosion at 180 ns and 4 MA with peak argon K-shell yields exceeding 15 kJ. As previously seen with short (∼100 ns) implosion times, the K-shell yield scales as the fourth power of peak current, I4. Limited testing with a 10 cm nozzle, which is appropriate for a >250 ns driver, has also achieved a stable implosion. [ABSTRACT FROM PUBLISHER]