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Two energies are identified that define the x-ray emission characteristics of Z-pinch array implosions. One, the kinetic energy per ion, is intensive, and the other, the kinetic energy per centimeter, is extensive. From a series of one-dimensional axisymmetric hydrodynamic calculations, we have calculated the dependence of the x-ray emission from aluminum implosions above 1 keV on these energies. These calculations are carried out for a specially chosen theoretical case where the kinetic energy that is generated during implosion is converted to thermal energy and x rays during the plasma collision on axis in the absence of current. In this case, we determine the I4 to I2 transition of the scaling of emission with peak current, I, as a parametric function of the kinetic energy per ion. We also determine a functional dependence of the emission on this energy when the mass of the imploded aluminum array is held fixed. It is seen that the ability of the plasma to radiate large amounts of energy in either I4 or I2 regimes is strongly dependent on the mass loading. Finally, some arguments are presented on how the breakpoint between I4 and I2 scaling is expected to scale when the atomic number of the array load is varied.

Annular Al-wire Z-pinch implosions on the Saturn accelerator ͓D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference ͑Institute of Electrical and Electronics Engineers, New York, 1987͒, p. 310͔ that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh-Taylor ͑RT͒ instability prior to the first compression seeds an mϭ0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional ͑2-D͒ radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These simulations agree qualitatively with the measured characteristics of the first x-ray pulse and the overall energetics, and they provide a 2-D view into the plasma hydrodynamics of the implosion.

A 90-wire, aluminum, z-pinch experiment was conducted on the Saturn accelerator at the Sandia National Laboratories that exhibited azimuthally symmetric implosions and two x-ray bursts, a main burst and a subsidiary one. These bursts correlated with two consecutive radial implosions and are consistent with predicted magnetohydrodynamics behavior. A variety of time-resolved, accurately timed, spectroscopic measurements were made in this experiment and are described in this paper. These measurements include ͑1͒ the pinch implosion time, ͑2͒ time-resolved pinhole pictures that give sizes of the K-shell emission region, ͑3͒ timeresolved K-series spectra that give the relative amounts of hydrogenlike to heliumlike to continuum emission, ͑4͒ the total and the K-shell x-ray power outputs, and ͑5͒ time-resolved photoconducting diode measurements from which continuum slopes and time-resolved electron temperatures can be inferred. Time-resolved Ly-␣ and Ly-␤ linewidths are obtained from the spectra and inferences about time-resolved ion temperatures are also made. All of these data correlate well with one another. A method is then presented of analyzing this data that relies on the complete set of time-resolved measurements. This analysis utilizes one-dimensional radiative magnetohydrodynamic simulations of the experiments, which drive z-pinch implosions using the measured Saturn circuit parameters. These simulations are used to calculate the same x-ray quantities as were measured. Then, comparisons of the measured and calculated data are shown to define a process by which different dynamical assumptions can be invoked or rejected in an attempt to reproduce the ensemble of data. This process depends on the full data set and provides insight into the structure of the radial temperature and density gradients of the on-axis pinch. It implies that the first implosion is composed of a hot plasma core, from which the kilovolt emissions emanate, surrounded by a cooler, denser shell, and it provides details about the structure of the temperature and density gradients between the core and shell regions. These results are found to be broadly consistent with an earlier, less detailed, data analysis in which plasma gradients are ignored. However, the ability to reproduce the full spectroscopic data in the present analysis is found to be sensitively dependent on the radial gradients that are calculated. ͓S1063-651X͑97͒06609-9͔

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Radiation transport redistributes energy within a medium through the emission and reabsorption of photons. These processes also have a pronounced effect on the spectrum of radiation that escapes the medium. As the deliverable energies of plasma drivers such as lasers and pulsed-power generators steadily increase, denser and/or more massive plasmas can be created. Such plasmas are more absorptive to their own emitted radiation, with portions of the line spectrum frequently being highly opaque. Thus, radiation transport becomes more important, along with the need to consider its impact on the design of experiments and their diagnosis. This tutorial paper covers the basic theory and equations describing radiation transport, its physical effects, experimental examples of transport phenomena, and current challenges and issues. Among the specific topics discussed are requirements for local thermodynamic equilibrium (LTE), conditions for diffusion and the use of the diffusion approximation, the formation of emission and absorption lines, the approach of an emitted spectrum to the Planck limit, and diagnostic applications of transport effects.

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