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Radial wire array $Z$ -pinches, where wires are positioned radially outward from a central cathode to a concentric anode, can act as a compact bright X-ray source that could potentially be used to drive a hohlraum. Experiments were performed on the 7-MA Saturn generator using radial wire arrays. These experiments studied a number of potential risks in scaling radial wire arrays up from the 1-MA level, where they have been shown to be a promising compact X-ray source. Data indicate that at 7 MA, radial wire arrays can radiate $\sim 9$ TW with 10-ns full-width at half-maximum from a compact pinch.

By varying current-loss circuit parameters, the Mach2-tabular collisional radiative equilibrium 2-D radiation magnetohydrodynamic model was tuned to reproduce the radiative and electrical properties of three recent argon gas-puff experiments (same initial conditions) performed on the Z machine at Sandia National Laboratories. The model indicates that there were current losses occurring near or within the diode region of the Z machine during the stagnation phase of the implosion. The “good” simulation reproduces the experimental K-shell powers, K-shell yields, total powers, percentage of emission radiated in $\alpha $ lines, size of the K-shell emission region, and the average electron temperature near the time-of-peak K-shell power. The calculated atomic populations, ion temperatures, and radial velocities are used as input to a detailed multifrequency ray-trace radiation transport model that includes the Doppler effect. This model is employed to construct time-, space-, and energy-resolved synthetic spectra. The role the Doppler effect likely plays in the experiments is demonstrated by comparing synthetic spectra generated with and without this effect.

In developing stainless-steel (SS) and copper wire-array X-ray sources on the Z machine, we consider the optimization of K-shell yield as a function of load height. Theory, numerical modeling, and experimental data suggest that an optimum exists corresponding to a tradeoff between the increase in radiating mass and the decrease in coupled current with increasing pinch height. A typical load height of 20 mm used on many previous Z wire-array X-ray sources is found to be near optimal for K-shell yield production in SS and copper implosions. Electrical data, pinhole imaging, and spectroscopy are used to study plasma conditions in wire-array z pinches corresponding to the variation in K-shell power and yield per unit length as the pinch height is changed from 12 to 24 mm.

A comprehensive gas puff capability is being developed on the Z pulsed power generator. We describe the methodology employed for developing a gas puff load on Z, which combines characterization and modeling of the neutral gas mass flow from a supersonic nozzle, numerical modeling of the implosion of this mass profile, and experimental evaluation of these magnetic implosions on Z. We are beginning a multiyear science program to study gas puff z-pinch physics at high current, starting with an 8-cm diameter double-shell nozzle, which delivers a column of Ar gas that is imploded by the machine's fast current pulse. The initial shots have been designed using numerical simulation with two radiation-magnetohydrodynamic codes. These calculations indicate that 1 mg/cm should provide optimal coupling to the driver and 1.6:1 middle:outer shell mass ratio will best balance the need for high implosion velocity against the need to mitigate the magnetic Rayleigh–Taylor instability. The models suggest 300–500-kJ Ar K-shell yield should be achievable on Z, and we report an initial commissioning shot at lower voltage in which 250 kJ was measured. Future experiments will pursue optimization of Ar and Kr K-shell X-ray sources, study fusion in deuterium gas puffs, and investigate the physics of gas puff implosions including energy coupling, instability growth, and radiation generation.

Two-dimensional (r, z) magnetohydrodynamic simulations with nonlocal thermodynamic equilibrium ionization and radiation transport are used to investigate the K-shell radiation output from doubly nested large-diameter (> 60 mm) stainless-steel arrays fielded on the refurbished Z pulsed-power generator. The effects of the initial density perturbations, wire ablation rate, and current loss near the load on the total power, K-shell power, and K-shell yield are examined. The broad mass distribution produced by wire ablation largely overcomes the deleterious impact on the K-shell power and yield of 2-D instability growth. On the other hand, the possible current losses in the final feed section lead to substantial reductions in K-shell yield. Following a survey of runs, the parameters for the perturbation level, ablation rate, and current loss are chosen to benchmark the simulations against existing 65-mm-diameter radiation data. The model is then used to predict the K-shell properties of larger diameter (70 mm) arrays to be imploded on the Z generator.

Z-pinch experiments at pulsed-power facilities generally field a wide range of diagnostics, designed to make measurements and study the relevant physics during all phases of the Z-pinch. Optical and X-ray images are collected to study ablation, implosion, and the stagnated pinch; yield and power measurements are made at various photon energies; and time-integrated and time-resolved spectroscopies are recorded to infer plasma conditions before, during, and after stagnation. Typically, diagnostics fielded are dispersed azimuthally around the Z-pinch and at varying distances relative to the Z-pinch. The data are then analyzed and interpreted as a single entity to provide detailed information about a particular process or physics effect. Correlation of the results of the individual diagnostics in this fashion assumes that each diagnostic would measure the same result regardless of its azimuthal or radial location. Assessments of diagnostics to measure K-shell yield at the Z accelerator, which are fielded at varying azimuthal locations and radial distances, are presented. These measurements illustrate that the K-shell emission is azimuthally uniform for titanium, stainless-steel, and copper wire arrays and that the standard correction for radial distance is valid. The importance of view-factor corrections is also discussed.

Compact tungsten wire array Z-pinches imploded on the Z generator at Sandia National Laboratories have proven to be a powerful reproducible X-ray source. Wire arrays have also been used in dynamic hohlraum radiation flow experiments and as an intense K-shell source, while the generator has been used extensively for isentropic compression experiments. A problem shared by all these applications is current loss, preventing the ~20-MA drive current from being reliably coupled to the load. This potentially degrades performance, while uncertainties in how this loss is described limit our predictive capability. We present details of a transmission line equivalent circuit model of the Z generator for use in driving 3-D resistive MHD simulations of wire array loads. We describe how power delivery to these loads is affected by multiple current losses and demonstrate how these may be calculated or reconstructed from available electrical data for inclusion in the circuit model. We then demonstrate how the circuit model and MHD load calculation may be combined to infer an additional current loss that has not been directly diagnosed for wire arrays.