Y. K. Chong

Deuterium gas-puff Z-pinch implosions on the Z acceleratora)

Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34MeV (±0.10MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility.

Initial results for an argon Z pinch using a double-shell gas puff

Recent observations are given for an argon double-shell gas puff imploded with up to 4 MA in 200 ns on the Double Eagle generator [G. B. Frazier et al., Digest of Technical Papers, Fourth IEEE Pulsed Power Conference (IEEE, Piscataway, NJ, 1983), p. 583]. Good K-shell x-ray output with good pinch quality was observed. A novel experimental procedure was used to selectively seed the inner or outer gas plenums with a chlorine tracer. The tracer data provide the first direct experimental evidence that the mass initially closest to the axis is the dominant contributor to the hot core of the radiating pinch.

Stabilized radiative Z-pinch loads with tailored density profiles

Mitigation of the Rayleigh–Taylor (RT) instability of Z-pinch loads imploded from large initial radii through tailoring initial load density profiles in radial and axial directions is studied numerically. These methods could be helpful for a variety of applications of high-power Z-pinches, from producing large amounts of K-shell radiation to imploding inertial confinement fusion pellets. Radial density tailoring is demonstrated to delay the onset of the RT instability development at the expense of reducing the energy available for conversion into radiation. Axial density tailoring can fully stabilize acceleration of a fraction of the initial load mass. For a better tradeoff between stability and radiative performance of the loads, the density profiles could be tailored in two dimensions, combining the advantages of both methods. Post-processing of the radiation-magnetohydrodynamic simulation results demonstrates that an appreciable K-shell argon radiation power could be generated with a stabilized argon l...

Scaling of K-Shell Emission From

Experiments in the last few years at the 20-MA Z Accelerator have produced significant K-shell X-ray output from a variety of initial load materials, including aluminum (1.7-keV photons, 400-kJ yield), argon (3.1-keV photons, 300-kJ yield), titanium (4.8-keV photons, 100-kJ yield), stainless steel (6.7-keV photons, 50-kJ yield), and copper (8.4-keV photons, 20-kJ yield). K-shell scaling theories developed at the Naval Research Laboratory [K. G. Whitney , Phys. Rev. E 50, 2166 (1994)] in the 1990s were benchmarked against the Al K-shell emission data from 10-MA facilities. The experiments at Z have not only led to a heuristic validation of this original theory but have also provided the data to fine tune the models for application to higher photon energies and for extension to higher current generators. The upgrade of the Z Accelerator to ZR, which will provide 26 MA to a -pinch load, should increase the radiated K-shell output for sources previously fielded at Z and will extend the range of photon energies where measurable radiation can be observed, which is likely up to 13 keV. A summary of the K-shell experiments at Z is presented, as well as an overview of the modified empirical-scaling theory. Proposed load configurations for ZR are discussed, as well as predictions for K-shell output.

Z

Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.

-Pinches: Z to ZR

In this paper, a theoretical model is described and demonstrated that serves as a useful tool for understanding K-shell radiating Z-pinch plasma behavior. Such understanding requires a self-consistent solution to the complete nonlocal thermodynamic equilibrium kinetics and radiation transport in order to realistically model opacity effects and the high-temperature state of the plasma. For this purpose, we have incorporated into the MACH2 two-dimensional magnetohydrodynamic (MHD) code [R. E. Peterkin et al., J. Comput. Phys. 140, 148 (1998)] an equation of state, called the tabular collisional radiative equilibrium (TCRE) model [J. W. Thornhill et al., Phys. Plasmas 8, 3480 (2001)], that provides reasonable approximations to the plasma’s opacity state. MACH2 with TCRE is applied toward analyzing the multidimensional implosion behavior that occurred in Decade Quad (DQ) [D. Price et al., Proceedings of the 12th IEEE Pulsed Power Conference, Monterey, CA, edited by C. Stallings and H. Kirbie (IEEE, New York, ...

Opacity and gradients in aluminum wire array z-pinch implosions on the Z pulsed power facility

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 machines 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.

One-and two-dimensional modeling of argon K-shell emission from gas-puff Z-pinch plasmas

Argon gas puffs have produced 330 kJ ± 9% of x-ray radiation above 3 keV photon energy in fast z-pinch implosions, with remarkably reproducible K-shell spectra and power pulses. This reproducibility in x-ray production is particularly significant in light of the variations in instability evolution observed between experiments. Soft x-ray power measurements and K-shell line ratios from a time-resolved spectrum at peak x-ray power suggest that plasma gradients in these high-mass pinches may limit the K-shell radiating mass, K-shell power, and K-shell yield from high-current gas puffs.

A Renewed Capability for Gas Puff Science on Sandia's Z Machine

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.

The effect of gradients at stagnation on K-shell x-ray line emission in high-current Ar gas-puff implosions

The implosion dynamics of an array of stainless steel (SS) wires on the Z and/or ZR accelerator produces an abundance of radiation from the K‐ and L‐shell ionization stages. As the plasma assembles on axis, a number of time resolved snapshots provide temperature and density profiles and size of the emitting region. The non‐LTE populations will be obtained by using detailed atomic models that include all important excitation, ionization, and recombination processes. In particular, we will investigate the effects of dielectronic recombination (DR) which is the most important recombination process for moderate to high Z plasma such as iron at moderate densities. We will analyze the ionization dynamics and generate K‐ and L‐shell spectra using the temperature and density conditions generated in the Z and/or ZR accelerator describing the implosion with a 1‐D non‐LTE radiation hydrodynamics model.