J. F. Seamen

Tungsten wire-array Z-pinch experiments at 200 TW and 2 MJ

Here Z, a 60 TW/5 MJ electrical accelerator located at Sandia National Laboratories, has been used to implode tungsten wire-array Z pinches. These arrays consisted of large numbers of tungsten wires (120–300) with wire diameters of 7.5 to 15 μm placed in a symmetric cylindrical array. The experiments used array diameters ranging from 1.75 to 4 cm and lengths from 1 to 2 cm. A 2 cm long, 4 cm diam tungsten array consisting of 240, 7.5 μm diam wires (4.1 mg mass) achieved an x-ray power of ∼200 TW and an x-ray energy of nearly 2 MJ. Spectral data suggest an optically thick, Planckian-like radiator below 1000 eV. One surprising experimental result was the observation that the total radiated x-ray energies and x-ray powers were nearly independent of pinch length. These data are compared with two-dimensional radiation magnetohydrodynamic code calculations.

Time-dependent electron temperature diagnostics for high-power, aluminum z-pinch plasmas

Time-resolved x-ray pinhole photographs and time-integrated radially resolved x-ray crystal-spectrometer measurements of azimuthally symmetric aluminum-wire implosions suggest that the densest phase of the pinch is composed of a hot plasma core surrounded by a cooler plasma halo. The slope of the free-bound x-ray continuum, provides a time-resolved, model-independent diagnostic of the core electron temperature. A simultaneous measurement of the time-resolved K-shell line spectra provides the electron temperature of the spatially averaged plasma. Together, the two diagnostics support a one-dimensional radiation–hydrodynamic model prediction of a plasma whose thermalization on axis produces steep radial gradients in temperature, from temperatures in excess of 1 kV in the core to below 1 kV in the surrounding plasma halo.

K-shell radiation physics in the ultrahigh optical depth pinches of the Z generator

Al:Mg alloy wire arrays of mass loads 1.3–3.6 mg/cm have been imploded with peak currents of 19 MA on the 60 TW Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The large mass loads have resulted in the highest K-shell x-ray line optical depths (∼103) produced to date in Z-pinches. Analysis of the time-resolved spectrum of a 2.1 mg/cm shot near the time of peak compression has yielded a temperature–density profile of the pinch that approximately reproduces all features of the x-ray data except the continuum above 5 keV, which is underpredicted. The Ly α/He α ratio for Al is shown to be enhanced relative to that of Mg by two mechanisms: photopumped ladder ionization and absorption of the Al He-like line in a cool outer halo. This analysis and comparisons to some Ti shots demonstrates that the K-shell yield of Al is significantly reduced by line and continuum self-absorption, but that of Ti is not.

Streaked laser shadowgraphy of tungsten wire array implosions on the Saturn generator

A combination of a 400 ns, 300 mJ, 640 nm dye laser, and an optical streak camera have been used to demonstrate that time-resolved shadowgrams can be made of the implosion phase of tungsten wire arrays. Initial experiments have shown that mirror lifetime and spatial resolution are issues for this diagnostic technique. Nonetheless, these experiments have provided new information on wire array dynamics; specifically, they show that even with a 0.46 mm wire spacing, the high density regions formed by the wires, are separate until 30 ns into the main drive current. Peak currents of 6.6 MA were obtained 40 ns after the start of the current, while peak radiated powers of 85 TW were measured at 50 ns.

Wire fixturing in high wire-number z pinches critical for high radiation power and reproducibility

The quality of high wire-number z-pinch implosions on Z using a dynamic hohlraum (DH) configuration [Sanford, et al., Phys. Plasmas 9, 3573 (2002)] is significantly affected by the method of holding the wires. The three arrangements discussed here have led to differences in radial and axial x-ray powers of factors of 1.6±0.2 and 1.5±0.2, respectively. An increase in power is accompanied by reductions in rise time and pulse width, and improvements in shot-to-shot reproducibility. Higher powers are produced by fixtures that enable the wires to be maintained taut, which also produce superior current contacts at the electrodes (and in particular at the cathode) prior to implosion. The increased axial power, and decreased variation in power and pulse shape, correlate with decreased wire-plasma material observed at the axial radiation exit holes of the DH.

Fast z-pinches as dense plasma, intense x-ray sources for plasma physics and fusion applications

As a result of advances in fast pulsed-power technology and cylindrical load fabrication, the Z pulsed-power accelerator at Sandia National Laboratories drives currents approaching 20 MA with a rise time of approximately 100 ns through cylindrically-symmetric loads (typically a cylindrical array consisting of a few hundred wires) to produce plasma densities in excess of , x-ray output energies approaching 2 MJ, radiation pulses as short as 4 ns and peak x-ray powers as high as . More than 15% of the stored electrical energy in the Z pulsed-power accelerator is converted into x-rays. The plasma pressures at peak compression are several TPa with electron temperatures that can exceed 3 keV at containment magnetic fields exceeding 1000 T. Depending on the atomic number and composition of the imploding plasma, these z-pinches can be tailored to produce intense sources of thermal x-rays, keV x-rays or neutrons. Although applications of these x-ray sources have included research in radiation material interaction, equations of state, opacity, astrophysics and x-ray lasers, the principal focus of the present research is to use them for indirect-drive inertial confinement fusion (ICF).

Spatially and temporally resolved crystal spectrometer for diagnosing high-temperature pinch plasmas on Z

We have developed a spatially and temporally resolved crystal spectrometer for analyzing a variety of pinch experiments on Z. The spectrometer uses a convex curved crystal to disperse spectra onto a flat microchannel plate (MCP) framing camera detector. A single wide, 1 cm, strip on the MCP is gated to provide temporal resolution. The spectral range governed by the 4 cm length of the MCP strip varies with the central Bragg angle and crystal. For a KAP crystal a typical range is 1500–2000 eV. This range can be shifted by translating the crystal along the optical axis to access different Bragg angles. The spectrometer can therefore measure K shell spectra of a wide variety of elements such as Al, Ti, and Fe. The short 1 cm width of the strip is spatially resolved with an imaging cross slit. With a 500 μm cross slit and magnification 1 the spatial resolution at the pinch is 1 mm. The instrument may also be fielded with seven time frames using a seven strip-line microchannel plate as the detector by sacrifici...

Implications of high-energy photons and electrons on target preheat at the Sandia “Z” facility

High-energy photons and electrons on the Sandia National Laboratories “Z” accelerator, a z-pinch device, will deposit energy into a capsule and fuel; this may create a potential preheat problem for inertial confinement infusion (ICF). In this article we discuss heating of the capsule and fuel by high-energy photons and electrons. The fuel is heated to <2 eV, in a time-integrated sense, on Z by these particles. Because peak implosion occurs at the peak in the soft x-ray emission on Z, the heating at times of interest is reduced roughly an order of magnitude to ∼0.2 eV for times of interest and fuel preheat from this mechanism is concluded to be small. These estimates are generated from time-integrated bremsstrahlung measurements. The uncertainty in the heating is high because the electron spectrum is not known directly, but inferred. In addition, the influence of photons and electrons at energies between 5 and 60 keV is not known. Given the uncertainties at this point, we do not know the impact on the feasibility of internal dynamic hohlraums for a z-pinch-driven ICF implosion. We discuss these issues and suggest directions for further study.

D-dot and B-dot monitors for Z-vacuum section power-flow measurements

New differential D-dot and B-dot monitors were developed for the Z vacuum section of an accelerator pulsed power supply. The D-dots measure voltage at the insulator stack. The B-dots measure current at the stack and in the outer magnetically-insulated transmission lines. Each monitor has two outputs that allow common-mode noise to be cancelled to the first order. The differential D-dot has one signal and one noise channel; the differential B-dot has two signal channels with opposite polarities. Each of the two B-dot sensors in the differential B-dot monitor has four 3 mm diameter loops and is encased in copper to reduce flux penetration. For both types of probes, two 2.2 mm diameter coaxial cables connect the outputs to a Prodyn balun for common-mode-noise rejection. The cables provide reasonable bandwidth and generate acceptable levels of Compton drive in the bremsstrahlung field of the Z accelerator. A new cavity B-dot is being developed to measure the total Z current 4.3 cm from the axis of the z-pinch load. All of the sensors are calibrated with 2-4% accuracy. The monitor signals are reduced with Barth or Weinschel attenuators, recorded on Tektronix 0.5 ns/sample digitizing oscilloscopes, and software cable compensated and integrated.

Uniform fill improves K-shell power relative to annular fill for argon gas puffs on Saturn

Summary form only given. The radiation from uniform-fill argon gas puffs on the Saturn accelerator with a 4.5-cm diameter nozzle are compared with that generated from a previously optimized 2.5-cm diameter annular nozzle. The pressure range of the uniform fill spanned 1300 to 2900 Torr and that of the annular nozzle was set to 1650 Torr-the pressure that previously maximized the K-shell radiation yield. B-dot monitors measured current in the MITLs and 4.5 cm upstream of the load. A bolometer and duplicate sets of filtered XRDs and PCDs, spanning the energy range of 200 eV to 6 keV, monitored the temporal characteristics of the radiation. A suite of time-integrated and time-resolved, filtered, fast-framing, X-ray pinhole cameras, and crystal spectrometers monitored the spatial and spectral structure of the radiation. The radial density profile of the initial gas profile was measured on a test stand at NRL using a two-color interferometer.