George C. Idzorek

Development and characterization of a Z-pinch-driven hohlraum high-yield inertial confinement fusion target concept

Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling (“hohlraum energetics”) is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et...

Dynamics and characteristics of a 215-eV dynamic-hohlraum x-ray source on Z

A radiation source has been developed on the 20-MA Z facility that produces a high-power x-ray pulse, generated in the axial direction primarily from the interior of a collapsing dynamic hohlraum (DH). The hohlraum is created from a solid cylindrical CH2 target centered within an imploding tungsten wire-array Z pinch. Analyses and interpretation of measurements made of the x-ray generation within and radiated from the hohlraum target have been done using radiation-magnetohydrodynamic-code simulations in the r-z plane that take account of the magnetic Rayleigh–Taylor (RT) instability. These analyses suggest that a significantly reduced RT seed (relative to that used to explain targetless Z-pinch data on Z) is required to explain the observations. Although some quantitative and qualitative agreement with the measurements is obtained with the reduced RT seed, differences remain. Initial attempts to include into the simulations a precursor plasma, arising from wire material driven ahead of the main implosion,...

Target diagnostic system for the national ignition facility (invited)

A review of recent progress on the design of a diagnostic system proposed for ignition target experiments on the National Ignition Facility (NIF) will be presented. This diagnostic package contains an extensive suite of optical, x ray, gamma ray, and neutron diagnostics that enable measurements of the performance of both direct and indirect driven NIF targets. The philosophy used in designing all of the diagnostics in the set has emphasized redundant and independent measurement of fundamental physical quantities relevant to the operation of the NIF target. A unique feature of these diagnostics is that they are being designed to be capable of operating in the high radiation, electromagnetic pulse, and debris backgrounds expected on the NIF facility. The diagnostic system proposed can be categorized into three broad areas: laser characterization, hohlraum characterization, and capsule performance diagnostics. The operating principles of a representative instrument from each class of diagnostic employed in t...

Unexpected axial asymmetry in radiated power from high-temperature dynamic-hohlraum x-ray sources

Radiation from the interior of a dynamic hohlraum within a wire-array Z pinch is used to generate high-power x-ray pulses in both the up and down axial directions through radiation exit holes (REHs) in the anode and cathode, respectively. Despite a concerted effort to ensure a symmetrical up-down configuration, the measured peak top radiated power remained about twice that of the bottom (with similar total radiated energies from each REH), as compared to current simulations that predict equal powers. This large asymmetry suggests the need for improved physics models and simulation capabilities.

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.

Soft x-ray measurements of z-pinch-driven vacuum hohlraums

This article reports the experimental characterization of a z-pinch driven-vacuum hohlraum. The authors have measured soft x-ray fluxes of 5 x 10{sup 12} W/cm{sup 2} radiating from the walls of hohlraums which are 2.4--2.5 cm in diameter by 1 cm tall. The x-ray source used to drive these hohlraums was a z-pinch consisting of a 300 wire tungsten array driven by a 2 MA, 100 ns current pulse. In this hohlraum geometry, the z-pinch x-ray source can produce energies in excess of 800 kJ and powers in excess of 100 TW to drive these hohlraums. The x-rays released in these hohlraums represent greater than a factor of 25 in energy and more than a factor of three in x-ray power over previous laboratory-driven hohlraums.

Blast wave radiation source measurement experiments on the Z Z-pinch facility

The Dynamic Hohlraum (DH) radiation on the Z facility at Sandia National Laboratories [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceeding of the 11th International Pulsed Power Conference, Baltimore, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] is a bright source of radiant energy that has proven useful for high energy density physics experiments. But the radiation output from a DH on Z needs to be well known. In this paper, a new method is presented for measuring the radiation fluence deposited in an experiment, specifically, an experiment driven by a Z DH. This technique uses a blast wave produced in a SiO2 foam, which starts as supersonic but transitions to subsonic, producing a shock at the transition point that is observable via radiography. The position of this shock is a sensitive measure of the radiation drive energy from the Z DH. Computer simulations have been used to design and analyze a Z foam blast wave experiment.

Measurement and simulation of apertures on Z hohlraums

We have performed aperture measurements and simulations for vacuum hohlraums heated by wire array implosions. A low-Z plastic coating is often applied to the aperture to create a high ablation pressure which retards the expansion of the gold hohlraum wall. However this interface is unstable and may be subject to the development of highly nonlinear perturbations (“jets”) as a result of shocks converging near the edge of the aperture. These experiments have been simulated using Lagrangian and Eulerian radiation hydrodynamics codes.

Properties of plasma radiation diagnostics

A number of diagnostics utilizing the radiation emitted from high-temperature plasmas have been developed at Los Alamos. Photoemissive x-ray diodes with photon energy bandpass filters provide time-resolved rough spectral data from about 6 eV to >10 keV photon energy. Filtered silicon photodiodes can be used down to 1 eV and offer the advantages of nominally flat response and ability to operate in poor vacuum conditions. Both types of diodes will provide a rough time-resolved spectrum and both are relatively inexpensive, reliable, and passive (i.e., no synchronization problems). For higher-energy resolution, bent crystal spectrographs are used in the x-ray region. With the addition of streak cameras or gated microchannel plates these systems provide data with high energy and high time resolution. To measure the total energy output, a thin foil bolometer is used that measures the change in foil resistance as it is heated by the plasma radiation. By combining these diagnostics into a complementary set good diagnostic information can be guaranteed on any plasma experiment. A number of diagnostics utilizing the radiation emitted from high-temperature plasmas have been developed at Los Alamos. Photoemissive x-ray diodes with photon energy bandpass filters provide time-resolved rough spectral data from about 6 eV to >10 keV photon energy. Filtered silicon photodiodes can be used down to 1 eV and offer the advantages of nominally flat response and ability to operate in poor vacuum conditions. Both types of diodes will provide a rough time-resolved spectrum and both are relatively inexpensive, reliable, and passive (i.e., no synchronization problems). For higher-energy resolution, bent crystal spectrographs are used in the x-ray region. With the addition of streak cameras or gated microchannel plates these systems provide data with high energy and high time resolution. To measure the total energy output, a thin foil bolometer is used that measures the change in foil resistance as it is heated by the plasma radiation. By combining these diagnostics into a complementary set good d...

High-count-rate proportional counter for soft x-ray position detection

The design of a multiwire gas proportional counter for soft x‐ray detection is described. Position information is obtained over a length of 110 mm from the signal induced in the cathode plane consisting of a delay line. A position resolution of better than 80 μm is demonstrated in tests performed with fluorescent x rays of well‐defined energies in the range 1.6–6 keV. Count‐rate capabilities reaching 1 MHz are demonstrated in crystal spectrometer measurements of the x‐ray emission from tokamak plasmas. The use of this type of detector in other applications is also discussed, especially in view of the potential of measuring the xy position over areas up to 110×160 mm2.