40 Matching Results

Search Results

Advanced search parameters have been applied.

Feasibility of measuring 3He bubble diameter populations in deuterium-tritium ice layers using Mie scattering

Description: In this report, I assess the feasibility of using Mie scattering to quantify the diameter distribution of {sup 3}He bubbles in DT ice layers. Mie scattering methods are often used for He diameter m measurements of particulates in emulsions like pigments and ink products. These measurements suggests that similar techniques could be used to measure the distribution of {sup 3}He bubbles He in DT ice layers, which is important for NIF ICF capsules. To investigate the achievable performance of bubble diameter measurements using Mie scattering, I performed numerical modeling using exact analytical expressions.
Date: January 22, 2007
Creator: Izumi, N
Partner: UNT Libraries Government Documents Department

Simulations of X-ray Emission from Omega Fill Tube Experiments

Description: The capsules used in ignition experiments on the National Ignition Facility (NIF) laser will have a layer of frozen DT inside a low-Z shell. Liquid DT will be injected through a narrow fill tube that penetrates the shell and frozen in place. The fill tube is a perturbation on the surface of the capsule and hydrodynamic instabilities will cause this perturbation to grow during an implosion. Experiments to investigate the growth of perturbations due to fill tubes have been carried out on the Omega laser. The goal of these experiments was to validate simulations at Omega energy scales and thus increase confidence in the use of simulations in planning for NIF experiments. Simulations show that the fill tube leads to a jet of shell material that penetrates into the DT fuel. Simulations will be used to pick experimental conditions in which the jet is small enough that it does not significantly reduce the yield of a NIF implosion. This paper compares experiments in which bumps and stalks were used as fill tube surrogates to 2D simulations of x-ray emission from Omega capsule implosions. Experiments and simulations are in reasonable agreement on the size of a bump or stalk required to produce a jet that is visible above the emission from a (nominally) smooth capsule.
Date: November 13, 2006
Creator: Langer, S; Izumi, N; Dittrich, T & Haan, S
Partner: UNT Libraries Government Documents Department

Using X-Rays to Test CVD Diamond Detectors for Areal Density Measurement at the National Ignition Facility

Description: At the National Ignition Facility (NIF), 192 laser beams will compress a target containing a mixture of deuterium and tritium (DT) that will release fusion neutrons, photons, and other radiation. Diagnostics are being designed to measure this emitted radiation to infer crucial parameters of an ignition shot. Chemical Vapor Deposited (CVD) diamond is one of the ignition diagnostics that will be used as a neutron time-of-flight detector for measuring primary (14.1 MeV) neutron yield, ion temperature, and plasma areal density. This last quantity is the subject of this study and is inferred from the number of downscattered neutrons arriving late in time, divided by the number of primary neutrons. We determine in this study the accuracy with which this detector can measure areal density, when the limiting factor is detector and electronics saturation. We used laser-produced x-rays to reproduce NIF signals in terms of charge carriers density, time between pulses, and amplitude contrast and found that the effect of the large pulse on the small pulse is at most 8.4%, which is less than the NIF accuracy requirement of {+-} 10%.
Date: May 6, 2008
Creator: Dauffy, L S; Koch, J A; Tommasini, R & Izumi, N
Partner: UNT Libraries Government Documents Department

Recovery of a CVD diamond detection system from strong pulses of laser produced x-rays

Description: We are studying the response of a CVD diamond detector to a strong x-ray pulse followed by a second weaker pulse arriving 50 to 300 ns later, with a contrast in amplitude of about 1000. These tests, performed at the LLNL Jupiter laser facility, are intended to produce charge carrier densities similar to those expected during a DT implosion at NIF, where a large 14.1 MeV neutron pulse is followed by a weak downscattered neutron signal produced by slower 6-10 MeV neutrons. The number of downscattered neutrons must be carefully measured in order to obtain an accurate value for the areal density, which is proportional to the ratio of downscattered to primary neutrons. The effects of the first strong pulse may include saturation of the diamond wafer, saturation of the oscilloscope, or saturation of the associated power and data acquisition electronics. We are presenting a double pulse experiment that will use a system of several polycrystalline CVD diamond detectors irradiated by 8.6 keV x-rays emitted from a zinc target. We will discuss implication for a NIF areal density measurement.
Date: April 25, 2006
Creator: Dauffy, L S; Koch, J A; Izumi, N; Tommasini, R & Lerche, R A
Partner: UNT Libraries Government Documents Department

Pulsed Laser-Based X-Ray Sources for Rapid-Cool DT Layer Characterization

Description: Ignition targets for the National Ignition Facility (NIF) will contain a cryogenically cooled {approx} 75 {micro}m-thick deuterium/tritium (DT) ice layer surrounded by a {approx} 150 {micro}m-thick beryllium (Be) shell [1]. Ignition target design optimization depends sensitively on the achievable inner surface quality of the ice layer and on the pressure of the DT gas inside the ice, which is determined by the temperature of the ice. The inner ice layer surface is smoothest at temperatures just below the DT ice/liquid/gas triple point (3T), but current ignition target designs require central gas pressures of 0.3 mg/cm3, corresponding to an ice layer temperature 1.5 K below the triple point (3T-1.5). At these lower temperatures, the ice layer quality degrades due to the formation of cracks and other features.
Date: August 29, 2007
Creator: Koch, J A; Dewald, E; Izumi, N; Kozioziemski, B; Landen, O & Siders, C
Partner: UNT Libraries Government Documents Department

Application of imaging plate to x-ray imaging and spectroscopy in laser plasma experiments

Description: We report recent progress of x-ray diagnostic techniques in laser plasma experiment with using imaging plates. Imaging plate is a photo-stimulable phosphor screen (BaF(Br0.85,10.15):Eu{sup 2+}) deposited on flexible metal or plastic substrate. We applied the imaging plate to x-ray microscopy in laser fusion experiment experiments. Self-emission x-ray images of imploded core were obtained successfully with using imaging plate and high magnification target mounted pinhole arrays. The imaging plates were applied also in ultra-intense laser experiment at the Rutherford Appleton Laboratory. Small samarium foil was irradiated by high intensity laser pulse from the Vulcan laser system. The k shell x-rays from the foil ({approx}40keV) was used as a line x-ray source for microscopic radiography. Performance of imaging plate on high-energy x-ray backlit radiography was demonstrated by imaging sinusoidal grooves of 6um amplitude on a Au foil. Detailed spectrum of k shell x-ray from Cu embedded foil target was successfully observed by fully coupling imaging plate with a highly ordered pyrolytic graphite crystal spectrometer. The performances of the imaging plates evaluated in actual laser plasma experiments will be presented.
Date: April 25, 2006
Creator: Izumi, N; Snavely, R; Gregori, G; Koch, J A; Park, H & Remington, B A
Partner: UNT Libraries Government Documents Department

Reconstruction of Quasi-Monochromatic Images for Multispectral X-ray Imaging with a Pinhole Array and a Flat Bragg Mirror

Description: We have developed a software package for reconstruction of quasi-monochromatic images from a multiple monochromatic x-ray imager for inertial confinement fusion implosions. The instrument consists of a pinhole array, a multi-layer Bragg mirror, and a image detector. The pinhole array projects hundreds of images onto the detector after reflection off the multi-layer Bragg mirror, which introduces spectral dispersion along the reflection axis. The quasi-monochromatic images of line emissions and continuum emissions can be used for measurement of temperature and density maps of implosion plasmas. In this paper, we describe a computer-aided processing technique for systematic reconstruction of quasi-monochromatic images from raw data. This technique provides flexible spectral bandwidth selection, and allows systematic subtraction of continuum emission from line emission images.
Date: August 4, 2005
Creator: Izumi, N; Barbee, T W; Koch, J A; Mancini, R C & Welser, L A
Partner: UNT Libraries Government Documents Department

Development of Compton Radiography Diagnostics for Inertial Confinement Fusion Implosions

Description: An important diagnostic tool for inertial confinement fusion will be time-resolved radiographic imaging of the dense cold fuel surrounding the hot spot. The measurement technique is based on point-projection radiography at photon energies from 60-200 keV where the Compton effect is the dominant contributor to the opacity of the fuel or pusher. We have successfully applied this novel Compton Radiography technique to the study of the final compression of directly driven plastic capsules at the OMEGA facility. The radiographs have a spatial and temporal resolution of {approx}10 {micro}m and {approx}10ps, respectively. A statistical accuracy of {approx}0.5% in transmission per resolution element is achieved, allowing localized measurements of areal mass densities to 7% accuracy. The experimental results show 3D non-uniformities and lower than 1D expected areal densities attributed to drive asymmetries and hydroinstabilities.
Date: November 16, 2010
Creator: Tommasini, R; Hatchett, S P; Hey, D S; Izumi, N; Koch, J A; Landen, O L et al.
Partner: UNT Libraries Government Documents Department

Multispectral X-Ray Imaging With A Pinhole Array And A Flat Bragg Mirror

Description: We describe a multiple monochromatic x-ray imager designed for implosion experiments. This instrument uses an array of pinholes in front of a flat multilayered Bragg mirror to provide many individual quasi-monochromatic x-ray pinhole images spread over a wide spectral range. We discuss design constraints and optimizations, and we discuss the specific details of the instrument we have used to obtain temperature and density maps of implosion plasmas.
Date: March 17, 2005
Creator: Koch, J A; Barbee, Jr., T W; Izumi, N; Tommasini, R; Welser, L A; Mancini, R C et al.
Partner: UNT Libraries Government Documents Department

Multispectral X-ray Imagaing for Core Temperature and Density Maps Retrieval in Direct Drive Implosions

Description: We report on the experiments aimed at obtaining core temperature and density maps in direct drive implosions at the OMEGA Laser Facility using multi-monochromatic X-ray imagers. These instruments use an array of pinholes and a flat multilayer mirror to provide unique multi-spectral images distributed over a wide spectral range. Using Argon as a dopant in the DD-filled plastic shells produces emission images in the Ar He-b and Ly-b spectral regions. These images allow the retrieval of temperature and density maps of the plasma. We deployed three identical multi-monochromatic X-ray imagers in a quasi-orthogonal line-of-sight configuration to allow tomographic reconstruction of the structure of the imploding core.
Date: April 26, 2006
Creator: Tommasini, R; Koch, J A; Izumi, N; Welser, L A; Mancini, R C; Delettrez, J et al.
Partner: UNT Libraries Government Documents Department

A Review of X-ray Diagnostic Calibrations in the 2 to 100 keV Region Using the High Energy X-ray Calibration Facility (HEX)

Description: The precise and accurate measurement of X-rays in the 2 keV to 100 keV region is crucial to the understanding of HED plasmas and warm dense matter in general. With the emergence of inertially confined plasma facilities as the premier platforms for ICF, laboratory astrophysics, and national security related plasma experiments, the need to calibrate diagnostics in the high energy X-ray regime has grown. At National Security Technologies High Energy X-ray Calibration Facility (HEX) in Livermore, California, X-ray imagers, filter-fluorescer spectrometers, crystal spectrometers, image plates, and nuclear diagnostics are calibrated. The HEX can provide measurements of atomic line radiation, X-ray flux (accuracy within 10%), and X-ray energy (accuracy within 1%). The HEX source is comprised of a commercial 160 kV X-ray tube, a fluorescer wheel, a filter wheel, and a lead encasement. The X-ray tube produces a Tungsten bremsstrahlung spectrum which causes a foil to fluoresce line radiation. To minimize bremsstrahlung in the radiation for calibration we also provide various foils as filters. For experimental purposes, a vacuum box capable of 10{sup -7} Torr, as well as HPGe and CdTe radiation detectors, are provided on an optical table. Most geometries and arrangements can be changed to meet experimental needs.
Date: May 19, 2010
Creator: Ali, Zaheer; Pond, T; Buckles, R A; Maddox, B R; Chen, C D; DeWald, E L et al.
Partner: UNT Libraries Government Documents Department

CVD Diamond Detectors for Current Mode Neutron Time-of-Flight Spectroscopy at OMEGA/NIF

Description: We have performed pulsed neutron and pulsed laser tests of a CVD diamond detector manufactured from DIAFILM, a commercial grade of CVD diamond. The laser tests were performed at the short pulse UV laser at Bechtel Nevada in Livermore, CA. The pulsed neutrons were provided by DT capsule implosions at the OMEGA laser fusion facility in Rochester, NY. From these tests, we have determined the impulse response to be 250 ps fwhm for an applied E-field of 500 V/mm. Additionally, we have determined the sensitivity to be 2.4 mA/W at 500 V/mm and 4.0 mA/W at 1000 V/mm. These values are approximately 2 to 5x times higher than those reported for natural Type IIa diamond at similar E-field and thickness (1mm). These characteristics allow us to conceive of a neutron time-of-flight current mode spectrometer based on CVD diamond. Such an instrument would sit inside the laser fusion target chamber close to target chamber center (TCC), and would record neutron spectra fast enough such that backscattered neutrons and x-rays from the target chamber wall would not be a concern. The acquired neutron spectra could then be used to extract DD fuel areal density from the downscattered secondary to secondary ratio.
Date: July 1, 2001
Creator: Schmid, G. J.; Glebov, V. Yu.; Friensehner, A. V.; Hargrove, D. R.; Hatchett, S. P.; Izumi, N. et al.
Partner: UNT Libraries Government Documents Department

Using Laser Entrance Hole Shields to Increase Coupling Efficiency in Indirect Drive Ignition Targets for the National Ignition Facility (NIF)

Description: Coupling efficiency, the ratio of the capsule absorbed energy to the driver energy, is a key parameter in ignition targets. The hohlraum originally proposed for NIF coupled {approx}11% of the absorbed laser energy to the capsule as x-rays. We describe here a second generation of hohlraum target which has higher coupling efficiency, {approx}16%. Because the ignition capsule's ability to withstand 3D effects increases rapidly with absorbed energy, the additional energy can significantly increase the likelihood of ignition. The new target includes laser entrance hole (LEH) shields as a principal method for increasing coupling efficiency while controlling symmetry in indirect-drive ICF. The LEH shields are high Z disks placed inside the hohlraum to block the capsule's view of the cold LEHs. The LEH shields can reduce the amount of laser energy required to drive a target to a given temperature via two mechanisms: (1) keeping the temperature high near the capsule pole by putting a barrier between the capsule and the pole, (2) because the capsule pole does not have a view of the cold LEHs, good symmetry requires a shorter hohlraum with less wall area. Current integrated simulations of this class of target couple 140 kJ of x-rays to a capsule out of 865 kJ of absorbed laser energy and produce {approx}10 MJ of yield. In the current designs, which are not completely optimized, the addition of the LEH shields saves {approx}95 kJ of energy (about 10%) over hohlraums without LEH shields.
Date: November 3, 2005
Creator: Callahan, D A; Amendt, P A; Dewald, E L; Haan, S W; Hinkel, D E; Izumi, N et al.
Partner: UNT Libraries Government Documents Department

Development of backlighting sources for a Compton radiography diagnostic of Inertial Confinement Fusion targets

Description: We present scaled demonstrations of backlighter sources, emitting Bremsstrahlung x-rays with photon energies above 75 keV, that we will use to record x-ray Compton radiographic snapshots of cold dense DT fuel in inertial confinement fusion implosions at the National Ignition Facility (NIF). In experiments performed at the Titan laser facility at Lawrence Livermore National Laboratory, we measured the source size and the Bremsstrahlung spectrum as a function of laser intensity and pulse length, from solid targets irradiated at 2e17-5e18 W/cm{sup 2} using 2-40 ps pulses. Using Au planar foils we achieved source sizes down to 5.5 {micro}m, and conversion efficiencies of about 1e-3 J/J into x-ray photons with energies in the 75-100 keV spectral range. We can now use these results to design NIF backlighter targets and shielding, and to predict Compton radiography performance as a function of the NIF implosion yield and associated background.
Date: May 7, 2008
Creator: Tommasini, R; MacPhee, A; Hey, D; Ma, T; Chen, C; Izumi, N et al.
Partner: UNT Libraries Government Documents Department

High aspect ratio hard x-ray (> 100 keV) imager to measure hot electron preheat for indirectly driven capsule implosions on the National Ignition Facility

Description: We have fielded a multi-pinhole, hard x-ray (> 100 keV) imager to measure the spatially-resolved bremsstrahlung emission from energetic electrons slowing in a plastic ablator shell during indirectly driven implosions at the National Ignition Facility. These electrons are generated in laser plasma interactions, and are a source of preheat to the deuterium-tritium fuel that could limit the compressibility required for ignition and burn. Our hard x-ray imaging measurements allow to set an upper limit to the DT fuel preheat, which we find is acceptable in current capsule implosions on the NIF.
Date: May 1, 2012
Creator: Doppner, T; Dewald, E; Divol, L; Burns, S; Izumi, N; Kline, J et al.
Partner: UNT Libraries Government Documents Department