Session CO5: Nuclear Diagnostics for NIF and HEDP

Using Exploding Pusher Targets to Commission Diagnostics for the National Ignition Facility

The National Ignition Campaign is preparing to start cryogenic Deuterium-Tritium (DT) implosions on the National Ignition Facility (NIF) in the Fall of 2010. Before these experiments can take place the diagnostic suite must be commissioned. This will be achieved using exploding pusher targets to produce MeV neutrons, protons and x-rays from gas filled capsules that are directly illuminated by NIF laser beams using Polar Direct Drive (PDD). Simulations predict neutron yields in range of 1e13 -- 1e15 for DT gas fills. This talk will describe the first PDD exploding pusher experiments on the NIF and discuss progress in commissioning the NIC diagnostics. LLNL-ABS-442792. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.   

Numerical Investigation of NIF Diagnostic Commissioning Experiments on OMEGA

To assist in validating neutron-yield-target designs, which will be used to commission new diagnostics on the NIF, a series of experiments were performed on OMEGA investigating the ability of polar-drive (PD) illumination to provide a reliable and flexible source of fusion products. These experiments used thin D$_{2}$- and DT-gas--filled glass-shell, ``exploding-pusher'' targets that were imploded either in 40-beam PD geometry or in 60-beam symmetric geometry with the same total laser energy. The 2-D, radiation-hydrodynamics code \textit{DRACO} was used to investigate the reduction in drive symmetry caused by PD and its effect on target performance by comparing with experimental observables including neutron yield, neutron-production-rate history, and self-emission images. Initial simulations are in good agreement with the experimental yield and indicate a modest 30{\%} to 40{\%} yield reduction for the PD implosions when compared with the equal-energy, symmetric drive implosions. The effect of SSD (both on and off) on the implosion characteristics will also be evaluated. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

South-Pole Bang-Time X-Ray Diagnostic for the NIF

An x-ray bang-time diagnostic is essential for the National Ignition Facility (NIF) to compare implosions with simulations as part of ignition tuning. The south-pole bang-time (SPBT) x-ray diagnostic is located 3 m directly below target chamber center, viewing the implosion through the hohlraum laser entrance hole. SPBT consists of five chemical-vapor-deposition (CVD) diamond detectors with different sensitivities. Wavelength-selecting highly oriented pyrolytic graphite (HOPG) crystal mirrors increase the signal-to-background ratio. SPBT electronics are placed near the NIF target chamber, minimizing the cable lengths and their effects on the detector time response. The SPBT is designed to provide 30-ps accuracy on the bang-time measurement and to operate at neutron yields up to 10$^{17}$. Characterization of the detector components and assembly are presented along with data from NIF implosions. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

NIF Gamma Reaction History

The primary objective of the NIF Gamma Reaction History (GRH) diagnostics is to provide bang time and burn width information based upon measurement of fusion gamma-rays. This is accomplished with energy-thresholded Gas Cherenkov detectors that convert MeV gamma-rays into UV/visible photons for high-bandwidth optical detection. In addition, the GRH detectors can perform $\gamma $-ray spectroscopy to explore other nuclear processes from which additional significant implosion parameters may be inferred (e.g., plastic ablator areal density). Implementation is occurring in 2 phases: 1) four PMT-based channels mounted to the outside of the NIF target chamber at $\sim $6 m from TCC (GRH-6m) for the 3e13-3e16 DT neutron yield range expected during the early ignition-tuning campaigns; and 2) several channels located just inside the target bay shield wall at $\sim $15 m from TCC (GRH-15m) with optical paths leading through the wall into well-shielded streak cameras and PMTs for the 1e16-1e20 yield range expected during the DT ignition campaign. This suite of diagnostics will allow exploration of interesting $\gamma $-ray physics well beyond the ignition campaign. Recent data from OMEGA and NIF will be shown.   

Investigation of the DT Fusion Gamma Spectrum with an Energy Thresholding Gas Cherenkov Detector

In addition to alphas and neutrons, the DT fusion reaction produces gamma rays from the intermediate $^{5}$He nucleus with a small branching ratio (BR) of several e-5 $\gamma $/n. The excited $^{5}$He can decay to the ground state emitting a 16.75 MeV $\gamma $ (width $\sim $0.5 MeV) or to the broad first excited state emitting a $\sim $12 MeV $\gamma $ (width $\sim $5 MeV). Knowledge of the BR between these two states is important to making absolutely calibrated measurements of the overall gamma-ray spectrum on the NIF. We have carried out an energy thresholding experiment for DT ICF implosions on the Omega laser using a Gas Cherenkov Detector, and have compared the relative intensities at various thresholds with possible theoretical gamma spectra folded with detector response as calculated by ACCEPT and GEANT4 codes. We present the results of this experiment, our estimate of the precision of the DT fusion gamma spectrum and the implications for the future determination of the DT $\gamma $/n BR.   

GRH Characterization using 4.4 MeV $^{12}$C Gamma-Rays

The OMEGA Gamma Reaction History (GRH) diagnostic has been characterized using a relatively well-known source of 4.43 MeV gamma rays produced from inelastic scattering of DT-neutrons off of a graphite puck placed near an imploding capsule at the OMEGA laser facility. An independently measured neutron yield, combined with the known $^{12}$C density and $^{12}$C(n,n'$\gamma )^{12}$C cross-section, allows an in-situ calibration of the GRH detection efficiency at 4.43 MeV. GRH data were collected at two different $^{12}$C target locations to confirm the published angular distribution of gamma rays and were compared with MCNP modeling predictions. These in-situ calibrations were used to validate the GRH simulation code based on a coupled MCNP/ACCEPT Monte-Carlo method. By combining these results with other absolute calibration methods, we are able to infer a DT branching ratio for gamma to neutron production and to make an accurate plastic ablator areal density measurement.   

Using gamma-ray emission to measure ablator areal density of imploded capsules at the Omega laser

We have measured the ablator areal density of plastic-shell implosions at the Omega laser, using gamma-ray emission from the capsules detected by the prototype Gamma Reaction History (GRH) diagnostic. The intensity of 4.44-MeV gamma emission from $^{12}$C nuclei in the ablator is proportional to the product of ablator areal density and yield of fusion neutrons, so by detecting the gammas we can infer the ablator areal density, provided we also have a measurement of total neutron yield. Neutron yield is determined from the nTOF experiment at Omega in our approach; alternatively one could use 16.7-MeV gammas from DT fusion. Inferred values of time-averaged carbon areal density are in the range 10-30 mg/cm$^{2}$, for a range of implosions. These values are smaller than predicted values based on 1D simulations, which are typically in the range 30-40 mg/cm$^{2}$. We discuss possible reasons for the discrepancy, primarily related to mixing.   

Neutron Imager for the National Ignition Facility

The goal of the National Ignition Campaign at the National Ignition Facility is to obtain ignition of an inertially confined fusion capsule. In this work, we describe the neutron imaging system that we are installing at the National Ignition Facility to provide information on the spatial distribution of material in the compressed capsule and any uncompressed fuel. The imager uses an array of 37 gold and tungsten apertures 20-cm long with an apex at 26.5 cm from the source to produce images in a scintillator array at 28-m. By imaging the front and back of the scintillator and gating those images with properly gated microchannel plate gated intensifiers, we expect to obtain two images: one of the primary neutrons from 13-17 MeV and one of the downscattered neutrons from 10-12 MeV. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Neutron Image Analysis for the National Ignition Facility

To obtain ignition in inertial confinement fusion experiments (ICF) at the National Ignition Facility (NIF), sited at Lawrence Livermore National Laboratory, Livermore, CA, will require a careful balance of drive conditions created by the 192 beam, 1.8 MJ laser. Drive symmetry and strength in ICF experiments will be reflected in the shape and density of the deuterium and tritium fuel, and therefore collection and analysis of neutron images produced by thermonuclear reactions, or resultant scatters, will provide this diagnostic information. A description of the NIF neutron imaging system (NIS) analysis methods will be presented, including an overview of the imaging system, a theoretical description of the image formation process, current system models, resultant analysis methods, examples from recent sensitivity and validation studies, as well as imaging data obtained to date.   

First measurements of the absolute neutron spectrum using the Magnetic Recoil Spectrometer (MRS) at the NIF

Proper assembly of capsule mass, as manifested through evolution of fuel areal density (\textit{$\rho $R}), is fundamentally important for achieving hot-spot ignition planned at the National Ignition Facility (NIF). Experimental information about \textit{$\rho $R} and \textit{$\rho $R} asymmetries, $T_{i}$ and yield is therefore essential for understanding how this assembly occurs. To obtain this information, a neutron spectrometer, called the Magnetic-Recoil Spectrometer (MRS) has been implemented on the NIF. Its primary objective is to measure the absolute neutron spectrum in the range 5 to 30 MeV, from which \textit{$\rho $R}, $T_{i}$ and yield can be directly inferred for both low-yield tritium-hydrogen-deuterium (THD) and high-yield DT implosions. In this talk, the results from the first measurements of the absolute neutron spectrum produced in exploding pusher and THD implosions will be presented. This work was supported in part by the U.S. DOE, LLNL and LLE.   

High-Speed Diamond Detectors for nTOF Plasma Diagnostics at the NIF

Neutron time-of-flight (nTOF) spectrometers are integral diagnostics at the National Ignition Facility (NIF) to extract neutron yield, ion temperature and bang time of the implosion. For measurements of the fuel areal density ($\rho R)$, one of these nTOF diagnostics will be operated with low shielding at a comparably close distance of 3.9 m to the hohlraum target to minimize the scattering contribution of the intense 14 MeV neutron signal to the spectral background. This nTOF spectrometer uses CVD diamond semiconductor detectors with sub-ns decay times and without the long tails that often affect the response of fast scintillators. It will measure the fraction of down-scattered neutrons that arrives only $\sim $10 ns after the large pulse of 14 MeV DT neutrons and that provides a measure of the areal density $\rho R$ and thus the ignition threshold function. We discuss the instrument design, Monte Carlo simulations of its response function, and measurements of the detector response to X-ray and neutron signals at the Laboratory for Laser Energetics (LLE). Special emphasis will be placed on discussing the contributions to the background for the neutrons down-scattered in the fuel into the spectral range of $\sim $10 to $\sim $12 MeV.   

Neutron Time-of-Flight Diagnostic Performance for the National Ignition Facility's 2010 Campaign

Installation of the neutron time-of-flight (nTOF) diagnostic at the National Ignition Facility (NIF) was completed in 2010. It consists of 18 data channels from 8 detectors along 6 flight paths. Two detector types are used: (1) scintillators coupled to gated photomultiplier tubes or vacuum photodiodes, and (2) chemical-vapor-deposition diamonds. Target-to-detector distances are nominally 4.5 and 22 m. These detectors were calibrated for yield and ion temperature at LLE's OMEGA Laser Facility prior to installation on the NIF. This presentation describes nTOF diagnostic performance in measuring neutron yield, ion temperature, and bang time in D$_{2}$ and THD (tritium, hydrogen, and deuterium) NIF implosions in 2010. This work is supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Monte Carlo Simulations of Neutron Scattering in Current-Mode Neutron Time-of-Flight Detectors

Current-mode neutron time-of-flight detectors are used in inertial confinement fusion experiments to measure the neutron yield and neutron-averaged ion temperature. The neutron-averaged ion temperature can be inferred from the temporal broadening of the neutron signal. Neutron scattering in the detector housing and nearby structures can significantly modify the temporal structure of the neutrons arriving at the detector. Monte Carlo neutron-transport simulations that assess the influence of neutron scattering on the detector signals as a function of detector geometry and location will be presented. This work is supported by the U.S. Department of Energy Office of Inertial Confinement No. DE-FC52-08NA28302.   

Active image readout system for extreme neutron envirorments for NIF

The National Ignition Facility is expected to start producing x-ray flux and neutron yields higher than any produced in laser driven implosion experiments in the past. Tuning of non-igniting capsule will require x-ray imaging of near burning plasmas that are generating yields of 10$^{17}$ neutrons. X-ray recording systems need to work in more hostile conditions than we have encountered in past laser facilities. We will present modeling, experimental data, and design concepts for x-ray imaging with electronic recording systems for this environment.   

Ultrafast 25 keV backlighting for experiments on Z

To extend the backlighting capabilities for Sandia's Z- Accelerator, Z-Petawatt, a laser which can provide laser pulses of 500 fs length and up to 120 J (100TW target area) or up to 450 J (Z / Petawatt target area) has been built over the last years. The main mission of this facility focuses on the generation of high energy X-rays, such as tin K$\alpha$ at 25 keV in ultra-short bursts. Achieving 25 keV radiographs with decent resolution and contrast required addressing multiple problems such as blocking of hot electrons, minimization of the source, development of suitable filters, and optimization of laser intensity. Due to the violent environment inside of Z, an additional very challenging task is finding massive debris and radiation protection measures without losing the functionality of the backlighting system. We will present the first experiments on 25 keV backlighting including an analysis of image quality and X-ray efficiency.