Session NO6: ICF Diagnostics: Nuclear

Nuclear diagnostic commissioning for the National Ignition Campaign

Nuclear diagnostics aiming at measuring neutron yield, ion temperature, neutron bang time and down scattered ratio are a main component of the National Ignition Campaign. Indeed as the neutron yield increases, neutron diagnostics will be the last ones to measure the performance of the cryogenic DT target, as X ray diagnostics become unusable due to neutron induced noise at high yield Therefore in order to commission these diagnostics, polar direct drive experiments on exploding pusher target have been taking place on the National Ignition Facility (NIF). Results of the exploding pusher performance on the NIF as well as progress on the neutron diagnostic commissioning will be presented in this talk. 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.   

First Results from the South Pole Bang Time (SPBT) Diagnostic on the NIF

The south pole bang time (SPBT) x-ray diagnostic has been successfully fielded on the NIF. SPBT consists of chemical-vapor-deposition diamond detectors, with different filtrations, located 3 m directly below target chamber center, viewing the implosion through the hohlraum laser entrance hole. The diamond detectors are sensitive to both x rays and neutrons. HOPG crystal mirror monochromators increase the x-ray signal to background ratio. SPBT is designed to measure the x-ray bang time with an accuracy of a few tens of picoseconds. SPBT x-ray and neutron results from NIF implosions are presented along with timing and error analysis. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Neutron Spectra Measured with Time-of-Flight Detectors at the National Ignition Facility

Neutron time-of-fight (nTOF) instruments are used to provide data on the performance of National Ignition Facility fusion experiments. nTOF detectors are used to measure the total neutron emission, temperature of the fuel, time of peak emission (bang time), and areal density of the compressed fuel (\textit{$\rho $R}). These instruments are precision diagnostics with sufficient dynamic range and high signal-to-noise so that the neutron spectrum from inertial confinement fusion implosions can be measured. This talk will focus on data from the scintillation detectors located at 20 m. Analysis techniques using both time-domain and energy-domain data are discussed. The next-generation detector based on an organic crystal scintillator show that improvements to scintillator decay, recording fidelity, and reduced scattering from the housing improve the precision of the neutron spectral measurement. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Measuring Areal Density Using n-T Elastic Scattering

We propose to infer the areal density (\textit{$\rho $R}) in cryogenic DT implosions on OMEGA by measuring the number of primary neutrons scattered off the dense triton (T) distribution during peak burn. Areal density measurements are currently inferred on OMEGA and the NIF by measuring the number of neutrons scattered off the DT plasma in the 10- to 12{\-}MeV range. Recent experiments show evidence that the low-energy portion of the elastic (n,T) scattering distribution (below 5 MeV) can be inferred above background components. We estimate that for a DT implosion with a yield of 5 $\times $ 10$^{12}$ and a burn-average \textit{$\rho $R} of 200 mg/cm$^{2}$, 1 $\times $ 10$^{5}$ (n,T) neutrons are produced between 3.2 and 5 MeV. This yield can be directly measured with good statistical accuracy using a standard neutron time-of-flight (nTOF) detector with an advanced scintillator that minimizes the delayed light emission from the primary neutrons. A description of this new ``kinematic endpoint'' \textit{$\rho $R} concept and the nTOF detector being developed to measure it will be discussed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Effects of Fluid Temperature and Velocity Distributions on Neutron Spectra

We commonly summarize the spectrum of 14 MeV neutrons produced by a laser fusion capsule by just a yield and a temperature, as if it were a uniform stationary fluid element. However, burn in a real fusion capsule occurs over a wide range of temperatures, and the velocity of the burning fuel is not negligible compared to thermal velocities. Even at low $\rho$r, absent any scattering, these effects cause the shape of the 14 MeV peak to depart significantly from a Gaussian, and cause its width, that is, the observed burn temperature, to vary depending on viewing direction. We describe our ongoing efforts to cope with these complexities in the analysis of data from the National Ignition Facility. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Recent results from measurements of ICF implosions with the neutron imaging diagnostic at NIF

A neutron imaging diagnostic has recently been commissioned at the National Ignition Facility (NIF). This new system is an important diagnostic tool for inertial fusion studies at the NIF for measuring the size and shape of the burning DT plasma during the ignition stage of ICF implosions. The imaging technique utilizes a ``pinhole'' neutron aperture, placed between the neutron source and a neutron detector. The detection system measures the two dimensional distribution of neutrons passing through the ``pinhole.'' This diagnostic has been designed to collect two images at two times. The long flight path for this diagnostic, 28 m, results in a chromatic separation of the neutrons, allowing the independently timed images to measure the source distribution for two neutron energies. Typically the first image measures the distribution of the 14 MeV neutrons and the second image of the 10-12 MeV neutrons. The combination of these two images has provided data on the size and shape of the burning plasma within the compressed capsule, as well as a measure of the quantity and spatial distribution of the cold fuel surrounding this core. The analysis of the data collected at NIF in June, 2011will be presented along with comparisons to hydrodynamic simulations of these implosions.   

RHO-R Measurements with the Neutron Imaging System at NIF

The first ever downscattered neutron images from ICF capsules were collected at NIF experiments by the neutron imaging system. The downscattered neutrons provide crucial information about the cold fuel areal density surrounding the hot fusion core. Analytical calculations together with simulations are used to estimate the areal density, rho-R, from the downscattered neutron intensities. We will present reconstructed intensity profiles of the hot fusion core and the cold fuel region surrounding it for three cryogenic DT implosions from Jun 2011, as well as the inferred areal densities from the analytical approach. This work was performed for the U.S. Department of Energy, National Nuclear Security Administration by the National Ignition Campaign partners. Prepared by LANL under Contract DE-AC-52-06-NA25396. Prepared by LLNL under Contract DE-AC52-07NA27344.   

NIF Neutron Image Reconstruction Techniques

Neutron imaging is an important diagnostic tool for inertial fusion studies at the National Ignition Facility (NIF) for measuring asymmetries in the burn region during the ignition stage of implosions. The technique for imaging of the spatial distribution of deuterium-tritium (DT) fusion neutrons utilizes an aperture - placed between the neutron source and a spatially sensitive neutron detector - which blocks the neutron flux and produces a shadow image of the neutron source at the detector. The recorded image is related to the 2D projection of the neutron source through a convolution with some, in general case spatially dependent, point spread function. The neutron source is reconstructed from the recorded image by solving a Fredholm-type integral equation of the first kind before we can estimate the shape and the size of the source. This inverse problem is notoriously ill-posed and presents certain difficulties in the context of NIF neutron imaging. Due to relatively low yield of neutrons the recorded images are expected to be noisy, so a special attention should be paid to the regularization of the equations to avoid the unacceptable amplification of the noise. We will present an overview of the developed techniques for reconstruction of source image from detector images and the results of reconstruction of the first neutron images obtained at NIF.   

ICF Gamma-Ray Yield Measurements on the NIF

The primary objective of the NIF Gamma Reaction History (GRH) diagnostic is to provide bang time and burn width information in order to constrain implosion simulation parameters such as shell velocity and confinement time. This is accomplished by measuring DT fusion $\gamma $-rays with energy-thresholded Gas Cherenkov detectors that convert MeV $\gamma $-rays into UV/visible photons for high-bandwidth optical detection. For yield determination, absolute uncertainties associated with the d(t,n)$\alpha $/d(t,$\gamma)^{5}$He branching ratio and detector response are removed by cross-calibrating the GRH signal against independent neutron yield measurements of directly-driven DT exploding pushers with negligible neutron downscatter. The GRH signal can then be used to make Total DTn Yield inferences on indirectly-driven, cryogenically-layered DT implosions which achieve high areal density and hence scatter a significant fraction of DTn out of the 14 MeV primary peak. By comparing the Total DTn Yield from $\gamma $-ray measurements with the Primary DTn Yield (13-15 MeV) from neutron measurements, the Total Downscatter Fraction (TDSF) can be inferred. Results of recent measurements will be presented.   

Estimates of the DT Fusion Gamma Spectrum Using an Energy Thresholding Gas Cherenkov Detector

In addition to alphas and neutrons, the DT fusion reaction also produces gamma rays from the intermediate excited 5He nucleus with a small branching ratio 10E-5 gamma/n. The very small branching ratio of the gamma-rays are mitigated by the very large yields that are expected on NIF (10E+19 ). The excited 5He can produce gamma-rays by decay to the ground state, emitting a 16.75 MeV gamma-ray (width 0.5 MeV), or to a broad first excited state emitting a 12 MeV gamma ray (width 5 MeV). Knowledge of the relative gamma-ray BR of these two states, from which we infer the DT gamma ray spectrum, is important to making absolutely calibrated measurements on a variety of experiments. We have carried out an energy thresh-holding experiment for DT ICF implosions on the Omega laser using a Gas Cherenkov Detector, and compared the relative intensities at various thresholds with theoretical gamma spectra folded with detector response as calculated by ACCEPT and GEANT4 codes. We present recent results from 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.   

Geant4 supplied parameters for gamma reaction history at NIF

The GRH diagnostics at NIF and Omega report ICF burn parameters through detection of multi-MeV $\gamma $ emissions. Of particular interest is `$\gamma $ bang-time' (GBT), defined as the temporal separation between light impacting the capsule and peak in the nuclear reaction history; GBT can constrain shock and compression parameters, and indicate fuel/ablator mix. Early NIF commissioning experiments have identified contributions to GRH signals from n,n'$\gamma $ reactions with remaining capsule ablator, hohlraum and thermo-mechanical package, outside the fuel hotspot region. Such contributions are mitigated by increasing the Cherenkov threshold above the energy of these emissions. The pressure adjustment modifies parameters important to GBT, such as cell time-of-flight and detector FWHM; corrections simulated using Geant4 are presented using models experimentally validated at Duke University. Beyond GBT, studies suggest GRH may be capable of recording ablator \textit{$\rho $R}, unfolding the DT $\gamma $ spectrum, and inferring the DT$_{\gamma }$ /DT$_{n}$ branching ratio. All calculations rely on the energy-resolved intensity response as a function of gas pressure. Geant4 response simulations, together with calculations by LANL using the experimentally validated ACCEPT code, are also presented.   

A high repetition rate plasma focus for neutron interrogation applications

A fast pulsed neutron source enables identification and ranging of contraband nuclear material using time-of-flight separation of the probe neutron pulse from the fission induced emission quanta. Alameda Applied Sciences Corporation has demonstrated a 1 Hz plasma focus neutron source that uses an impedance matching transformer to better couple the power from the driver to the dynamic pinch load. For a 24 kV primary charge, the system produces a 61 kA peak current with a neutron yield up to 5x10$^{5}$ neutrons/pulse at 1 Hz. Experiments are described in which induced 845keV gamma emission from iron targets (by 2.45MeV DD neutrons) was separated (by time of flight) from the 20-30ns probe neutron pulses. Monte Carlo simulations are used to optimize the concept for a fieldable system.   

The Laser Megajoule cryogenic program: filling and conformation of Deuterium/Tritium targets

In order to reach ignition on the French ``Laser MegaJoule'' (LMJ) facility, specific ICF (inertial confinement fusion) targets are designed. Theses targets are basically composed of a plastic microshell, which includes a solid cryogenic layer of deuterium and tritium. This layer must present a very homogeneous thickness, and its structure has to be very smooth to prevent from hydrodynamics instabilities. This presentation will describe the experimental strategy that has been chosen by CEA to study the technological facilities and the cryogenic targets needed. An integrating sphere cryostat is used to study the deuterium crystallisation and conformation. Theses experiments also contribute to build thermal numerical models of the target. Another cryostat (SFS, Studying Filling Station) is used to study scale one targets. In fact, the target is highly sensitive to its thermal environment, and cryogenic experiments with the real targets are absolutely necessary to prepare future shots on LMJ. The final operational system will be composed of more than ten glove boxes: gas handling systems, target filling process, filling cells maintenance{\ldots} Last results and last delivered cryogenic machines will be presented.   

LMJ ignition target, evolutions and fabrication developments

In order to prepare Inertial Confinement Fusion (ICF) experiments on the ``Laser Megajoule'' (LMJ) facility in France, CEA has developed for more than a decade a large amount of specific targets which generally meet very strict specifications. More over, target fabrication technologies have improved a lot in the world during these last few years, particularly to prepare and realize experiments on new facilities (NIF and LMJ). The development of new fabrication technologies in CEA, has allowed making new cryogenic targets, compatible with a very good thermal control, necessary for the experiments. In this presentation, a focus on the state of the art on the LMJ cryogenic target fabrication is exposed. Design evolutions are presented to obtain best compromise between target fabrication and physics requirements to achieve ICF experiments.