Experimental Evidence of a Variant Neutron Spectrum from the T(t,2n)α Reaction at Center-of-Mass Energies in the Range of 16-50 keV.

Full calculations of six-nucleon reactions with a three-body final state have been elusive and a long-standing issue. We present neutron spectra from the T(t,2n)α (TT) reaction measured in inertial confinement fusion experiments at the OMEGA laser facility at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (E_{c.m.}) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two E_{c.m.}, with the ^{5}He ground state resonant peak at 8.6 MeV being significantly stronger at the higher than at the lower energy. The data provide the first conclusive evidence of a variant TT-neutron spectrum in this E_{c.m.} range. In contrast to earlier available data, this indicates a reaction mechanism that must involve resonances and/or higher angular momenta than L=0. This finding provides an important experimental constraint on theoretical efforts that explore this and complementary six-nucleon systems, such as the solar ^{3}He(^{3}He,2p)α reaction.

First Liquid Layer Inertial Confinement Fusion Implosions at the National Ignition Facility.

The first cryogenic deuterium and deuterium-tritium liquid layer implosions at the National Ignition Facility (NIF) demonstrate D_{2} and DT layer inertial confinement fusion (ICF) implosions that can access a low-to-moderate hot-spot convergence ratio (1230) DT ice layer implosions. Although high CR is desirable in an idealized 1D sense, it amplifies the deleterious effects of asymmetries. To date, these asymmetries prevented the achievement of ignition at the NIF and are the major cause of simulation-experiment disagreement. In the initial liquid layer experiments, high neutron yields were achieved with CRs of 12-17, and the hot-spot formation is well understood, demonstrated by a good agreement between the experimental data and the radiation hydrodynamic simulations. These initial experiments open a new NIF experimental capability that provides an opportunity to explore the relationship between hot-spot convergence ratio and the robustness of hot-spot formation during ICF implosions.

Using Inertial Fusion Implosions to Measure the T+

Light nuclei were created during big-bang nucleosynthesis (BBN). Standard BBN theory, using rates inferred from accelerator-beam data, cannot explain high levels of ^{6}Li in low-metallicity stars. Using high-energy-density plasmas we measure the T(^{3}He,γ)^{6}Li reaction rate, a candidate for anomalously high ^{6}Li production; we find that the rate is too low to explain the observations, and different than values used in common BBN models. This is the first data directly relevant to BBN, and also the first use of laboratory plasmas, at comparable conditions to astrophysical systems, to address a problem in nuclear astrophysics.

First high-convergence cryogenic implosion in a near-vacuum hohlraum.

Recent experiments on the National Ignition Facility [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] demonstrate that utilizing a near-vacuum hohlraum (low pressure gas-filled) is a viable option for high convergence cryogenic deuterium-tritium (DT) layered capsule implosions. This is made possible by using a dense ablator (high-density carbon), which shortens the drive duration needed to achieve high convergence: a measured 40% higher hohlraum efficiency than typical gas-filled hohlraums, which requires less laser energy going into the hohlraum, and an observed better symmetry control than anticipated by standard hydrodynamics simulations. The first series of near-vacuum hohlraum experiments culminated in a 6.8 ns, 1.2 MJ laser pulse driving a 2-shock, high adiabat (α∼3.5) cryogenic DT layered high density carbon capsule. This resulted in one of the best performances so far on the NIF relative to laser energy, with a measured primary neutron yield of 1.8×10(15) neutrons, with 20% calculated alpha heating at convergence ∼27×.

Thin shell, high velocity inertial confinement fusion implosions on the national ignition facility.

Experiments have recently been conducted at the National Ignition Facility utilizing inertial confinement fusion capsule ablators that are 175 and 165 µm in thickness, 10% and 15% thinner, respectively, than the nominal thickness capsule used throughout the high foot and most of the National Ignition Campaign. These three-shock, high-adiabat, high-foot implosions have demonstrated good performance, with higher velocity and better symmetry control at lower laser powers and energies than their nominal thickness ablator counterparts. Little to no hydrodynamic mix into the DT hot spot has been observed despite the higher velocities and reduced depth for possible instability feedthrough. Early results have shown good repeatability, with up to 1/2 the neutron yield coming from α-particle self-heating.

First Report of Okra enation leaf curl virus and Associated Cotton leaf curl Multan betasatellite and Cotton leaf curl Multan alphasatellite Infecting Cotton in Pakistan: A New Member of the Cotton Leaf Curl Disease Complex.

Cotton (Gossypium hirsutum L.) is an important and widely cultivated crop in Pakistan, upon which many rely for economic security. Cotton leaf curl disease (CLCuD) is caused by a complex comprising of more than eight species in the genus Begomovirus (family Geminiviridae) with associated betasatellite and alphasatellites. During 2011, characteristic symptoms of leaf curl disease were widespread (>40%), and the whitefly Bemisia tabaci (Genn.) vector of the leaf curl complex was abundant in commercial cotton fields in Burewala, Pakistan. Symptoms included vein thickening, upward or downward leaf curling, and foliar enations. To test for the presence of a begomovirus(es), total DNA was extracted from 100 mg of symptomatic leaf tissues from five different plants (isolates CLCuDBur1 to 5) using the CTAB method (1). Total DNA extracts were used for rolling circle amplification (RCA) using TempliPhi DNA Amplification Kit (GE Healthcare). Of the five field isolates, the RCA product for only one, CLCuDBur3, digested with HindIII, produced an apparently full-length ~2.7 kb fragment, suggesting that CLCuD-Bur3 represented a distinct isolate. The 2.7-kb fragment was cloned into the plasmid vector pGEM-3Zf+ (Promega, Madison, WI). To test for the presence of associated alphasatellites and betasatellites, the PCR primers, AlphaF/R and BetaF/R (2), were used to amplify the putative 1.4-kbp molecules. The resultant 1.4-kb PCR products were ligated into the pGEMT-Easy vector and cloned. Cloned inserts for each were subjected to DNA sequencing, bidirectionally. The cloned monopartite, helper begomovirus genome (HF567945), one betasatellite (HF567946), and one alphasatellite (HF567947) sequences were determined and found to be 2,742, 1,358, and 1,376 bases long, respectively. Pairwise sequence comparisons were carried out for each using the 10 most closely related species or strains (identified in GenBank using BLASTn) using MEGA5 software. The CLCuDBur3 genome sequence shared its highest identity (99.6%) with Okra enation leaf curl virus (OELCuV) (KC019308), so CLCuDBur3 is a variant of OELCuV, a begomovirus reported previously from Abelmoschus esculentus (L.) (okra) plants in India. The betasatellite and alphasatellite shared their highest nt identity at 96 and 98.7% with Cotton leaf curl Multan betasatellite (CLCuMB) (AM774311) and Cotton leaf curl Multan alphasatellite (CLCuMA), respectively (misnamed as CLCuBuA in GenBank) (FN658728). Additionally, the HindIII-digested RCA products were analyzed by Southern blot hybridization using a DIG-labeled DNA probe specific for the intergenic region of either Cotton leaf curl Burewala virus (CLCuBuV) or OELCuV. The OELCuV, but not the CLCuBuV, probe hybridized with HindIII digested RCA products (CLCuDBur3 genome), confirming the presence of OELCuV and the absence of CLCuBuV, the latter being the most prevalent begomovirus species infecting cotton in Pakistan. This is the first report of OELCuV infecting cotton plants in Pakistan, underscoring the discovery of yet another begomovirus member of the CLCuD complex. Further, the possible co-infection of cotton by OELCuV and other recognized species of the CLCuD complex could facilitate further diversification (potentially, through recombination) and lead to the emergence of new variants with the potential to cause damage to the cotton crop in Pakistan. References: (1) J. J. Doyle and J. L. Doyle. Focus. 12:13, 1990. (2) M. Zia-Ur-Rehman et al. Plant Dis. 97:1122, 2013.

First Detection of Cotton leaf curl Burewala virus and Cognate Cotton leaf curl Multan betasatellite and Gossypium darwinii symptomless alphasatellite in Symptomatic Luffa cylindrica in Pakistan.

Cotton leaf curl disease (CLCuD) is the major plant viral constraint to cotton production on the Indian subcontinent (2). CLCuD is primarily caused by begomovirus, Cotton leaf curl Burewala virus (CLCuBuV), and Cotton leaf curl Multan betasatellite (CLCuMB). During 2011 in Burewala, Pakistan, plants in a production field of Luffa cylindrica (Ghia tori) were infested with the whitefly Bemisia tabaci (Genn.), and ~60% of the plants exhibited leaf curling and stunting symptoms, reminiscent of those caused by begomoviruses (Geminiviridae). Total DNA was extracted from five different symptomatic leaf samples using the CTAB method (1), and extracts were analyzed by Southern blot hybridization. As a probe, we used a 1.1-kbp fragment of CLCuBuV and a positive signal was obtained from all five samples. Total DNA was used as template for rolling circle amplification (RCA) using the TempliPhi DNA Amplification Kit (GE Healthcare, Little Chalfont, United Kingdom). The amplified RCA products were digested with EcoRI, and the resulting ~2.7-kbp fragments from each isolate were directionally cloned into the EcoRI digested, pGEM-3Zf+ (Promega, Madison, WI) plasmid vector. PCR was used to amplify the prospective, associated betasatellite and alphasatellite molecules using the primers BetaF5'-GGTACCGCCGGAGCTTAGCWCKCC-3' and BetaR5'-GGTACCGTAGCTAAGGCTGCTGCG-3', and AlphaF5'-AAGCTTAGAGGAAACTAGGGTTTC-3' and AlphaR5'-AAGCTTTTCATACARTARTCNCRDG-3', respectively. The putative satellite amplicons, at ~1.4 kbp each were cloned in the plasmid vector pGEMT-Easy (Promega, Madison, WI) and sequenced. BLASTn comparisons of the apparently full-length begomoviral genomes, at 2,753 nt, against the NCBI database revealed that all five isolates were most closely related to CLCuBuV (FR750321). In addition, one each of beta- and alpha-satellite were amplified from all five samples at 1,393 and 1,378 bases, respectively. The beta- and alpha-satellites were most closely related to CLCuMB (HE985228) and the Gossypium darwinii symptomless alphasatellite (GDaSA) (FR877533), respectively. Pairwise sequence comparisons of the top 10 BLASTn hits using MEGA5 indicated that the helper begomovirus shared 99.9% identity with CLCuBuV (FR750321), the most prevalent helper virus currently associated with the leaf curl complex in Pakistan. Based on the ICTV demarcation for begomoviral species at <89%, it is considered a variant of CLCuBuV. The resultant beta- and alpha-satellite sequences were 98.1% and 97.8% identical to CLCuMB (HE985228) and GDaSA (FR877533), respectively, and are the most prevalent satellites associated with the CLCuD complex in Pakistan and India (2). To our knowledge, this is first report of the CLCuBuV-CLCuMB-GDaSA complex infecting a cucurbitaceous species, and the first report of L. cylindrica as a host of the CLCuD complex. This discovery of CLCuBuV and associated satellites in a cucurbitaceous host that is widely grown in Pakistan and India where this complex infects cotton indicates that the host range of CLCuBuV is broader than expected. This new information will aid in better understanding of cotton leaf curl disease epidemiology in the current epidemic areas. References: (1) J. J. Doyle and J. L. Doyle. Focus 12:13, 1990. (2) S. Mansoor et al. Trends Plant Sci. 11:209, 2006.

Evidence for stratification of deuterium-tritium fuel in inertial confinement fusion implosions.

Measurements of the D(d,p)T (dd) and T(t,2n)(4)He (tt) reaction yields have been compared with those of the D(t,n)(4)He (dt) reaction yield, using deuterium-tritium gas-filled inertial confinement fusion capsule implosions. In these experiments, carried out on the OMEGA laser, absolute spectral measurements of dd protons and tt neutrons were obtained. From these measurements, it was concluded that the dd yield is anomalously low and the tt yield is anomalously high relative to the dt yield, an observation that we conjecture to be caused by a stratification of the fuel in the implosion core. This effect may be present in ignition experiments planned on the National Ignition Facility.

Measurements of the T(t,2n)4He neutron spectrum at low reactant energies from inertial confinement implosions.

Measurements of the neutron spectrum from the T(t,2n)4He (tt) reaction have been conducted using inertial confinement fusion implosions at the OMEGA laser facility. In these experiments, deuterium-tritium (DT) gas-filled capsules were imploded to study the tt reaction in thermonuclear plasmas at low reactant center-of-mass (c.m.) energies. In contrast to accelerator experiments at higher c.m. energies (above 100 keV), these results indicate a negligible n + 5He reaction channel at a c.m. energy of 23 keV.

Diagnostic signature of the compressibility of the inertial-confinement-fusion pusher.

Carbon shell areal density measurements from many types of inertial confinement fusion implosions at the National Ignition Facility (NIF) demonstrate that the final state of the outside portion of the shell is set primarily by capsule coast time, the coasting period between main laser shut off and peak fusion output. However, the fuel areal density does not correlate with the increasing carbon compression. While two-dimensional (2D) radiation-hydrodynamic simulations successfully capture the carbon compression, energy must be added to the simulated fuel-ice layer to reproduce fuel areal density measurements. The data presented demonstrates that the degradation mechanisms that reduce the compressibility of the fuel do not reduce the compressibility of the ablator.

Solid Cherenkov detector for studying nucleosynthesis in inertial confinement fusion.

Measuring gamma rays emitted from nuclear reactions gives insight into their nuclear structure. Notably, there are several nuclear reactions that produce gamma rays at ∼1 MeV-3 MeV energies such as T(4He, γ)7Li, 4He(3He, γ)7Be, and 12C(p, γ)13N, which may solve questions lingering about big-bang nucleosynthesis and stellar nucleosynthesis. To observe 1 MeV-3 MeV gamma rays in an inertial confinement fusion system, a new style of the Cherenkov detector was developed using aerogel and fused silica as a Cherenkov medium. Utilizing the OMEGA laser facility, both aerogel and fused silica media were compared with the existing gas-medium Cherenkov detector to validate the concept. Gamma ray measurements from high yield inertial confinement fusion implosions (deuterium-tritium and deuterium-3He) demonstrated that aerogel and fused silica were viable Cherenkov media, paving the way for a potential optimized detector to make these cross section measurements on OMEGA or the National Ignition Facility.

Improved calibration of the OMEGA gas Cherenkov detector.

Inertial fusion implosions are diagnosed using γ rays to characterize the implosion physics or measure basic nuclear properties, including cross sections. For the latter, previously reported measurements at laser facilities using gas Cherenkov detectors are limited by a large systematic uncertainty in the detector response. We present a novel in situ calibration technique using neutron inelastic scattering, which we apply to the new GCD-3 detector. The calibration accuracy is improved by ∼3× over the previous method.

Improved inertial confinement fusion gamma reaction history 12C gamma-ray signal by direct subtraction.

The Gamma Reaction History (GRH) diagnostic located at the National Ignition Facility (NIF) measures time resolved gamma rays released from inertial confinement fusion experiments by converting the emitted gamma rays into Cherenkov light. Imploded capsules have a bright 4.4 MeV gamma ray from fusion neutrons inelastically scattering with carbon atoms in the remaining ablator. The strength of the 4.4 MeV gamma ray line is proportional to the capsule's carbon ablator areal density and can be used to understand the dynamics and energy budget of a carbon-based ablator capsule implosion. Historically, the GRH's four gas cells use the energy thresholding from the Cherenkov process to forward fit an estimation of the experiment's complete gamma ray spectrum by modeling the surrounding environment in order to estimate the 4.4 MeV neutron induced carbon gamma ray signal. However, the high number of variables, local minima, and uncertainties in detector sensitivities and relative timing had prevented the routine use of the forward fit to generate carbon areal density measurements. A new, more straightforward process of direct subtraction of deconvolved signals was developed to simplify the extraction of the carbon areal density. Beryllium capsules are used as a calibration to measure the capsule environment with no carbon signal. The proposed method is then used to appropriately subtract and isolate the carbon signal on shots with carbon ablators. The subtraction algorithm achieves good results across all major capsule campaigns, achieving similar results to the forward fit. This method is now routinely used to measure carbon areal density for NIF shots.

Development of an ultra-fast photomultiplier tube for gamma-ray Cherenkov detectors at the National Ignition Facility (PD-PMT).

A new ultra-fast photomultiplier tube and associated drivers have been developed for use in the next generation of gamma-ray high pressure gas Cherenkov detectors for inertial confinement fusion experiments at the National Ignition Facility. Pulse-dilation technology has been applied to a standard micro-channel-plate-based photomultiplier tube to improve the temporal response by about 10×. The tube has been packaged suitably for deployment on the National Ignition Facility, and remote electronics have been designed to deliver the required non-linear waveforms to the pulse dilation electrode. This is achieved with an avalanche pulse generator system capable of generating fast arbitrary waveforms over the useful parameter space. The pulse is delivered via fast impedance-matching transformers and isolators, allowing the cathode to be ramped on a sub-nanosecond time scale between two high voltages in a controlled non-linear manner. This results in near linear pulse dilation over several ns. The device has a built-in fiducial system that allows easy calibration and testing with fiber optic laser sources. Results are presented demonstrating the greatly improved response time and other parameters of the device.

Cherenkov detector analysis for implosions with multiple nuclear reactions.

Nuclear reactions that produce γ rays occur in inertial fusion implosions and are commonly measured with Cherenkov detectors. Typically a detector is primarily sensitive to a single reaction, but in some implosions, multiple fusion reactions can occur and are combined in the data. We discuss an analysis technique using multiple thresholded detectors to reproduce the individual burn histories from reactions like DT and HT fusion, which is applicable to separated-reactant mix experiments. Requirements for this technique and resulting analysis uncertainties are quantified using synthetic data.

Diffusion-dominated mixing in moderate convergence implosions.

High-Z material mixed into the fuel degrades inertial fusion implosions and can prevent ignition. Mix is often assumed to be dominated by hydrodynamic instabilities, but we report Omega data, using shells with ∼150nm deuterated layers to gain unprecedented resolution, which give strong evidence that the dominant mix mechanism is diffusion for these moderate temperature (≲6 keV) and convergence (∼12) implosions. Small-scale instability-driven or turbulent mix is negligible.

Characterization of the Mercury pulsed power x-ray source spectrum using multichannel density aerogel Cherenkov detectors.

The Aerogel Cherenkov Detector for Cygnus (ACD/C) is a time-dependent, x-ray spectral detector that uses SiO2 aerogels spanning an index of refraction (n = 1.02-1.07) corresponding to a 1.1-2.3 MeV x-ray energy threshold. The ACD/C was developed for pulsed power x-ray sources like Cygnus located at the Nevada National Site and Mercury located at the Naval Research Laboratory (NRL). Aerogels sit between the measurement capabilities of gas (>2 MeV) and solids such as fused silica (>0.3 MeV). The detector uses an aluminum converter to Compton scatter incoming x-rays and create relativistic electrons, which produce Cherenkov light in an aerogel or a fused silica medium. The ACD/C was fielded at the NRL when Mercury was tuned to produce up to 4.8 MeV endpoint bremsstrahlung. Despite a high radiation and electromagnetic interference background, the ACD/C was able to achieve high signal over noise across five aerogel densities and fused silica, including a signal to noise for a 1.1 MeV aerogel threshold. Previous experiments at Cygnus observed a signal that was comparable to the noise (1×) at the same threshold. The ACD/C observed time-resolved rise and fall times for different energy thresholds of the photon spectrum. Monte Carlo simulations of the ACD/C's aerogel response curves were folded with a simulation of Mercury's photon energy spectrum and agree within the error to the observed result.

Characterisation of a sub-20 ps temporal resolution pulse dilation photomultiplier tube.

A pulse-dilation photomultiplier tube (PD-PMT) with sub-20 ps temporal resolution has been developed for use with γ-ray-sensitive gas Cherenkov detectors at the National Ignition Facility to improve the diagnosis of nuclear fusion burn history and the areal density of the remaining capsule ablator. The pulse-dilation mechanism entails the application of a time-dependent, ramp waveform to a photocathode-mesh structure, introducing a time-dependent photoelectron accelerating potential. The electric field imparts axial velocity dispersion to outgoing photoelectrons. The photoelectron pulse is dilated as it transits a drift region prior to amplification in a microchannel plate and read out with a digital oscilloscope. We report the first measurements with the prototype PD-PMT demonstrating nominal <20 ps FWHM across a 400 ps measurement window and <30 ps FWHM for an extracted charge up to 300 pC. The output peak areas are linear to within 20% over 3 orders of magnitude of input intensity. 3D particle in cell simulations, which included space charge effects, have been carried out to investigate the device temporal magnification, resolution, and linearity.

Pulse dilation gas Cherenkov detector for ultra-fast gamma reaction history at the NIF (invited).

The Cherenkov mechanism used in Gas Cherenkov Detectors (GCDs) is exceptionally fast. However, the temporal resolution of GCDs, such as the Gamma Reaction History diagnostic at the National Ignition Facility (NIF), has been limited by the current state-of-the-art photomultiplier tube technology to ∼100 ps. The soon-to-be deployed Pulse Dilation Photomultiplier Tube (PD-PMT) at NIF will allow for temporal resolution comparable to that of the gas cell or ∼10 ps. Enhanced resolution will contribute to the quest for ignition in a crucial way through precision measurements of reaction history and ablator areal density (ρR) history, leading to better constrained models. Features such as onset of alpha heating, shock reverberations, and burn truncation due to dynamically evolving failure modes may become visible for the first time. Test measurements of the PD-PMT at Atomic Weapons Establishment confirmed that design goals have been met. The PD-PMT provides dilation factors of 2 to 40× in 6 increments. The GCD-3 recently deployed at the NIF has been modified for coupling to a PD-PMT and will soon be making ultrafast measurements.

Progress on next generation gamma-ray Cherenkov detectors for the National Ignition Facility.

Fusion reaction history and ablator areal density measurements for Inertial Confinement Fusion experiments at the National Ignition Facility are currently conducted using the Gamma Reaction History diagnostic (GRH_6m). Future Gas Cherenkov Detectors (GCDs) will ultimately provide ∼100x more sensitivity, reduce the effective temporal response from ∼100 to ∼10 ps, and lower the energy threshold from 2.9 to 1.8 MeV, relative to GRH_6m. The first phase toward next generation GCDs consisted of inserting the existing coaxial GCD-3 detector into a reentrant well which puts it within 4 m of the implosion. Reaction history and ablator gamma measurement results from this Phase I are discussed here. These results demonstrate viability for the follow-on Phases of (II) the use of a revolutionary new pulse-dilation photomultiplier tube to improve the effective measurement bandwidth by >10x relative to current PMT technology; and (III) the design of a NIF-specific "Super" GCD which will be informed by the assessment of the radiation background environment within the well described here.