M. Farrell

Neutron spectrometry--an essential tool for diagnosing implosions at the National Ignition Facility (invited).

DT neutron yield (Y(n)), ion temperature (T(i)), and down-scatter ratio (dsr) determined from measured neutron spectra are essential metrics for diagnosing the performance of inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF). A suite of neutron-time-of-flight (nTOF) spectrometers and a magnetic recoil spectrometer (MRS) have been implemented in different locations around the NIF target chamber, providing good implosion coverage and the complementarity required for reliable measurements of Y(n), T(i), and dsr. From the measured dsr value, an areal density (ρR) is determined through the relationship ρR(tot) (g∕cm(2)) = (20.4 ± 0.6) × dsr(10-12 MeV). The proportionality constant is determined considering implosion geometry, neutron attenuation, and energy range used for the dsr measurement. To ensure high accuracy in the measurements, a series of commissioning experiments using exploding pushers have been used for in situ calibration of the as-built spectrometers, which are now performing to the required accuracy. Recent data obtained with the MRS and nTOFs indicate that the implosion performance of cryogenically layered DT implosions, characterized by the experimental ignition threshold factor (ITFx), which is a function of dsr (or fuel ρR) and Y(n), has improved almost two orders of magnitude since the first shot in September, 2010.

The magnetic recoil spectrometer for measurements of the absolute neutron spectrum at OMEGA and the NIF

The neutron spectrum produced by deuterium-tritium (DT) inertial confinement fusion implosions contains a wealth of information about implosion performance including the DT yield, ion-temperature, and areal-density. The Magnetic Recoil Spectrometer (MRS) has been used at both the OMEGA laser facility and the National Ignition Facility (NIF) to measure the absolute neutron spectrum from 3 to 30 MeV at OMEGA and 3 to 36 MeV at the NIF. These measurements have been used to diagnose the performance of cryogenic target implosions to unprecedented accuracy. Interpretation of MRS data requires a detailed understanding of the MRS response and background. This paper describes ab initio characterization of the system involving Monte Carlo simulations of the MRS response in addition to the commission experiments for in situ calibration of the systems on OMEGA and the NIF.

Measuring the absolute deuterium-tritium neutron yield using the magnetic recoil spectrometer at OMEGA and the NIF.

A magnetic recoil spectrometer (MRS) has been installed and extensively used on OMEGA and the National Ignition Facility (NIF) for measurements of the absolute neutron spectrum from inertial confinement fusion implosions. From the neutron spectrum measured with the MRS, many critical implosion parameters are determined including the primary DT neutron yield, the ion temperature, and the down-scattered neutron yield. As the MRS detection efficiency is determined from first principles, the absolute DT neutron yield is obtained without cross-calibration to other techniques. The MRS primary DT neutron measurements at OMEGA and the NIF are shown to be in excellent agreement with previously established yield diagnostics on OMEGA, and with the newly commissioned nuclear activation diagnostics on the NIF.

Experimental results of radiation-driven, layered deuterium-tritium implosions with adiabat-shaped drives at the National Ignition Facility

Radiation-driven, layered deuterium-tritium (DT) implosions were carried out using 3-shock and 4-shock “adiabat-shaped” drives and plastic ablators on the National Ignition Facility (NIF) [E. M. Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. The purpose of these shots was to gain further understanding on the relative performance of the low-foot implosions of the National Ignition Campaign [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] versus the subsequent high-foot implosions [T. Doppner et al., Phys. Rev. Lett. 115, 055001 (2015)]. The neutron yield performance in the experiment with the 4-shock adiabat-shaped drive was improved by factors ∼3 to ∼10, compared to five companion low-foot shots despite large low-mode asymmetries of DT fuel, while measured compression was similar to its low-foot companions. This indicated that the dominant degradation source for low-foot implosions was ablation-front instability growth, since adiabat shaping significantly stabilized this growth. For the experiment with the low-power 3-shock adiabat-shaped drive, the DT fuel compression was significantly increased, by ∼25% to ∼36%, compared to its companion high-foot implosions. The neutron yield increased by ∼20%, lower than the increase of ∼50% estimated from one-dimensional scaling, suggesting the importance of residual instabilities and asymmetries. For the experiment with the high-power, 3-shock adiabat-shaped drive, the DT fuel compression was slightly increased by ∼14% compared to its companion high-foot experiments. However, the compression was reduced compared to the lower-power 3-shock adiabat-shaped drive, correlated with the increase of hot electrons that hypothetically can be responsible for reduced compression in high-power adiabat-shaped experiments as well as in high-foot experiments. The total neutron yield in the high-power 3-shock adiabat-shaped shot N150416 was 8.5 × 1015 ± 0.2 × 1015, with the fuel areal density of 0.90 ± 0.07 g/cm2, corresponding to the ignition threshold factor parameter IFTX (calculated without alpha heating) of 0.34 ± 0.03 and the yield amplification due to the alpha heating of 2.4 ± 0.2. The performance parameters were among the highest of all shots on NIF and the closest to ignition at this time, based on the IFTX metric. The follow-up experiments were proposed to continue testing physics hypotheses, to measure implosion reproducibility, and to improve quantitative understanding on present implosion results.

Tailored ramp-loading via shock release of stepped-density reservoirsa)

The concept of a gradient piston drive has been extended from that of a single component reservoir, such as a high explosive, to that of a multi-component reservoir that utilizes low density foams and large shocks to achieve high pressures (∼3.5 mbar) and controlled pressure vs. time profiles on a driven sample. Simulated and experimental drives shaped through the use of multiple component (including carbonized resorcinol formaldehyde and SiO2 foam) reservoirs are compared. Individual density layers in a multiple component reservoir are shown to correlate with velocity features in the measured drive which enables the ability to tune a pressure drive by adjusting the components of the reservoir. Pre-shot simulations are shown to be in rough agreement with the data, but post-shot simulations involving the use of simulated plasma drives were needed to achieve an exact match. Results from a multiple component reservoir shot (∼3.5 mbar) at the National Ignition Facility are shown.

Hydrodynamic instability growth of three-dimensional modulations in radiation-driven implosions with “low-foot” and “high-foot” drives at the National Ignition Facility

Hydrodynamic instability growth has been studied using three-dimensional (3-D) broadband modulations by comparing “high-foot” and “low-foot” spherical plastic (CH) capsule implosions at the National Ignition Facility (NIF) [E. M. Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. The initial perturbations included capsule outer-surface roughness and capsule-mounting membranes (“tents”) that were similar to those used in a majority of implosions on NIF. The tents with thicknesses of 31-nm, 46-nm, and 109-nm were used in the experiments. The outer-surface roughness in the “low-foot” experiment was similar to the standard specification, while it was increased by ∼4 times in the “high-foot” experiment to compensate for the reduced growth. The ablation-front instability growth was measured using a Hydrodynamic Growth Radiography platform at a convergence ratio of ∼3. The dominant capsule perturbations, generated by the tent mountings, had measured perturbation amplitudes comparable to the capsule thickness with ...

Surface oxygen micropatterns on glow discharge polymer targets by photo irradiation

Recent simulations predict surface oxygen may be a significant source of disruptive perturbations in the implosion process of glow-discharge polymers (GDP) ablators at the National Ignition Facility. GDP material held in ambient atmospheric conditions showed an increase in mass when stored in light transparent containers, which suggests that photo exposure is a driving force for oxygen absorption. To investigate if surface oxygen is a contributing factor of disruptive perturbations during implosion, a method to imprint a periodic micropattern of oxygen on the surface of GDP was developed and used to fabricate a flat sample for empirical testing. Photo exposure using collimated blue light was used to generate micropatterns of surface oxygen on the GDP material. The periodic oxygen micropattern was confirmed by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy. A SIMS depth profile showed the atomic percent of oxygen ranged from 8 at. % near the surface to 1 at. % at a depth of 2 μm ...

Developments in Microcoining Rippled Metal Foils

Abstract Rippled metal foils are currently sought for high-strain-rate material strength studies at laser facilities. Because these metals typically cannot be diamond turned, we employ a microcoining process to imprint the [approximately]5-μm-deep by [approximately]50-μm-long ripples into the metal surface. This work details recent process developments to fabricate these rippled metal targets, specifically for iron and tantalum. The process consists of nitriding a steel die, diamond turning the die, and then pressing the die into a polished metal foil of choice. We show: advantages of deeper-nitrided dies, improved foil thickness uniformity and characterization, variation in coining stress over different materials, pattern quality characterization, bowing reduction, and patterning of multimode ripples.

Hydro-instability growth of perturbation seeds from alternate capsule-support strategies in indirect-drive implosions on National Ignition Facility

Hydrodynamic instability growth of the capsule support membranes (or “tents”) and fill tubes has been studied in spherical, glow discharge polymer plastic capsule implosions at the National Ignition Facility (NIF) [Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. In NIF implosions, the capsules are supported by tents because the nominal 10-μm thick fill tubes are not strong enough to support capsules by themselves. After it was recognized that the tents had a significant impact of implosion stability, new support methods were investigated, including thicker, 30-μm diameter fill tubes and cantilevered fill tubes, as described in this article. A new “sub-scale” version of the existing x-ray radiography platform was developed for measuring growing capsule perturbations in the acceleration phase of implosions. It was calibrated using hydrodynamic growth measurements of pre-imposed capsule modulations with Legendre modes of 60, 90, 110, and 140 at convergence ratios up to ∼2.4. Subsequent experiments with 3-D ...

Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics

This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(3He,np)α and 3He(3He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measur...