National Diagnostic Working Group (NDWG) for inertial confinement fusion (ICF)/high-energy density (HED) science: The whole exceeds the sum of its parts.

The National Diagnostic Working Group (NDWG) has led the effort to fully exploit the major inertial confinement fusion/high-energy density facilities in the US with the best available diagnostics. These diagnostics provide key data used to falsify early theories for ignition and suggest new theories, recently leading to an experiment that exceeds the Lawson condition required for ignition. The factors contributing to the success of the NDWG, collaboration and scope evolution, and the methods of accomplishment of the NDWG are discussed in this Review. Examples of collaborations in neutron and gamma spectroscopy, x-ray and neutron imaging, x-ray spectroscopy, and deep-ultraviolet Thomson scattering are given. An abbreviated history of the multi-decade collaborations and the present semiformal management framework is given together with the latest National Diagnostic Plan.

Design of a multi-detector, single line-of-sight, time-of-flight system to measure time-resolved neutron energy spectra.

In the dynamic environment of burning, thermonuclear deuterium-tritium plasmas, diagnosing the time-resolved neutron energy spectrum is of critical importance. Strategies exist for this diagnosis in magnetic confinement fusion plasmas, which presently have a lifetime of ∼1012 longer than inertial confinement fusion (ICF) plasmas. Here, we present a novel concept for a simple, precise, and scale-able diagnostic to measure time-resolved neutron spectra in ICF plasmas. The concept leverages general tomographic reconstruction techniques adapted to time-of-flight parameter space, and then employs an updated Monte Carlo algorithm and National Ignition Facility-relevant constraints to reconstruct the time-evolving neutron energy spectrum. Reconstructed spectra of the primary 14.028 MeV nDT peak are in good agreement with the exact synthetic spectra. The technique is also used to reconstruct the time-evolving downscattered spectrum, although the present implementation shows significantly more error.

Design of the high-yield time-gated x-ray hot-spot imager for OMEGA.

Time-resolved x-ray self-emission imaging of hot spots in inertial confinement fusion experiments along several lines of sight provides critical information on the pressure and the transient morphology of the hot spot on the University of Rochester's OMEGA Laser System. At least three quasi-orthogonal lines of sight are required to infer the tomographic information of the hot spots of deuterium-tritium cryogenic layered implosions. OMEGA currently has two time-gated x-ray hot-spot imagers: the time-resolved Kirkpatrick-Baez x-ray microscope and the single-line-of-sight, time-resolved x-ray imager (SLOS-TRXI). The time-gated x-ray hot-spot imager (XRHSI) is being developed for use on OMEGA as the third line of sight for the high-yield operation of up to 4 × 1014 neutrons. XRHSI follows the SLOS-TRXI concept; however, it will have improved spatial and temporal resolutions of 5 µm and 20 ps, respectively. The simultaneous operation of the three instruments will provide 3-D reconstructions of the assembled hot-spot fuel at various times through peak thermonuclear output. The technical approach consists of a pinhole array imager and demagnifying time-dilation drift tube that are coupled to two side-by-side hybrid complementary metal-oxide semiconductor (hCMOS) image sensors. To minimize the background and to harden the diagnostics, an angled drift-tube assembly shifting the hCMOS sensors out of the direct line of sight and neutron shielding will be applied. The technical design space for the instrument will be discussed and the conceptual design will be presented.

Phased plan for the implementation of the time-resolving magnetic recoil spectrometer on the National Ignition Facility (NIF).

The time-resolving magnetic recoil spectrometer (MRSt) is a transformative diagnostic that will be used to measure the time-resolved neutron spectrum from an inertial confinement fusion implosion at the National Ignition Facility (NIF). It uses a CD foil on the outside of the hohlraum to convert fusion neutrons to recoil deuterons. An ion-optical system positioned outside the NIF target chamber energy-disperses and focuses forward-scattered deuterons. A pulse-dilation drift tube (PDDT) subsequently dilates, un-skews, and detects the signal. While the foil and ion-optical system have been designed, the PDDT requires more development before it can be implemented. Therefore, a phased plan is presented that first uses the foil and ion-optical systems with detectors that can be implemented immediately-namely CR-39 and hDISC streak cameras. These detectors will allow the MRSt to be commissioned in an intermediate stage and begin collecting data on a reduced timescale, while the PDDT is developed in parallel. A CR-39 detector will be used in phase 1 for the measurement of the time-integrated neutron spectra with excellent energy-resolution, necessary for the energy calibration of the system. Streak cameras will be used in phase 2 for measurement of the time-resolved spectrum with limited spectral coverage, which is sufficient to diagnose the time-resolved ion temperature. Simulations are presented that predict the performance of the streak camera detector, indicating that it will achieve excellent burn history measurements at current yields, and good time-resolved ion-temperature measurements at yields above 3 × 1017. The PDDT will be used for optimal efficiency and resolution in phase 3.

Performance of a hardened x-ray streak camera at Lawrence Livermore National Laboratory's National Ignition Facility.

Electron tubes continue to provide the highest speeds possible for recording dynamics of hot high-energy density plasmas. Standard streak camera drive electronics and CCD readout are not compatible with the radiation environment associated with high DT fusion yield inertial confinement fusion experiments >1013 14 MeV DT neutrons or >109 n cm-2 ns-1. We describe a hardened x-ray streak camera developed for the National Ignition Facility and present preliminary results from the first experiment on which it has participated, recording the time-resolved bremsstrahlung spectrum from the core of an inertial confinement fusion implosion at more than 40× the operational neutron yield limit of the previous National Ignition Facility x-ray streak cameras.

Design of the ion-optics for the MRSt neutron spectrometer at the National Ignition Facility (NIF).

A new Magnetic Recoil Spectrometer (MRSt) is designed to provide time-resolved measurements of the energy spectrum of neutrons emanating from an inertial confinement fusion implosion at the National Ignition Facility. At present, time integrated parameters are being measured using the existing magnet recoil and neutron time-of-flight spectrometers. The capability of high energy resolution of 2 keV and the extension to high time resolution of about 20 ps are expected to improve our understanding of conditions required for successful fusion experiments. The layout, ion-optics, and specifications of the MRSt will be presented.

Observation of Hydrodynamic Flows in Imploding Fusion Plasmas on the National Ignition Facility.

Inertial confinement fusion implosions designed to have minimal fluid motion at peak compression often show significant linear flows in the laboratory, attributable per simulations to percent-level imbalances in the laser drive illumination symmetry. We present experimental results which intentionally varied the mode 1 drive imbalance by up to 4% to test hydrodynamic predictions of flows and the resultant imploded core asymmetries and performance, as measured by a combination of DT neutron spectroscopy and high-resolution x-ray core imaging. Neutron yields decrease by up to 50%, and anisotropic neutron Doppler broadening increases by 20%, in agreement with simulations. Furthermore, a tracer jet from the capsule fill-tube perturbation that is entrained by the hot-spot flow confirms the average flow speeds deduced from neutron spectroscopy.

Top-level physics requirements and simulated performance of the MRSt on the National Ignition Facility.

The time-resolving Magnetic Recoil Spectrometer (MRSt) for the National Ignition Facility (NIF) has been identified by the US National Diagnostic Working Group as one of the transformational diagnostics that will reshape the way inertial confinement fusion (ICF) implosions are diagnosed. The MRSt will measure the time-resolved neutron spectrum of an implosion, from which the time-resolved ion temperature, areal density, and yield will be inferred. Top-level physics requirements for the MRSt were determined based on simulations of numerous ICF implosions with varying degrees of alpha heating, P2 asymmetry, and mix. Synthetic MRSt data were subsequently generated for different configurations using Monte-Carlo methods to determine its performance in relation to the requirements. The system was found to meet most requirements at current neutron yields at the NIF. This work was supported by the DOE and LLNL.

The Crystal Backlighter Imager: A spherically bent crystal imager for radiography on the National Ignition Facility.

The Crystal Backlighter Imager (CBI) is a quasi-monochromatic, near-normal incidence, spherically bent crystal imager developed for the National Ignition Facility (NIF), which will allow inertial confinement fusion capsule implosions to be radiographed close to stagnation. This is not possible using the standard pinhole-based area-backlighter configuration, as the self-emission from the capsule hotspot overwhelms the backlighter signal in the final stages of the implosion. The CBI mitigates the broadband self-emission from the capsule hot spot by using the extremely narrow bandwidth inherent to near-normal-incidence Bragg diffraction. Implementing a backlighter system based on near-normal reflection in the NIF chamber presents unique challenges, requiring the CBI to adopt novel engineering and operational strategies. The CBI currently operates with an 11.6 keV backlighter, making it the highest energy radiography diagnostic based on spherically bent crystals to date. For a given velocity, Doppler shift is proportional to the emitted photon energy. At 11.6 keV, the ablation velocity of the backlighter plasma results in a Doppler shift that is significant compared to the bandwidth of the instrument and the width of the atomic line, requiring that the shift be measured to high accuracy and the optics aligned accordingly to compensate. Experiments will be presented that used the CBI itself to measure the backlighter Doppler shift to an accuracy of better than 1 eV. These experiments also measured the spatial resolution of CBI radiographs at 7.0 µm, close to theoretical predictions. Finally, results will be presented from an experiment in which the CBI radiographed a capsule implosion driven by a 1 MJ NIF laser pulse, demonstrating a significant (>100) improvement in the backlighter to self-emission ratio compared to the pinhole-based area-backlighter configuration.

Implementation of the foil-on-hohlraum technique for the magnetic recoil spectrometer for time-resolved neutron measurements at the National Ignition Facility.

The next-generation Magnetic Recoil Spectrometer, called MRSt, will provide time-resolved measurements of the deuterium-tritium-neutron spectrum from inertial confinement fusion implosions at the National Ignition Facility. These measurements will provide critical information about the time evolution of the fuel assembly, hot-spot formation, and nuclear burn. The absolute neutron spectrum in the energy range of 12-16 MeV will be measured with high accuracy (∼5%), unprecedented energy resolution (∼100 keV) and, for the first time ever, time resolution (∼20 ps). Crucial to the design of the system is a CD conversion foil for the production of recoil deuterons positioned as close to the implosion as possible. The foil-on-hohlraum technique has been demonstrated by placing a 1-mm-diameter, 40-µm-thick CD foil on the hohlraum diagnostic band along the line-of-sight of the current time-integrated MRS system, which measured the recoil deuterons. In addition to providing validation of the foil-on-hohlraum technique for the MRSt design, substantial improvement of the MRS energy resolution has been demonstrated.

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.

The single-line-of-sight, time-resolved x-ray imager diagnostic on OMEGA.

The single-line-of-sight, time-resolved x-ray imager (SLOS-TRXI) on OMEGA is one of a new generation of fast-gated x-ray cameras comprising an electron pulse-dilation imager and a nanosecond-gated, burst-mode, hybrid complementary metal-oxide semiconductor sensor. SLOS-TRXI images the core of imploded cryogenic deuterium-tritium shells in inertial confinement fusion experiments in the ∼4- to 9-keV photon energy range with a pinhole imager onto a photocathode. The diagnostic is mounted on a fixed port almost perpendicular to a 16-channel, framing-camera-based, time-resolved Kirkpatrick-Baez microscope, providing a second time-gated line of sight for hot-spot imaging on OMEGA. SLOS-TRXI achieves ∼40-ps temporal resolution and better than 10-µm spatial resolution. Shots with neutron yields of up to 1 × 1014 were taken without observed neutron-induced background signal. The implosion images from SLOS-TRXI show the evolution of the stagnating core.

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.

Implementing time resolved electron temperature capability at the NIF using a streak camera.

A new capability at the National Ignition Facility (NIF) has been implemented to measure the temperature of x-ray emitting sources. Although it is designed primarily for Inertial Confinement Fusion (ICF), it can be used for any hot emitting source that is well modeled. The electron temperature (Te) of the hot spot within the core of imploded ICF capsules is an effective indicator of implosion performance. Currently, there are spatially and temporally integrated Te inferences using image plates. A temporally resolved measurement of Te will help elucidate the mechanisms for hot spot heating and cooling such as conduction to fuel, alpha-heating, mix, and radiative losses. To determine the temporally resolved Te of hot spots, specific filters are added to an existing x-ray streak camera "streaked polar instrumentation for diagnosing energetic radiation" to probe the emission spectrum during the x-ray burn history of implosions at the NIF. One of the difficulties in inferring the hot spot temperature is the attenuation of the emission due to opacity from the shell and fuel. Therefore, a series of increasingly thick titanium filters were implemented to isolate the emission in specific energy regions that are sensitive to temperatures above 3 keV while not significantly influenced by the shell/fuel attenuation. Additionally, a relatively thin zinc filter was used to measure the contribution of colder emission sources. Since the signal levels of the emission through the thicker filters are relatively poor, a dual slit (aperture) was designed to increase the detected signal at the higher end of the spectrum. Herein, the design of the filters and slit is described, an overview of the solving technique is provided, and the initial electron temperature results are reported.

Developing an Experimental Basis for Understanding Transport in NIF Hohlraum Plasmas.

We report on the first multilocation electron temperature (T_{e}) and flow measurements in an ignition hohlraum at the National Ignition Facility using the novel technique of mid-Z spectroscopic tracer "dots." The measurements define a low resolution "map" of hohlraum plasma conditions and provide a basis for the first multilocation tests of particle and energy transport physics in a laser-driven x-ray cavity. The data set is consistent with classical heat flow near the capsule but reduced heat flow near the laser entrance hole. We evaluate the role of kinetic effects, self-generated magnetic fields, and instabilities in causing spatially dependent heat transport in the hohlraum.

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.

A high-speed two-frame, 1-2 ns gated X-ray CMOS imager used as a hohlraum diagnostic on the National Ignition Facility (invited).

A novel x-ray imager, which takes time-resolved gated images along a single line-of-sight, has been successfully implemented at the National Ignition Facility (NIF). This Gated Laser Entrance Hole diagnostic, G-LEH, incorporates a high-speed multi-frame CMOS x-ray imager developed by Sandia National Laboratories to upgrade the existing Static X-ray Imager diagnostic at NIF. The new diagnostic is capable of capturing two laser-entrance-hole images per shot on its 1024 × 448 pixels photo-detector array, with integration times as short as 1.6 ns per frame. Since its implementation on NIF, the G-LEH diagnostic has successfully acquired images from various experimental campaigns, providing critical new information for understanding the hohlraum performance in inertial confinement fusion (ICF) experiments, such as the size of the laser entrance hole vs. time, the growth of the laser-heated gold plasma bubble, the change in brightness of inner beam spots due to time-varying cross beam energy transfer, and plasma instability growth near the hohlraum wall.

Next generation gamma-ray Cherenkov detectors for the National Ignition Facility.

The newest generation of Gas Cherenkov Detector (GCD-3) employed in Inertial Confinement Fusion experiments at the Omega Laser Facility has provided improved performance over previous generations. Comparison of reaction histories measured using two different deuterium-tritium fusion products, namely gamma rays using GCD and neutrons using Neutron Temporal Diagnostic (NTD), have provided added credibility to both techniques. GCD-3 is now being brought to the National Ignition Facility (NIF) to supplement the existing Gamma Reaction History (GRH-6m) located 6 m from target chamber center (TCC). Initially it will be located in a reentrant well located 3.9 m from TCC. Data from GCD-3 will inform the design of a heavily-shielded "Super" GCD to be located as close as 20 cm from TCC. It will also provide a test-bed for faster optical detectors, potentially lowering the temporal resolution from the current ∼100 ps state-of-the-art photomultiplier tubes (PMT) to ∼10 ps Pulse Dilation PMT technology currently under development.

Development of a high resolution x-ray spectrometer for the National Ignition Facility (NIF).

A high resolution (E/ΔE = 1200-1800) Bragg crystal x-ray spectrometer is being developed to measure plasma parameters in National Ignition Facility experiments. The instrument will be a diagnostic instrument manipulator positioned cassette designed mainly to infer electron density in compressed capsules from Stark broadening of the helium-ß (1s2-1s3p) lines of krypton and electron temperature from the relative intensities of dielectronic satellites. Two conically shaped crystals will diffract and focus (1) the Kr Heß complex and (2) the Heα (1s2-1s2p) and Lyα (1s-2p) complexes onto a streak camera photocathode for time resolved measurement, and a third cylindrical or conical crystal will focus the full Heα to Heß spectral range onto an image plate to provide a time integrated calibration spectrum. Calculations of source x-ray intensity, spectrometer throughput, and spectral resolution are presented. Details of the conical-crystal focusing properties as well as the status of the instrumental design are also presented.