Session TO5: ICF: Neutron Diagnostics and Fast Ignition

Backscatter edge model to infer fuel layer conditions near stagnation in ICF implosions

Observations during the stagnation of ICF implosions are primarily focused on diagnosing the hot spot. The scattered neutron spectrum also contains a wealth of information about the cold dense fuel. Previous analysis has been restricted to an inference of fuel areal density. In this work we develop a method to extract additional properties, including fluid velocity and temperature. Scattering kinematics are affected by the velocity distribution of the ions with which the neutrons interact. Neutrons which undergo a single 180$^{\mathrm{o}}$ scattering event from D and T ions produce sharp edges in the spectrum. The spectral shape of the backscatter edge is especially sensitive to the ion velocity distribution. Similar to the DT primary spectrum, thermal and non-thermal broadening and bulk fluid velocity shifts govern the spectral form. A model to fit the edge shape has been derived in order to infer an average scattering medium velocity, temperature and acceleration. The 1D discrete ordinates neutron transport code Minotaur is used to post-process 1D Chimera and LILAC simulations of ICF experiments to produce synthetic neutron spectra. Inferred quantities from the backscatter edge model can then be related to implosion performance metrics such as hotspot pressure. Multidimensional effects are investigated through 3D Chimera simulations with the aim of identifying failure mechanisms brought about by asynchronous stagnation of the capsule. Comparisons are made to experimental data obtained at OMEGA with observations of both the nT and nD edges.   

Experimental Analysis of nT Kinematic Edge Data on OMEGA

Recent work [A. J. Crilly \textit{et al.}, Phys. Plasmas \textbf{25}, 122703 (2018)] has identified the shape of the nT kinematic edge present in the scattered neutron energy spectrum of DT cryogenic experiments as a useful diagnostic feature. The neutrons that populate the nT kinematic edge spectral feature have originated from scattering events with tritons of various velocities and temperatures, and therefore contain information on the triton velocity distributions. The mean energy of the nT edge is related to the mean of the scatter-weighted triton velocity distribution, while the slope of the edge is related to the variance of the scatter-weighted triton velocity distribution. An experimental analysis of the nT kinematic edge measured in cryogenic implosions on OMEGA will be presented and the mean and variance of the scatter-weighted triton velocity distribution inferred. A comparison to 1-D and 2-D radiation-hydrodynamic simulation results will be presented and provide insights into the interpretation of these values.   

Measurement of fuel-shell $\rho $R using the neutron spectrometer measurements of down scatter ratio (DSR) in NIF cryogenic layered implosions.

Large fuel-shell $\rho $R is one of the primary factors in capsule performance for inertial confinement fusion (ICF) implosions at the National Ignition Facility (NIF). Six diagnostic line-of-sight measurements of the down scatter ration (DSR), a ratio of the neutron yield in the neutron kinetic energy range from 10 - 12 MeV to the 13 - 15 MeV yield. The DSR is linearly proportional to the fuel shell $\rho $R in the current implosions. We report on the trends in the current implosions including an analysis of the low mode. We discuss the expected improvement of the DSR characterization with the planned addition of another spectrometer line-of-sight. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Measuring Neutron Angular Distributions with the National Ignition Facility's Real-Time Neutron Activation Diagnostic

The uniformity of the DT shell is important for successful ICF implosions and is reflected in the angular distribution of un-scattered 14 MeV neutrons. The neutron angular distribution is measured at NIF by the ``RT-NAD'', an array of 48 Gamma Ray Spectrometers that monitor the slow decay of $^{\mathrm{89}}$Zr isotopes produced by 14 MeV neutrons during the shot. The large number of detectors and the high statistical precision of the array allow the Spherical Harmonic modes of the neutron angular distribution to be measured up to $L =$ 4. We describe the RTNAD hardware and analysis procedures and present measured angular distributions for select ICF shots.   

Hot Spot and Fuel Imaging Using Nuclear Diagnostics on Direct-Drive Cryogenic Implosions on OMEGA

Achieving a symmetric implosion and fuel assembly is critical to maximizing hot-spot pressure and nuclear yield in inertial confinement fusion (ICF) implosions. Nuclear diagnostics provide a direct measurement of the hot-spot shape relevant to nuclear performance and an indirect measurement of the fuel morphology via neutron elastic scattering of fuel ions. We present recent advances in neutron and charged-particle imaging of directly driven cryogenic ICF implosions at the Omega Laser Facility. A neutron imaging system with penumbral and annular apertures was used to record neutron images of the hot spot. Charged-particle penumbral and pinhole imaging provides a complementary measurement of the hot spot via forward-scattered (high-energy) deuterons, as well as a new approach to measure the symmetry of the assembled fuel via sidescattered (low-energy) deuterons. Experimental results and plans for further implementation of these diagnostics will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Improving the DD and DT nToF environment for the bottom, axial line-of-sight detectors at Sandia National Laboratories' Z Machine Laboratories

Parameters such as neutron yield, ion temperature, and Be-liner areal density can be inferred from neutron time-of-flight (nToF) measurements and are essential to understanding the performance of a MagLIF implosion. It is important the measured signals accurately represent the source conditions. Simulations were performed using the Monte Carlo for N-Particle (MCNP) code to improve the quality of measured signals on the bottom, axial nToF detectors at the Z machine. The present Z-machine and detector shields give rise to significant amount of bremsstrahlung and scattered neutrons being incident on the detectors. This alters the shape of the signal and results in a poor signal-to-noise ratio and unresolved structure. By changing the geometry surrounding the detectors and adding a midpoint collimator the simulations suggest the contributions from neutron scattering and bremsstrahlung can be minimized, lowering the background and improving~the detector signal.   

\textbf{New Fast Neutron Time-of-Flight Detectors with Subnanosecond Instrument Response Function for DT Implosions on OMEGA}

Two new fast neutron time-of-flight (nTOF) detectors were recently deployed on the OMEGA laser. The detectors use 10-mm-diam Hamamatsu microchannel plate{\-}photomultiplier tubes (MCP-PMT's) without any scintillator. The elimination of the scintillator removes the scintillator decay from the instrument response function (IRF) and makes the IRF of the PMT nTOF faster than a traditional nTOF detector. The two PMT nTOF detectors are located along antipodal lines of sight at 4.9 m and 10.4 m from the target chamber center. The PMT nTOF detectors are designed to measure neutron yield, ion temperature, and bulk fuel velocity in DT cryogenic implosions on OMEGA. The results of the measurements in experiments with cryogenic and room-temperature implosions over a wide range of neutron yields and ion temperatures will be presented. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

\textbf{The Particle X-ray Temporal Diagnostic for time-resolved measurements of electron temperature in cryogenic DT implosions at OMEGA}

The Particle X-ray Temporal Diagnostic (PXTD) is currently being implemented on OMEGA for a time-resolved measurements of the electron temperature in cryogenic DT implosions. This will be done through time-resolved measurements of the x-ray continuum in the energy range of 10--20 keV. As this type of measurement is unaffected by residual fuel-bulk flows and other non-thermal effects, it provides valuable information about the thermal properties of the hot spot as well as the stagnation pressure in a cryogenic DT implosion. Combined with a neutron-time-of-flight measurement of the ion temperature, which is in contrast affected by flows and non-thermal effects, an understanding of the energy balance can be obtained. This type of data can also be used to assess when an implosion starts deviating from 1D behavior, or when asymmetries and residual fuel-bulk flows start to become significant in the implosion. The work was supported by DOE and LLE.   

\textbf{A new tri-particle mono-energetic backlighting platform for the NIF and OMEGA}

Inertial-confinement-fusion and laboratory-astrophysical experiments involving lasers create high-energy-density plasmas that are of complex nature involving mixtures of ions, electrons, and electric/magnetic fields. Measurements play a critical role in providing quantitative information in these experiments, but several challenges with the current diagnostics remain to be addressed. Building on previous experience, a new DT$^{\mathrm{3}}$He tri-particle backlighter offers a unique capability, which has all the characteristic features of the D$^{\mathrm{3}}$He backlighter, but will substantially advance the capability for diagnosing strong fields and high density in HED plasmas on Omega and the NIF. Radiographs made with the 9.5-MeV deuterons, combined with 3-MeV DD-proton and 15-MeV D$^{\mathrm{3}}$He-proton radiographs, provide further energy constrains and a third time-of-flight delay, allowing discriminatory, high-quality radiographs of electric and magnetic fields and plasma matter to be recorded. Experiments for studying laser-driven, transit phenomena of HED plasmas, such as plasma transport and dynamics, as well as hydrodynamic/kinetic instabilities would greatly benefit from the additional time-resolved radiograph. This work was supported in part by the U.S. DOE, NLUF, LLE and LLNL.   

Laser Plasma Interactions at Shock Ignition Intensities and in NIF Direct Drive Ignition-Scale Ablation-Plasma Conditions

Experiments performed at Omega and the National Ignition Facility have, for the first time, diagnosed laser plasma interactions and the associated hot-electrons at laser intensities of direct relevance to the Shock Ignition approach to laser fusion, and in the ablation plasma conditions expected for direct-drive NIF-ignition designs. The experiments indicate Stimulated Raman Scattering (SRS) is the dominant hot-electron production mechanism. Importantly, the measured hot-electron temperatures are sufficiently low that the hot-electrons should deposit their energy within the implosion shell in-flight, rather than pre-heating the fuel. This opens the possibility that hot-electrons will aid the shock-generation process. Large scale particle-in-cell simulations support the experimental findings.   

A planar geometry platform for accessing ignition length-scale coronal plasmas on Omega.

Experiments performed at Omega have achieved similar density length scales to those anticipated for ignition-scale shock-ignition direct drive inertial confinement fusion implosion using, for example, polar-direct geometry on the National Ignition Facility. This enables the study of parametric instabilities, and in particular SRS and the generation of hot electrons, in plasmas that approximate the conditions relevant to shock ignition. These experiments use an open-cone target to ensure efficient laser-to-target coupling with 20 Omega beams from the 23\textdegree 48\textdegree and 62\textdegree cones. The high angle cone beams were used to create a large focal spot and plasmas density length scales approaching 500 $\mu $m, whilst the 23\textdegree beams use smaller phase plates and drive a more intense beam into this plasma. Here we will discuss the design of the experiment and our analysis of the SRS and hot electron measurements.   

Kinetic Behaviour of SRS in Long Scale-length Plasmas Relevant to Shock-Ignition on the National Ignition Facility

The behaviour of Laser-plasma instabilities (LPI) at laser intensities of $10^{15}$-$10^{16}$Wcm$^{-2}$ is of fundamental importance to the shock-ignition scheme. A key issue is understanding the hot-electron distribution produced by LPIs and whether this will strengthen the ignitor shock sufficiently or cause an unacceptable level of preheat. We recently performed 2D PIC simulations relevant to shock-ignition in short ($L_n=170\mu$m) and long ($L_n=600\mu$m) scale-length plasmas characteristic of experiments at OMEGA and the NIF respectively. We previously reported our initial findings of a transition from the TPD-dominated OMEGA-scale regime to the SRS-dominated regime at NIF-scale. In this talk we focus on the behaviour of SRS at NIF-scale. SRS backscattered light is observed with a divergence half-angle ranging up to approximately 60° and a spectrum that includes significant contributions from densities below the Landau cutoff. Kinetic inflation plays a key role in enhancing SRS activity at low density, and we discuss the detailed interplay of this and other nonlinear effects. Finally, we comment on the hot-electron output, which is found to have a low characteristic temperature.