Session JO5: ICF Implosions and Diagnostics

The National Ignition Facility (NIF) User Program

The 192-beam National Ignition Facility (NIF) at LLNL is now operational and conducting experiments in ICF ignition and other areas of high energy density science. The NIF ignition program is conducted by the National Ignition Campaign (NIC). In addition to execution of the ignition program, the NIC will also provide the necessary infrastructure for operation of NIF as a user facility. As a user facility, NIF will execute experiments in support of stockpile stewardship/national security, fundamental science, and inertial fusion energy. An overview of the multi-mission NIF experimental program, with emphasis on development of fundamental science at NIF and the process for accessing NIF facility time, will be presented.   

Magnetized Spherical Implosions on the OMEGA Laser

In previous experiments, a line-average magnetic field between 30 to 40 MG was observed in cylindrical imploding targets.\footnote{O.V. Gotchev \textit{et al}., Phys. Rev. Lett. \textbf{103}, 215004 (2009)} The hot spot was magnetized and the heat conduction was suppressed according to Braginskii's formula. No clear evidence of neutron-yield enhancement was observed. Several conjectures have been proposed to explain the observations, such as the low hot-spot density that led to the ion mean free path to exceed the hot-spot size. In addition, the high shot-to-shot variations in the neutron-yield measurements make it difficult to accurately determine the correct neutron output. We have carried out a set of spherical implosions on OMEGA with embedded seed magnetic fields. The hot-spot density and collisionality are more than an order of magnitude higher than in cylindrical implosions. Furthermore, less shot-to-shot variations are expected. Results from the measurements of the hot-spot temperature and neutron yield of spherical implosions are reported to shed some light on the issue of magnetic insulation of magnetized inertial fusion targets. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement Nos. DE-FC52-08NA28302 and DE-FC02-04ER54789.   

Study of Self-Generated Magnetic Fields in Implosion Experiments on OMEGA

Proton radiography of directly driven inertial fusion implosions has revealed the development of filamentary electromagnetic fields in outflowing corona of plastic-shell targets.\footnote{ C. K. Li \textit{et al}., Phys. Rev. Lett. \textbf{100}, 225001 (2008).} To explain these experiments, the dynamics of self-generated magnetic fields that originated near the critical surface of the targets is investigated using 2-D MHD simulations. Laser imprint is considered as the source of plasma nonuniformities that generate seed fields via the thermoelectric effect. The predicted fields ($\sim $1 MG) show good agreement with the fields inferred from the experiments. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Reduced Compression and Yield for Direct Drive ICF Capsules

We report on a series of experiments conducted at the LLE's Omega Laser to study the effect of high z dopant gases on the performance of D2 filled capsules. The experiments all consisted of thin, $\sim $ 4.5 microns, glass capsules filled with 7-10 atm. of gas that were imploded using the direct drive laser beams at Omega. The laser conditions for these experiments were 1 ns flat pulses with a total energy of $\sim $ 23 kJ. The typical implosion times were $\sim$ 1.3 ns and yields ranged from 10$^9$ neutrons to $>$ 10$^{11}$ neutrons. The capsules were doped with varying levels of Kr gas and also in several cases contained He3. A key finding of these experiments was that the capsules did not reach the full compression predicted by simulation. Specifically, we found that the capsule diameter matched the simulation until the time when the main reflected shock reached the shell. After that time, the capsule diameter is measured to be larger than predicted. In this presentation, we quantify the effect of this reduced compression on the yield of the capsule.   

Areal Density and Ion-Temperature Measurements in Cryogenic-DT Implosions on OMEGA

High areal densities (\textit{$\rho $R}) were recently reported from low-adiabat cryogenic deuterium--tritium (DT) implosions on the OMEGA laser.\footnote{V. N. Goncharov \textit{et al}., Phys. Rev. Lett. \textbf{104}, 165001 (2010)} These implosions used a multiple-shock drive pulse to minimize shock preheating. The \textit{$\rho $R} was measured using a magnetic recoil spectrometer to infer the neutron fraction scattered in the dense fuel. The measured yields, however, were only a few percent of the 1-D prediction. Considerable effort is underway to minimize the sources of nonuniformity in these implosions. This includes improved ice- and capsule-surface quality, improved beam-pointing accuracy, and advances in target mounting and alignment. These improvements are expected to measurably increase the ion temperature and primary neutron yield. This talk will report on the latest target-performance results. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Hard X-Ray Compton Radiography of Cryogenic Implosions on OMEGA

Compton scattering radiography of hard backlight x rays is being developed to measure the mass density distribution and areal density of cryogenic implosions on OMEGA. This work builds on previous success with Compton radiography of warm polymer-shell implosions using bremsstrahlung backlighters driven by the OMEGA EP short-pulse laser. Analyses of simulated radiographic data, based on simulated cryogenic implosions and the weak spectral and material dependence of Compton scattering, demonstrates measurable signal levels. Mass distributions are obtained by Abel inverting the radiographs of spherically symmetric models and spherical ice-block models with offset cores. The results of recent Compton radiography measurements are analyzed. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Advanced processing of spectrally-resolved core images from OMEGA direct-drive implosions

We discuss the processing of spectrally-resolved core images from argon-doped, deuterium-filled OMEGA direct-drive implosions. Spectrally-resolved images were recorded by a DDMMI instrument which consists of a pinhole array, a multi-layer Bragg mirror, and an x-ray framing camera with micro-channel plates. The pinhole array creates a large number of object images each one of them characteristic of a slightly different wavelength range, which are then recorded by a gated framing camera. Thus, DDMMI yields data that are resolved in wavelength, space, and time. DDMMI data can be processed to extract broad- and narrow-band core images, as well as space-integrated and space-resolved spectra. These data are important for determining the spatial structure of the implosion core.   

Yield, Ion Temperature, fuel-$\rho $R and Burn-history Measurements in Exploding Pusher Experiments at OMEGA and the NIF

In preparation for the planned DD- and D$^{3}$He-exploding-pusher experiments at the NIF, we conducted similar experiments at OMEGA in which yield, ion temperature, fuel-$\rho $R, and burn history were measured by a variety of diagnostic techniques. The resulting data from these measurements provide, in combination with simulations, a comprehensive understanding of these implosions. In this presentation, we report the result from these experiments and their potential implications for the NIF experiments. A status report on the NIF activities will be presented as well. This work was supported in part by the US DOE, LLNL and LLE.   

Development of a Spherical Crystal X-Ray-Imaging Diagnostic for OMEGA and OMEGA EP

Bent Bragg crystal x-ray imagers have become routine diagnostics in various laser, magnetically confined, and astrophysical plasmas. The Laboratory for Laser Energetics is developing a monochromatic x-ray imager for backlit and self-emission plasma imaging at Cu K$_{\alpha }$ line radiation at 1.541~{\AA} (8.048~keV). The image will be formed by a spherically bent quartz crystal with a 2131 lattice cut. The crystal 2$d$ lattice spacing~of 3.082~{\AA} corresponds to an 88.7\r{ } angle (1.3\r{ } from normal). The crystal has a 25-mm diameter and the radius of curvature radius is $R_{c}$~=~500~mm. The resulting spectral bandpass is $\Delta $\textit{$\lambda $}/\textit{$\lambda $} = 10$^{-3}$. Optical ray tracing shows that a spatial resolution of about 6 to 8~\textit{$\mu $}m is possible, limited by astigmatism and coma. The imager prototype will be tested on the MTW Laser Facility. The x-ray radiation will be formed by interaction of short ($<$1-ps), high-power ($>$10$^{19}$~W/cm$^{2})$ laser pulses with thin Cu foils. Details of design, testing, and the schedule for implementation on OMEGA EP will be reported. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302.   

Utilizing proton bang time and areal density measurements for diagnosing NIF implosions

J.KILKENNY, A.NIKROO, \textit{GA}, R.OLSON, R.LEEPER, SNL, D.WILSON, \textit{LANL} -- In NIF D$^{3}$He implosions, measurements of shock-bang time and associated areal density, including possible P$_{2}$ asymmetries, will lead to important insights for guiding the NIF-capsule-tuning campaign. In concert with simulations and essential information from other diagnostics, such as those measuring the remaining ablator mass, implosion velocity, compression-bang time, ion temperature, etc, a comprehensive understanding of these implosions can be discerned. In this presentation, we report on the status of this work, which is supported in part by US DOE, LLNL, LLE and FSC.   

A CVD-diamond based proton-bang-time detector for OMEGA and the NIF

J.KILKENNY, A.NIKROO, \textit{GA} -- A chemical vapor deposition (CVD) diamond detector has been used for the first time to measure $\sim $15 MeV protons generated by D$^{3}$He-fusion reactions in exploding-pusher implosions on OMEGA. The results indicate that a proton-bang time can be measured accurately. The CVD response has also been characterized using D$^{3}$He implosions on OMEGA and $\sim $10 ps x-ray pulses on LLE's Multi-Terawatt Laser facility (MTW). The motivation for this work is that measurements of the shock-proton-bang time in upcoming SymCap and ConvAbl campaigns at the NIF will provide information that will constrain LASNEX modeling of the implosions and possibly address effects such as preheat. The results from the measurements at OMEGA, MTW and the NIF will be presented. Some modeling of these measurements will be presented as well. This work was performed in part at the NLUF, and was supported in part by US DOE, LLNL, LLE and FSC.   

Diagnosing Inertial Confinement Fusion Implosions Using the D$^{3}$He Spectrum Line Width at OMEGA and the NIF

Wedge Range Filter (WRF) spectrometers are used to measure the proton spectrum due to the D+$^{3}$He $\to $ p (14.7 MeV) + $^{4}$He (3.6 MeV) reactions produced in implosions containing D and $^{3}$He gas. The line width of the measured spectrum is due to the thermal Doppler broadening, instrumental broadening, and several capsule effects such as a finite source size and implosion asymmetries. Models for these broadening sources are presented. Using these models we calculate an ion temperature in OMEGA and NIF exploding pusher shots. This Doppler-derived temperature is compared to independent measurements. Alternatively we use this model to constrain the amplitude of high-mode $\rho $R asymmetries in NIF indirect-drive CH shell implosions. This work was supported in part by the U.S. DoE, LLNL, LLE, FSC, and NLUF. A.Zylstra is supported by the DoE NNSA Stewardship Science Graduate Fellowship.   

Calibration of a Thomson parabola ion spectrometer and Fujifilm imaging plates for energetic protons, deuterons, and alpha particles

A Thomson parabola ion spectrometer (TPIS) has been designed and built to study energetic ions accelerated from the rear surface of targets irradiated by ultra-intense laser light from the Multiterawatt (MTW) laser facility at the Laboratory for Laser Energetics (LLE). The device uses a permanent magnet and a pair of electrostatic deflector plates to produce parallel magnetic and electric fields, which cause ions of a given charge-to-mass ratio to be deflected onto parabolic curves on the detector plane. The position of the ion along the parabola can be used to determine its energy. Fujifilm imaging plates (IP) are placed in the rear of the device and are used to detect the incident ions. The energy dispersion of the spectrometer has been calibrated using monoenergetic ion beams from the SUNY Geneseo 1.7 MV pelletron accelerator. The IP sensitivity has been measured for protons and deuterons with energies between 0.6 MeV and 3.4 MeV, and for alpha particles with energies between 1.5 MeV and 5.1 MeV.