S. Friedrich

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.

Diagnosing implosion performance at the National Ignition Facility (NIF) by means of neutron spectrometry

The neutron spectrum from a cryogenically layered deuterium?tritium (dt) implosion at the National Ignition Facility (NIF) provides essential information about the implosion performance. From the measured primary-neutron spectrum (13?15?MeV), yield (Yn) and hot-spot ion temperature (Ti) are determined. From the scattered neutron yield (10?12?MeV) relative to Yn, the down-scatter ratio, and the fuel areal density (?R) are determined. These implosion parameters have been diagnosed to an unprecedented accuracy with a suite of neutron-time-of-flight spectrometers and a magnetic recoil spectrometer implemented in various locations around the NIF target chamber. This provides good implosion coverage and excellent measurement complementarity required for reliable measurements of Yn, Ti and ?R, in addition to ?R asymmetries. The data indicate that the implosion performance, characterized by the experimental ignition threshold factor, has improved almost two orders of magnitude since the first shot taken in September 2010. ?R values greater than 1?g?cm?2 are readily achieved. Three-dimensional semi-analytical modelling and numerical simulations of the neutron-spectrometry data, as well as other data for the hot spot and main fuel, indicate that a maximum hot-spot pressure of ?150?Gbar has been obtained, which is almost a factor of two from the conditions required for ignition according to simulations. Observed Yn are also 3?10 times lower than predicted. The conjecture is that the observed pressure and Yn deficits are partly explained by substantial low-mode ?R asymmetries, which may cause inefficient conversion of shell kinetic energy to hot-spot thermal energy at stagnation.

Charged-particle spectroscopy for diagnosing shock ρR and strength in NIF implosions

The compact Wedge Range Filter (WRF) proton spectrometer was developed for OMEGA and transferred to the National Ignition Facility (NIF) as a National Ignition Campaign diagnostic. The WRF measures the spectrum of protons from D-(3)He reactions in tuning-campaign implosions containing D and (3)He gas; in this work we report on the first proton spectroscopy measurement on the NIF using WRFs. The energy downshift of the 14.7-MeV proton is directly related to the total ρR through the plasma stopping power. Additionally, the shock proton yield is measured, which is a metric of the final merged shock strength.

Engineering architecture of the neutron Time-of-Flight (nToF) diagnostic suite at the National Ignition Facility

This paper describes the engineering architecture and function of the neutron Time-of-Flight (nToF) diagnostic suite installed on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL). These instruments provide key measures of neutron yield, ion temperature, drift velocity, neutron bang-time, and neutron downscatter ratio. Currently, there are five nToFs on three collimated lines-of-site (LOS) from 18m to 27m from Target Chamber Center, and three positioned 4.5m from TCC, within the NIF Target Chamber but outside the vacuum and confinement boundary by use of re-entrant wells on three other LOS. NIF nToFs measure the time history and equivalent energy spectrum of reaction generated neutrons from a NIF experiment. Neutrons are transduced to electrical signals, which are then carried either by coaxial or Mach-Zehnder transmission systems that feed divider assemblies and fiducially timed digitizing oscilloscopes outside the NIF Target Bay (TB) radiation shield wall. One method of transduction employs a two-stage process wherein a neutron is converted to scintillation photons in hydrogen doped plastic (20x40mm) or bibenzyl crystals (280x1050mm), which are subsequently converted to an electrical signal via a photomultiplier tube or a photo-diode. An alternative approach uses a single-stage conversion of neutrons-to-electrons by use of a thin (0.25 to 2 mm) Chemical Vapor Deposition Diamond (CVDD) disc (2 to 24mm radius) under high voltage bias. In comparison to the scintillator method, CVDDs have fast rise and decay times (

Observation of strong electromagnetic fields around laser-entrance holes of ignition-scale hohlraums in inertial-confinement fusion experiments at the National Ignition Facility

Energy spectra and spectrally resolved one-dimensional fluence images of self-emitted charged-fusion products (14.7MeV D 3 He protons) are routinely measured from indirectly driven inertial-confinement fusion (ICF) experiments utilizing ignition-scaled hohlraums at the National Ignition Facility (NIF). A striking and consistent feature of these images is that the fluence of protons leaving the ICF target in the direction of the hohlraums laser entrance holes (LEHs) is very nonuniform spatially, in contrast to the very uniformfluence of protons leaving through the hohlraum equator. In addition, the measured nonuniformities are unpredictable, and vary greatly from shot to shot. These

In-flight observations of low-mode ρR asymmetries in NIF implosionsa)

Charged-particle spectroscopy is used to assess implosion symmetry in ignition-scale indirect-drive implosions for the first time. Surrogate D3He gas-filled implosions at the National Ignition Facility produce energetic protons via D+3He fusion that are used to measure the implosion areal density (ρR) at the shock-bang time. By using protons produced several hundred ps before the main compression bang, the implosion is diagnosed in-flight at a convergence ratio of 3–5 just prior to peak velocity. This isolates acceleration-phase asymmetry growth. For many surrogate implosions, proton spectrometers placed at the north pole and equator reveal significant asymmetries with amplitudes routinely ≳10%, which are interpreted as l=2 Legendre modes. With significant expected growth by stagnation, it is likely that these asymmetries would degrade the final implosion performance. X-ray self-emission images at stagnation show asymmetries that are positively correlated with the observed in-flight asymmetries and compar...

The effect of shock dynamics on compressibility of ignition-scale National Ignition Facility implosions

The effects of shock dynamics on compressibility of indirect-drive ignition-scale surrogate implosions, CH shells filled with D3He gas, have been studied using charged-particle spectroscopy. Spectral measurements of D3He protons produced at the shock-bang time probe the shock dynamics and in-flight characteristics of an implosion. The proton shock yield is found to vary by over an order of magnitude. A simple model relates the observed yield to incipient hot-spot adiabat, suggesting that implosions with rapid radiation-power increase during the main drive pulse may have a 2× higher hot-spot adiabat, potentially reducing compressibility. A self-consistent 1-D implosion model was used to infer the areal density (ρR) and the shell center-of-mass radius (Rcm) from the downshift of the shock-produced D3He protons. The observed ρR at shock-bang time is substantially higher for implosions, where the laser drive is on until near the compression bang time (“short-coast”), while longer-coasting implosions have lowe...

Neutron time-of-flight measurements on the national ignition facility

Neutron time-of-flight (nToF) instruments are used to provide data on the performance of National Ignition Facility (NIF) fusion experiments. Detectors are located at 4.5 and 20 meters from the center of the target chamber with a 3.9 meter detector to be installed soon. These instruments are used to measure the total neutron emission, temperature of the fuel, time of peak emission (bang time), and areal density of the compressed fuel ρr). This talk will focus on data from the 20 meter detectors which are Xylene based liquid scintillators coupled to micro-channel plate photomultipliers. A figure of merit defined by the ratio of number of neutrons from 10 to 12 MeV divided by the number of neutrons between 13 and 15 MeV, called the down-scatter-ratio (DSR), is used to infer ρr. Analysis techniques using both time domain and energy domain data are discussed showing limitations and error analysis of both methods. Simulated data for an improved detector based on an organic crystal scintillator show that improvements to both scintillator decay and recording fidelity improve the precision of the DSR measurement

Nuclear diagnostic commissioning for the national ignition campaign

Nuclear diagnostics aiming at measuring neutron yield, ion temperature, neutron bang time and down scattered ratio are a main component of the National Ignition Campaign. Therefore in order to commission these diagnostic, polar direct drive experiments on exploding pusher target have been taking place on the National Ignition Facility (NIF) to produce neutron yield up to about 2e14 neutron. Results of the exploding pusher performance on the NIF as well as progress on the neutron diagnostic commissioning will be presented in this talk.