Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility

Abstract

An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T ion are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T ion are observed and the difference is seen to increase with increasing apparent DT T ion . The line-of-sight rms variations of both DD and DT T ion are small, ∼150 eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T ion . Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T ion greater than the DD T ion , but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results. DOI: 10.1103/PhysRevE.94.021202 At the National Ignition Facility (NIF) [1], cryogenically layered capsules of deuterium (D) and tritium (T) fuel contained in 2-mm-diam carbon-based shells are imploded through laser irradiation of a surrounding high-Z hohlraum [2,3]. The imploding DT fuel assembles and "stagnates" in a configuration with a cold high-density shell surrounding a low-density hot spot. Efficient conversion of shell kinetic energy to hot-spot thermal energy is an essential requirement to achieving ignition at the NIF [4,5]. At peak convergence, this ideally results in a spherically symmetric, cold, dense DT fuel shell with an areal density ρR of ∼1.5 g/cm 2 surrounding a ∼5-keV hot spot with ρR ∼ 0.3 g/cm 2 . Although the word "stagnation" is often used for this phase of the implosion, it is inappropriate as the DT and DD neutron spectra indicate significant remaining kinetic energy. Neutron spectrometers [6][7][8][9][10][11][12][13][14][15] provide directional measurements of DT and DD neutron spectra from which yield, burn-averaged ion temperatures T ion and areal densities ρR are obtained. Neutron activation detectors (NADs) [16] measure the unscattered DT yield Y DT . In this paper we focus on the ion "temperatures" from a more extensive set of experiments than previously published [2] and conclude that the fuel assembly during burn in layered DT implosions is not well described by detailed one-dimensional (1D) physics models and simulations. The leading hypothesis for the observed discrepancy between the data and the 1D description is significant disordered motion and the highly 3D nature of the assembly at burn.For a homogeneous stationary DT plasma in thermal equilibrium at ion temperature T thermal , the variance of the * Corresponding author: gatu@psfc.mit.edu DT neutron spectrum (in units of neutron energy) is given bywhere E n is th...

Synthetic nuclear diagnostics for inferring plasma properties of inertial confinement fusion implosions

“…Fluid velocity introduced by drive asymmetries was found to increase the inferred ion temperature and cause variation in this temperature along different lines of sight. This variation is also observed within experiments 4,6 .…”

“…For fuel areal densities relevant to current indirect drive ICF (≤ 1 g/cm 2 ), the 10-12 MeV spectrum is dominated by single scattering from DT. This leads to fitted relations between fuel areal density and DSR 6,45 . Based on the capsule model used, ρR-DSR relations can be inferred from areal density scans in Minotaur.…”

“…Spectroscopy is performed experimentally by the Neutron Time of Flight detectors (nToFs) 11,12 and the Magnetic Recoil Spectrometer (MRS) 13 . Measurements by these diagnostics are used to infer ion temperatures, areal densities and fluid motion 3,6,8,9,14,15 . These are generally calculated from primary and low scattering angle features of the spectra.…”

Kinetic studies of ICF implosions

“…This mechanism has a stronger impact on the DD reaction than on DT, thereby making the apparent DD temperature lower than DT [32]. Such a trend is often observed at NIF [34] and explanations available to date include the bulk fluid motion [35,36], neutron scattering and burn weighting due to reactivity dependence on temperature [34]. None of these mechanisms alone seems capable of giving the observed 25% difference between the DD and DT temperatures, so the newly predicted effect may contribute.…”

Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions

“…15,16 Uniform radial or turbulent flows will result in an overall enhancement in inferred T ion , while asymmetric flows will lead to line-of-sight (LOS) variations in T ion . 17 For this reason, measurements of the neutron spectra from various angles around an ICF implosion can be used to infer the properties of the flow field in the implosion.…”