The effect of turbulent kinetic energy on inferred ion temperature from neutron spectra

Murphy, T. J.

doi:10.1063/1.4885342

Cited by 112 publications

(64 citation statements)

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“…Also shown are simulated ratios from 1D HYDRA simulations (black crosses) and predicted ratios in apparent T ion due to a nonthermal contribution from flows according to Ref. [23], assuming a thermal T ion = 0 keV (solid purple line), thermal T ion = 1.5 keV (dot-dashed line), and thermal T ion = 2 keV (double-dot-dashed line).…”

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“…It should be noted that the strong coupling of deuterons and tritons will result in equality of the true ion temperatures. In a recent paper, Murphy [23] articulates that the quantity inferred from the neutron spectrum variance, which we will call apparent T ion , for a homogeneous nonstationary plasma is the sum of the thermal broadening and a macroscopic broadening from the line-of-sight-projected bulk fluid velocity variance σwhere k B is Boltzmann's constant, m X = m t for DT neutrons, and m X = m d for DD (see also [24]). If the variance of the bulk velocity dominates over T thermal , then apparent T ion as inferred from the neutron spectrum variance for DD neutrons will be 80% of the apparent T ion inferred from DT neutrons.…”

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

Johnson¹,

Knauer²,

Cerjan³

et al. 2016

Phys. Rev. E

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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...

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

Johnson¹,

Knauer²,

Cerjan³

et al. 2016

Phys. Rev. E

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“…Furthermore, the 3D simulations continue the trend of under predicting the hot spot ion temperatures and over predicting the implosion DSRs, although the 3D DSR results are clearly closer to agreement. This is notable in that the 3D simulations fully account for the current best prediction of the 3D hot spot flows that are hypothesized to account for the measured ion temperatures [38,39]. That is, the realistic stagnation process in the presence of hohlraum asymmetries and capsule perturbations is modeled fully in these simulations and yet continues to disagree with the measured hot spot ion temperature.…”

Section: D Simulationsmentioning

confidence: 83%

Capsule modeling of high foot implosion experiments on the National Ignition Facility

Clark

Kritcher

Milovich

et al. 2017

Plasma Phys. Control. Fusion

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This paper summarizes the results of detailed, capsule-only simulations of a set of high foot implosion experiments conducted on the National Ignition Facility (NIF). These experiments span a range of ablator thicknesses, laser powers, and laser energies, and modeling these experiments as a set is important to assess whether the simulation model can reproduce the trends seen experimentally as the implosion parameters were varied. Two-dimensional (2D) simulations have been run including a number of effects-both nominal and off-nominal-such as hohlraum radiation asymmetries, surface roughness, the capsule support tent, and hot electron pre-heat. Selected three-dimensional simulations have also been run to assess the validity of the 2D axisymmetric approximation. As a composite, these simulations represent the current state of understanding of NIF high foot implosion performance using the best and most detailed computational model available. While the most detailed simulations show approximate agreement with the experimental data, it is evident that the model remains incomplete and further refinements are needed. Nevertheless, avenues for improved performance are clearly indicated.

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“…Recent theoretical work has suggested that bulk fluid motion of a TN plasma can have a noticeable broadening effect on neutron spectrum width in inertial confinement fusion, [42][43][44] potentially causing an erroneously high ion temperature to be inferred from the spectrum width. This effect was included in the simulations of the 15 MA gas puff and 2 MA DPF.…”

Section: B Thermonuclear Spectra Broadened By Bulk Fluid Motionmentioning

confidence: 99%

Neutron spectra from beam-target reactions in dense Z-pinches

Appelbe

Chittenden

2015

Physics of Plasmas

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The energy spectrum of neutrons emitted by a range of deuterium and deuterium-tritium Z-pinch devices is investigated computationally using a hybrid kinetic-MHD model. 3D MHD simulations are used to model the implosion, stagnation, and break-up of dense plasma focus devices at currents of 70 kA, 500 kA, and 2 MA and also a 15 MA gas puff. Instabilities in the MHD simulations generate large electric and magnetic fields, which accelerate ions during the stagnation and break-up phases. A kinetic model is used to calculate the trajectories of these ions and the neutron spectra produced due to the interaction of these ions with the background plasma. It is found that these beam-target neutron spectra are sensitive to the electric and magnetic fields at stagnation resulting in significant differences in the spectra emitted by each device. Most notably, magnetization of the accelerated ions causes the beam-target spectra to be isotropic for the gas puff simulations. It is also shown that beam-target spectra can have a peak intensity located at a lower energy than the peak intensity of a thermonuclear spectrum. A number of other differences in the shapes of beam-target and thermonuclear spectra are also observed for each device. Finally, significant differences between the shapes of beam-target DD and DT neutron spectra, due to differences in the reaction cross-sections, are illustrated.

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The effect of turbulent kinetic energy on inferred ion temperature from neutron spectra

Cited by 112 publications

References 27 publications

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

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

Capsule modeling of high foot implosion experiments on the National Ignition Facility

Neutron spectra from beam-target reactions in dense Z-pinches

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