Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Plasma Physics
Volume 64, Number 11
Monday–Friday, October 21–25, 2019; Fort Lauderdale, Florida
Session DI3: Invited: ICF I |
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Chair: Russ Follett Room: Floridian Ballroom CD |
Monday, October 21, 2019 3:00PM - 3:30PM |
DI3.00001: 3D isobaric hotspot reconstruction from multiple neutron and Xray views on the NIF: quantifying radiative loss impact on DT implosion and other insights Invited Speaker: Laurent Divol Moving beyond spatially averaged measurements of temperature and density for the central reacting deuterium and tritium plasma in inertial confinement fusion experiments gives a more complete understanding of the properties of the assembled hot spot and provide insight into mechanisms that degrade fusion energy production. In this work, the multi-axis time-integrated neutron imaging of a layered implosion on the NIF, under an isobaric assumption, allows the three-dimensional reconstruction of the density and temperature profile of the central hotspot during burn. The measured neutron yield and spectral width constrain the stagnation pressure. One can then look for signatures of various degradation mechanisms, such as spatial temperature gradients or neutron spectrum deviation from pure Gaussian. Multi-axis synthetic Xray images are calculated using DCA DT emissivity and compared to measurements, giving a 3D reconstruction of the ablator mix penetrating the hotspot. We quantify the absolute radiative loss due to discrete, spatially resolved features such as the fill-tube induced jet and meteors. The method is validated with HYDRA simulations and applied to a series of recent high-performing HDC and CH implosions. Finally, alpha heating, effective rho-r and pressure are compared to the usual 0/1D isobaric conduction-limited approach. 3D values are 10-20{\%} lower on average. [Preview Abstract] |
Monday, October 21, 2019 3:30PM - 4:00PM |
DI3.00002: Modeling the 3D structure of ignition experiments at the NIF Invited Speaker: Ryan Nora Inertial confinement fusion ignition experiments at the National Ignition Facility (NIF) have shown strong self-heating and are approaching the burning plasma regime. However, detailed experimental diagnostics show a variety of limiting degradations that must be removed on the push for ignition. These limiters have both axisymmetric (2D) or fully three-dimensional (3D) characteristics as revealed by a host of spatially resolved x-ray and neutron diagnostics. To understand these limiters, researchers routinely use two complementary approaches – one using forward modeling and a limited number of high-fidelity 3D simulations, another using inverse modeling and large numbers of moderate-fidelity 2D simulations.\newline \newline We introduce here a new, 3D inverse technique that uses the full complement of spatially resolved NIF data to estimate the low mode number 3D implosion structure. Our goal is to reproduce a heavily coupled set of diagnostic measures including multiple x-ray images, neutron images, and other spectrally resolved neutron diagnostics. We begin by developing a large ensemble of 3D HYDRA simulations perturbed by long wavelength x-ray drive asymmetries as are likely produced by imbalances in the laser and target. Then, using a machine learning surrogate for our simulations and a companion optimizer, we select simulations whose diagnostic outputs are near matches for experimental observations. Our end product is both a candidate radiation drive model and an associated 3D implosion structure that replicates the experimental observations.\newline \newline We detail in our talk the results garnered from application of this technique to a high-yield NIF ignition campaign. We explore the implications of the findings, including a characterization of the perturbations, their impacts, and the potential benefits of their mitigation. We also place the results in the context of the previous approaches and carefully sketch the conditions under which our techniques are applicable. [Preview Abstract] |
Monday, October 21, 2019 4:00PM - 4:30PM |
DI3.00003: Impact of Non-Maxwellian Electron Distribution Functions on Crossed-Beam Energy Transfer Invited Speaker: David Turnbull Energy transfer between crossed laser beams is an important process in both the direct- and indirect-drive approaches to inertial confinement fusion (ICF), and unreliable predictions in numerous contexts have raised questions as to the validity of models. Typically, those models require state variable inputs (i.e., $n_e$, $T_e$, and $T_i$) that are computed in radiation-hydrodynamic simulations, which assume Maxwellian electron distribution functions (EDF). However, laser plasma heating is predicted to distort the EDF away from Maxwellian\footnote{A. B. Langdon \textit{et al.}, Phys. Rev. Lett. {\bf 44}, 575-579 (1980).}. Here, measurements of the complete Thomson scattering spectrum indicate the presence of super-Gaussian EDF's that are consistent with existing theory\footnote{J. P. Matte \textit{et al.}, Plas. Phys. & Cont. Fus. {\bf 30}, 1665 (1988).}. In such plasmas, ion acoustic wave (IAW) frequencies increase monotonically with super-Gaussian exponent\footnote{B. B. Afeyan \textit{et al.}, Phys. Rev. Lett. {\bf 80}, 2322-2325 (1998).}. To match experiments that measured power transfer between crossed laser beams mediated by IAW's, accounting for the measured non-Maxwellian EDF is required\footnote{D. Turnbull \textit{et al.}, in review (2019).}. This effect is estimated to decrease energy transfer in indirectly-driven hohlraums at the National Ignition Facility by $\approx27\%$; this will reduce (and may eliminate) the \textit{ad hoc} saturation clamp that has previously been used to match observables like shape, thereby improving the predictive capability of integrated modeling. [Preview Abstract] |
Monday, October 21, 2019 4:30PM - 5:00PM |
DI3.00004: Orbital Angular Momentum in Photon-Photon Scattering Invited Speaker: Ramy Aboushelbaya Photon-photon scattering in vacuum is one of the oldest and most intriguing predictions of quantum electrodynamics, as it would confirm what is called "vacuum polarization" and change our perception of the electromagnetic vacuum. However, experimental verification of scattering between real photons in vacuum hasn't materialized yet. This is due, in part, to the relative weakness of this interaction. Several proposals have been put forth to attempt to detect this effect, including using high-power lasers which compensate the relatively low energy of their photons with the ultra-high intensities they can achieve. With the advent of new multi-petawatt laser facilities, such as ELI and APOLLON, an experiment to detect photon-photon scattering using high-power lasers is looking increasingly feasible. However, these types of experiments still need to find a way to increase the relatively low signal-to-noise ratio caused by the large amount of background radiation coming from unwanted effects such as inverse Compton scattering. To this end, we have investigated the effect of orbital angular momentum (OAM) on elastic photon-photon scattering in vacuum for the first time. We defined exact solutions to the vacuum electromagnetic wave equation which carry OAM. Using those, the expected coupling between three initial waves has been derived in the framework of an effective field theory based on the Euler-Heisenberg Lagrangian which has shown that OAM adds a signature to the generated photons thereby greatly improving the signal-to-noise ratio. This forms the basis for a proposed high-power laser experiment utilizing quantum optics techniques to filter the generated photons based on their OAM state. This would allow the detection of these rare scattering events on the previously mentioned multi-petawatt systems thereby finally providing experimental proof for elastic photon-photon scattering in vacuum. [Preview Abstract] |
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