Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session KI02: HED/ICF: Plasma DiagnosticsInvited Session Live
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Chair: Carolyn Kuranz, University of Michigan Room: Ballroom C |
Tuesday, November 9, 2021 3:00PM - 3:30PM |
KI02.00001: Ultrafast time-resolved imaging of void collapse in ICF ablator materials Invited Speaker: Silvia Pandolfi Decades of research have been devoted to the realization of inertial confinement fusion (ICF) [1]. However, hydrodynamic instabilities seeded by mesoscale imperfections, such as micro-voids, remain a persistent problem for achieving the conditions required for fusion ignition [2]. It is thus crucial to understand the process by which voids seed instabilities in ICF ablator materials under extreme conditions, and to visualize the evolution of these hydrodynamic instabilities. Here, we demonstrate a novel experimental platform for ultra-fast imaging of materials under dynamic compression developed at the Matter in Extreme Conditions (MEC) instrument at the LCLS x-ray free electron laser (XFEL). |
Tuesday, November 9, 2021 3:30PM - 4:00PM |
KI02.00002: Measuring Characteristic Differences between High- and Low-Performing Discharges on the MegaJoule Neutron Imaging Radiography (MJOLNIR) DPF Invited Speaker: Andrea E Schmidt A dense plasma focus (DPF) is a relatively compact coaxial plasma gun which completes its discharge as a Z-pinch. These devices are designed to operate at a variety of scales to produce short (<100 ns) pulses of ions, X-rays, and/or neutrons. LLNL recently constructed and commissioned a new device, the MJOLNIR (MegaJOuLe Neutron Imaging Radiography) DPF, which is designed for radiography and high-yield operations. MJOLNIR is one of the first DPF devices to be designed along with numerical simulations to optimize ion acceleration and target formation. In particular, particle-in-cell (PIC) simulations of discharges with the Chicago code have provided significant insight into the various physical factors that influence neutron yield. Operations in the original pulsed power configuration rated at 1 MJ of stored energy achieved neutron yields of up to 3.8E11 neutrons/pulse at 2.5 MA peak current. The pulsed power system has now been upgraded to a rating of 2 MJ and commissioning is underway at the higher stored energy. MJOLNIR is equipped with a wide range of diagnostics, including nuclear activation detectors, neutron time-of-flight detectors, a fast-framing camera, optical light gates, and a time-gated neutron and x-ray imager. Optical light gates and the framing camera have enabled run-down and run-in velocity measurements of the plasma sheath. We find run-down speeds steadily increase during the process of electrode clean-up. We present trends in these velocities, as well as an assessment of sheath symmetry and asymmetry, for high- and low-performing discharges. Current traces show evidence that is consistent with low-performance shots being plagued by early-in-time current re-strikes. We describe key insights from modeling that have influenced electrode design and enabled full voltage operations. Comparisons between modeling predictions and measurements are presented. Prepared by LLNL under Contract DE-AC52-07NA27344. |
Tuesday, November 9, 2021 4:00PM - 4:30PM |
KI02.00003: Thermal decoupling of deuterons and tritons during the shock-convergence phase in Inertial Confinement Fusion implosions Invited Speaker: Neel Kabadi A series of thin glass-shell shock-driven DT gas-filled capsule implosions were conducted at the OMEGA laser facility. These experiments generate plasmas with similar collisionality to the central hot-spot plasma during the shock-convergence phase of ignition-relevant ablatively-driven Inertial Confinement Fusion (ICF) implosions. Hydrodynamic simulations with DUED [1] match the measured DTn and DDn yields as well as burn duration for implosions with a high initial gas-fill density (4 mg/cm3), but greatly over predict the yield for implosions with lower initial densities down to 0.2 mg/cm3. Kinetic simulations with the implicit Fokker-Planck (iFP) code reproduce the measured nuclear yields and burn duration at both low and high initial density. The temperatures inferred from the DTn and DDn spectra are most consistent with a two-ion-temperature plasma, where the initial temperature ratio Tt/Td is 1.5. This is the first experimental confirmation of the long-standing conjecture that plasma shocks couple energy proportional to the species mass in multi-ion plasmas. The temperature ratio dependence on equilibration time matches expected thermal equilibration described by hydrodynamic theory. This measured trend is reproduced in the kinetic iFP simulations and indicates that deuterons and tritons have different energy distributions during the shock-convergence phase in ignition-relevant ICF implosions. This work was supported in part by the US DOE, LLE, LLNL, and DOE NNSA Center of Excellence. |
Tuesday, November 9, 2021 4:30PM - 5:00PM |
KI02.00004: Neutron backscatter edges – a novel diagnostic for Inertial Confinement Fusion Invited Speaker: Aidan C Crilly Observations during the stagnation of ICF implosions are primarily focused towards diagnosing the hotspot, with the cold dense fuel shell properties largely unmeasured. Efficient energy conversion between the kinetic energy of the shell and the internal energy of the hotspot is a key requirement for ignition. Residual kinetic and excess internal energy in the shell are energy sinks which will prevent ignition. Direct measurement of the energy content of the shell is therefore critically important to the understanding of implosion performance. |
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