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 GI3: Invited: ICF II |
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Chair: David Ampleford Room: Floridian Ballroom CD |
Tuesday, October 22, 2019 9:30AM - 10:00AM |
GI3.00001: Understanding the impact of ablator micro-structure and fuel-ablator mixing on ICF implosions Invited Speaker: Christopher Weber ICF implosions on the National Ignition Facility (NIF) have used high-density carbon (HDC) ablators to achieve record fusion performance, but the final compression of the fuel is less than expected. Compression will need to be increased to improve confinement and achieve a robust fusion burn. A leading hypothesis for reduced compression is that the hotter ablator material is mixing with the cold fuel, increasing its entropy and lowering its density. Mixing at this interface may be worse than originally predicted due to HDC's micro-structure, which is composed of anisotropic nano- to micro-crystalline grains with lower density between grains. Measurements using the two-dimensional VISAR diagnostic at the Omega laser facility revealed velocity modulations on the shock wave that are larger with HDC ablators than with amorphous plastic. Simulations that directly model the HDC micro-structure can reproduce these velocity modulations and show that, when applied to an implosion, more fuel-ablator mixing is observed than previously expected. Implosion radiographs using the Crystal Backlighter Imager (CBI), which provides images of the shell near peak velocity, are better matched when using this micro-structure model to increase internal mixing. Experiments are planned to use this platform to further measure mixing and test mitigation schemes. With this increased level of mixing, the compression of DT-layered implosions is in better agreement with the simulations. [Preview Abstract] |
Tuesday, October 22, 2019 10:00AM - 10:30AM |
GI3.00002: Using X-ray Spectroscopy to Quantify Mix and Plasma Conditions in Ignition Experiments Using W-doped HDC Capsules at the NIF Invited Speaker: Edward Marley Experiments at the National Ignition Facility (NIF) seek to produce fusion burn using thermal x-rays from a gold-lined uranium hohlraum to heat and compress a capsule filled with deuterium (D) and tritium (T). The DT fuel must be shielded from the Au M-band x-rays to prevent preheat, so tungsten (W) is doped into the high-density carbon (HDC) ablator. The W-doped HDC from the ablator can mix into the DT hot spot from fill tube-induced or other instabilities. X-ray emission of W L-band transitions (9-12 keV) is observed on most ICF implosions, indicating there is mix. This mix reduces the hotspot energy and temperature through ionization energy and radiative losses. Quantifying the total energy loss is important to understand the impact on the ignition boundary and our proximity to it. The spectra contain Mg- to F-like 3d-2p W emission features from the hot spot, Ti- and V-like 3d-2p emission features from the outgoing shock, and W L-shell fluorescents (L-alpha and -beta) and 3$\rightarrow$2 absorption features from the cooler and denser surrounding fuel and remaining shell. The hot spot portion of the spectra are analyzed to infer temperature and mass distribution of the mix in the hotspot, and total radiative losses escaping the hotspot in both M-band, L-band, and continuum radiation. Results of radiative losses as a function of laser energy and W dopant-fraction in the “Big Foot” scaling series are presented. The measurements show a strong inverse correction between radiative losses from mix and neutron yield. A detailed analysis of the entire spectrum, including radiation transport and density effects, is in development. This uses the atomic kinetics/radiation transfer code CRETIN coupled to sufficiently detailed atomic physics models to reconstruct a complete and self-consistent picture of the temperature and density profiles and the absorption and emission processes. Planned improvements in data quality include high resolution and time-resolved measurements. This is a multi-year project to understand mix and its effects on hotspot physics through x-ray spectroscopy and atomic physics. [Preview Abstract] |
Tuesday, October 22, 2019 10:30AM - 11:00AM |
GI3.00003: Gamma measured ablator areal density observations, trends and time shifts on the National Ignition Facility Invited Speaker: Kevin Meaney In inertial confinement fusion, few diagnostics are able to look at the final state of the ablator in integrated, cyro-layered implosions. The Gamma Reaction History diagnostic measures fusion gammas as well as the neutron induced 4.4 MeV carbon ablator. Recently, a new analysis routine was developed to isolate this carbon gamma line from other neutron induced background, giving the areal density ($\rho $R) of the ablator as well as the time shift between the carbon gammas and the DT fusion. Now carbon $\rho $R values have been generated for a database across many National Ignition Facility (NIF) shots. The values can be compared and contrasted across the NIF campaigns. They reveal that the ablator is surprisingly not set by velocity, DT cold fuel radius or picket intensity, as one might expect. But instead is sensitive to the coast time, mass remaining and dopants. This verifies that coast time is a vital metric that continues to apply pressure onto the capsule late time, improving performance, as well as the effectiveness of dopants to reduce mix and preheat and increase compression. The data is also suggestive that fill tube jets increase the effective carbon $\rho $R but the signal is within the uncertainty level. Future improvements could use carbon $\rho $R in combination with x-ray diagnostics to constrain the mass and density of all mixed material -- cold or hot. The carbon gammas are observed to systematically arrive \textasciitilde 15ps later than the DT fusion peak, implying the ablator areal density is increasing across the burnwidth of the reaction. This shift allows estimation of the amount of kinetic energy and velocity in the ablator at bangtime and may reflect a slower final ablator speed than what is predicted by simulations, consistent with a slower implosion and observed longer burnwidth. Simulations predict that an igniting capsule has the carbon gamma signal arrive before the DT fusion peak, suggesting the carbon gamma timing could be a metric that could be used to quantify future ignition. [Preview Abstract] |
Tuesday, October 22, 2019 11:00AM - 11:30AM |
GI3.00004: Performance scaling with drive parameters in Magnetized Liner Inertial Fusion experiments Invited Speaker: Matthew Gomez The long-standing challenge of confining a fusion plasma can become easier by leveraging the benefits of both inertial and magnetic confinement schemes. A magneto-inertial fusion concept called Magnetized Liner Inertial Fusion (MagLIF) [S. A. Slutz, et al., Phys. Plasmas 17, 056303 (2010)] has recently demonstrated significant promise in experiments on the Z machine. In MagLIF, current from the Z-machine is used to implode a metal cylinder containing magnetized and preheated fusion fuel. The initial MagLIF experiments established the viability of magneto-inertial fusion by demonstrating thermonuclear neutron generation from fusion-relevant fuel temperatures and densities and the ability to trap charged fusion products in a highly-magnetized fuel column [M. R. Gomez, et al., Phys. Rev. Lett. 113, 155003 (2014), P. F. Schmit, et al., Phys. Rev. Lett. 113, 155004 (2014)]. These experiments were conducted with 10 T, approximately 0.5 kJ of preheat, and a 16-18 MA peak load current, and they generated 1-2e12 primary DD neutrons with ion temperatures between 1.8 and 2.5 keV. Recent efforts have been focused on developing a platform that allows for increased applied B-field (\textgreater 15 T), laser energy coupling (\textgreater 1 kJ), and current (\textgreater 19 MA) to be delivered to the target. These improvements increased the primary neutron yield by nearly an order of magnitude to 1e13 DD neutrons and the ion temperature to 3.1 keV. The observed increase in performance follows the predicted scaling in 2D simulations, which also indicate that further gains are possible with additional improvements to the platform. Development of a \textgreater 20 T, \textgreater 2 kJ, and \textgreater 20 MA capability is underway, and there is a path to 25-30 T, 4-6 kJ, and 21-23 MA on the Z machine, which could produce up to 100 kJ of DT-equivalent yield. *Sandia National Laboratories is a multimission laboratory managed and operated by NTESS, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. DOE's NNSA under contract DE-NA0003525. [Preview Abstract] |
Tuesday, October 22, 2019 11:30AM - 12:00PM |
GI3.00005: Neutron yield enhancement and suppression by magnetization in laser-driven cylindrical implosions Invited Speaker: Edward Hansen An externally applied, axial magnetic field can increase neutron yield in inertial confinement fusion by reducing heat losses from the compressing fuel. On the other hand, magnetic pressure is detrimental to achieving a high fuel pressure, so it must remain negligible for magnetization to be beneficial. Experiments and three-dimensional magneto-hydrodynamic simulations of cylindrical implosions on the OMEGA laser show yield enhancement of up to 60{\%} and then yield degradation as an applied axial magnetic field is increased from 0 to nearly 30 T. The results demonstrate that maximizing the benefit of magnetization in cylindrical implosions requires the fuel convergence ratio to be limited, which requires the fuel to be preheated. The results also show that it is possible to produce a plasma with an ion temperature greater than 1 keV and a density of order 1 g/cm$^{\mathrm{3}}$ with a magnetic pressure comparable to the thermal pressure, a new regime for laser-produced plasmas on OMEGA. [Preview Abstract] |
Tuesday, October 22, 2019 12:00PM - 12:30PM |
GI3.00006: Kinetic simulations of power flow in the Z accelerator Invited Speaker: Nichelle Bennett The challenge for the Terawatt-class accelerators driving $Z$-pinch experiments, such as Sandia National Laboratories' $Z$ machine, is to efficiently couple power from multiple storage banks into a single multi-mega amp (MA) transmission line. The physical processes that degrade efficiency are identified in the first-ever, multi-dimensional simulations of the $Z$ machine. Kinetic models follow the range of physics occurring during a pulse, from vacuum pulse propagation to charged-particle emission and insulated flow to electrode plasma expansion. Simulations demonstrate that current is diverted from the load through a combination of standard and anomalous transport. Standard transport occurs in the adder region where the electrode current density is a few $10^4-10^5$~A/cm$^2$ and current is diverted from the load via uninsulated charged-particle flows. In regions with $>10^6$~A/cm$^2$, electrode surface plasmas develop velocity-shear instabilities and a Hall-field-related transport which scales with electron density. These results provide the physics basis for designing future pulsed-power systems. [Preview Abstract] |
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