Bulletin of the American Physical Society
59th Annual Meeting of the APS Division of Plasma Physics
Volume 62, Number 12
Monday–Friday, October 23–27, 2017; Milwaukee, Wisconsin
Session GI3: Hotspots, Applied Fields, and MagLIF |
Hide Abstracts |
Chair: Ryan McBride, University of Michigan Room: 103ABC |
Tuesday, October 24, 2017 9:30AM - 10:00AM |
GI3.00001: First liquid-layer implosion experiments at the NIF Invited Speaker: Alex Zylstra Replacing the standard ice layer in an ignition design with a liquid layer allows fielding the target with a higher central vapor pressure, leading to reduced implosion convergence ratio (CR). At lower CR, the implosions are expected to be more robust to instabilities and asymmetries than standard ice-layer designs, and are also unique in that the hot spot can be primarily formed from material originating in the central fuel vapor. The first liquid-layer implosions on the National Ignition Facility (NIF) have been performed by wicking the liquid fuel into a supporting foam that lines the inside surface of the capsule [T. Braun et al., ACS Appl. Mater. Interfaces 8, 2600 (2016)]. A series of shots has been conducted between CR of 12 and 20 using a HDC ablator driven by a 3-shock pulse in a near-vacuum Au hohlraum [R.E. Olson et al., Phys. Rev. Lett. 117, 245001 (2016)]. At the lowest CR the implosion performance is well predicted by 2-D radiation-hydrodynamics calculations. However, as the CR is increased the nominal simulations do not capture the experimentally observed trends. Data-based models suggest that the hot spot formation is unexpectedly suppressed at higher convergence. The data could be explained by reduced hydrodynamic coupling efficiency, or an anomalously enhanced thermal conductivity in the mixed DT/foam material. We show that the latter hypothesis can explain observed trends in several experimental metrics, including the yield, ion temperature, and burn duration. [Preview Abstract] |
Tuesday, October 24, 2017 10:00AM - 10:30AM |
GI3.00002: The high velocity, high adiabat, ``Bigfoot'' campaign and tests of indirect-drive implosion scaling Invited Speaker: Daniel Casey To achieve hotspot ignition, inertial confinement fusion (ICF) implosions must achieve high hotspot internal energy that is inertially confined by a dense shell of DT fuel. To accomplish this, implosions are designed to achieve high peak implosion velocity, good energy coupling between the hotspot and imploding shell, and high areal-density at stagnation. However, experiments have shown that achieving these simultaneously is extremely challenging, partly because of inherent tradeoffs between these three interrelated requirements. The Bigfoot approach is to intentionally trade off high convergence, and therefore areal-density, in favor of high implosion velocity and good coupling between the hotspot and shell. This is done by intentionally colliding the shocks in the DT ice layer. This results in a short laser pulse which improves hohlraum symmetry and predictability while the reduced compression improves hydrodynamic stability. The results of this campaign will be reviewed and include demonstrated low-mode symmetry control at two different hohlraum geometries (5.75 mm and 5.4 mm diameters) and at two different target scales (5.4 mm and 6.0 mm hohlraum diameters) spanning 300-430 TW in laser power and 0.8-1.7 MJ in laser energy. Results of the \textasciitilde 10{\%} scaling between these designs for the hohlraum and capsule will be presented. Hydrodynamic instability growth from engineering features like the capsule fill tube are currently thought to be a significant perturbation to the target performance and a major factor in reducing its performance compared to calculations. Evidence supporting this hypothesis as well as plans going forward will be presented. Ongoing experiments are attempting to measure the impact on target performance from increase in target scale, and the preliminary results will also be discussed. *This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, October 24, 2017 10:30AM - 11:00AM |
GI3.00003: Development of new platforms for hydrodynamic instability and asymmetry measurements in deceleration phase of indirectly-driven implosions on NIF Invited Speaker: Louisa Pickworth Hydrodynamic instabilities and asymmetries are a major obstacle in the quest to achieve ignition as they cause pre-existing capsule perturbations to grow and ultimately quench the fusion burn in experiments at the National Ignition Facility (NIF). This talk will review recent developments of the experimental platforms and techniques to measure high-mode instabilities and low-mode asymmetries in the deceleration phase of implosions. These new platforms provide a natural link between the acceleration-phase experiments and neutron performance of layered deuterium-tritium implosions. In one innovative technique, self-emission from the hot spot was enhanced with argon dopant to ``self-backlight'' the shell in-flight around peak compression. Experiments with pre-imposed 2-D perturbations measured instability growth factors [1], while experiments with 3-D, ``native-roughness'' perturbations measured shell integrity in the deceleration phase of implosions. In a complimentary technique, the inner surface of the shell, along with its low-mode asymmetries and high-mode perturbations were visualized in implosions using x-ray emission of a high-Z dopant added to the inner surface of the capsule. These new measurements were instrumental in revealing unexpected surprises and providing improved understanding of the role of instabilities and asymmetries on implosion performance. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [1] L. A. Pickworth \textit{et al.,} Phys. Rev. Lett. \textbf{117}, 035001 (2016). [Preview Abstract] |
Tuesday, October 24, 2017 11:00AM - 11:30AM |
GI3.00004: 3D Stagnation instabilities in MagLIF loads on the Z Generator. Invited Speaker: Christopher Jennings Experiments with Magnetized Liner Inertial Fusion (MagLIF) [S.A. Slutz et al. Phys. Plasmas 17, 056303 (2010)] loads have successfully demonstrated the premise of magnetized fusion [M.R. Gomez et al Phys. Rev. Lett. 113, 155003(2014)]. While these experiments are increasingly well diagnosed, many of the measurements (particularly during stagnation) are time integrated, limited in spatial resolution or require additional assumptions to interpret in the context of a structured, rapidly evolving system. As such, there is some ambiguity over what may be limiting performance. Poor laser coupling in preheating the fuel prior to implosion has been suggested as a mechanism [A.B. Sefkow et al. Phys. Plasmas 21, 072711(2014)]. Mix of high Z contaminants that cool the fuel is also a significant concern [S.B. Hansen et al. Phys. Plasmas 22,056313(2015)] In addition, time integrated crystal imaging has shown significant structure in the final fuel assembly indicating potential disruption from instabilities. Understanding the balance between these degradation mechanisms is vital to progress with MagLif. We compare several sets of experimental data with synthetically generated data from systematically varied 3D resistive-MHD simulations to gain insight into the relative contributions of different degradation mechanisms. We demonstrate how some measurements strongly indicate disruption from liner material penetrating into the fuel at stagnation, and discuss the implications this has for how MagLif targets work and scale to larger drive currents. We then explore the extent to which different combinations of instability development, current delivery, high-Z mix into the fuel and initial laser deposition can be differentiated in our existing measurements. Better determining the dominant degradation mechanisms can directly influence the direction we take to improve performance, or our confidence in scaling these targets to higher currents. [Preview Abstract] |
Tuesday, October 24, 2017 11:30AM - 12:00PM |
GI3.00005: Experiments on helical modes in magnetized thin foil-plasmas* Invited Speaker: David Yager-Elorriaga This paper gives an in-depth experimental study of helical features on magnetized, ultrathin foil-plasmas driven by the 1-MA linear transformer driver at University of Michigan. Three types of cylindrical liner loads were designed to produce: (a) pure magneto-hydrodynamic (MHD) modes (defined as being void of the acceleration-driven magneto-Rayleigh-Taylor instability, MRT) using a non-imploding geometry, (b) pure kink modes using a non-imploding, kink-seeded geometry, and (c) MRT-MHD coupled modes in an unseeded, imploding geometry. For each configuration, we applied relatively small axial magnetic fields of $B_{z} =$ 0.2-2.0 T (compared to peak azimuthal fields of 30-40 T). The resulting liner-plasmas and instabilities were imaged using 12-frame laser shadowgraphy and visible self-emission on a fast framing camera. The azimuthal mode number was carefully identified with a tracking algorithm of self-emission minima. Our experiments show that the helical structures are a manifestation of discrete eigenmodes. The pitch angle of the helix is simply $m/kR$, from implosion to explosion, where $m$, $k$, and $R$ are the azimuthal mode number, axial wavenumber, and radius of the helical instability. Thus, the pitch angle increases (decreases) during implosion (explosion) as $R$ becomes smaller (larger). We found that there are one, or at most two, discrete helical modes that arise for magnetized liners, with no apparent threshold on the applied $B_{z} $ for the appearance of helical modes; increasing the axial magnetic field from zero to 0.5 T changes the relative weight between the $m=0$ and $m=1$ modes. Further increasing the applied axial magnetic fields yield higher $m$ modes. Finally, the seeded kink instability overwhelms the intrinsic instability modes of the plasma. These results are corroborated with our analytic theory on the effects of radial acceleration on the classical sausage, kink, and higher $m$ modes. \\ \par*Work supported by US DOE award DE-SC0012328, Sandia National Laboratories, and the National Science Foundation. D.Y.E. was supported by NSF fellowship grant number DGE 1256260. The fast framing camera was supported by a DURIP, AFOSR Grant FA9550-15-1-0419. [Preview Abstract] |
Tuesday, October 24, 2017 12:00PM - 12:30PM |
GI3.00006: Direct observation of electrothermal instability structures on intensely Ohmically heated aluminum with current flowing in a surface skin layer Invited Speaker: Thomas Awe Implosions on the Z Facility assemble high-energy-density plasmas for radiation effects and ICF experiments, but achievable stagnation pressures and temperatures are degraded by the Magneto-Rayleigh-Taylor (MRT) instability. While the beryllium liners (tubes) used in Magnetized Liner Inertial Fusion (MagLIF) experiments are astonishingly smooth (10 to 50 nm RMS roughness), they also contain distributed micron-scale resistive inclusions, and large MRT amplitudes are observed. Early in the implosion, an electrothermal instability (ETI) may provide a perturbation which greatly exceeds the initial surface roughness of the liner. Resistive inhomogeneities drive nonuniform current density and Joule heating, resulting in locally higher temperature, and thus still higher resistivity. Such unstable temperature and pressure growth produce density perturbations which seed MRT. For MagLIF liners, ETI seeding of MRT has been inferred by evaluating late-time MRT, but a direct observation of ETI is not made. ETI is directly observed on the surface of 1.0-mm-diameter solid Al rods pulsed to 1 MA in 100 ns via high resolution gated optical imaging (2 ns temporal and 3 micron spatial resolution). Aluminum 6061 alloy rods, with micron-scale resistive inclusions, consistently first demonstrate overheating from distinct, 10-micron-scale, sub-eV spots, which 5-10 ns later merge into azimuthally stretched elliptical spots and discrete strata (40-100 microns wide by 10 microns tall). Axial plasma filaments form shortly thereafter. Surface plasma can be suppressed for rods coated with dielectric, enabling extended study of the evolution of stratified ETI structures, and experimental inference of ETI growth rates. This fundamentally new and highly 3-dimensional dataset informs ETI physics, including when the ETI seed of MRT may be initiated. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700