Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session CI2: Z Pinches |
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Chair: Radu Pasura, University of Nevada, Reno Room: Plaza E |
Monday, November 11, 2013 2:00PM - 2:30PM |
CI2.00001: Design of MagLIF experiments using the Z facility Invited Speaker: Adam Sefkow The MagLIF (\textit{Mag}netized $L$iner $I$nertial $F$usion) concept has been presented as a path toward obtaining substantial fusion yields using the Z facility [S.A. Slutz,\textit{ et. al.}, Phys. Plasmas 17, 056303 (2010)], and related experiments have begun in earnest at Sandia National Laboratories. We present fully integrated numerical magnetohydrodynamic simulations of the MagLIF concept, which include laser preheating of the fuel, the presence of electrodes, and end loss effects. These simulations have been used to design neutron-producing integrated MagLIF experiments on the Z facility for the capabilities that presently exist, namely, D$_{\mathrm{2}}$ fuel, peak currents of I$_{\mathrm{max}}=$15-18 MA, pre-seeded axial magnetic fields of B$_{\mathrm{z0}}=$7-10 T, and laser preheat energies of E$_{\mathrm{laser}}=$2-3 kJ delivered in 2 ns. The first fully integrated experiments, based on these simulations, are planned to occur in 2013. Neutron yields in excess of 10$^{\mathrm{11}}$ are predicted with the available laser preheat energy and accelerator drive energy. In several years, we plan to upgrade the laser to increase E$_{\mathrm{laser}}$ by several more kJ, provide B$_{\mathrm{z0}}$ up to 30 T, deliver I$_{\mathrm{max}}=$22 MA or more to the load, and develop the capability to use DT fuel. [Preview Abstract] |
Monday, November 11, 2013 2:30PM - 3:00PM |
CI2.00002: Observations of altered instability structure for imploding z-pinch liners that are premagnetized with a uniform axial field Invited Speaker: Thomas Awe Magnetically driven implosions provide an energy-rich platform for inertial confinement fusion. The magnetized liner inertial fusion concept (MagLIF, Slutz \textit{et al. }Phys. Plasmas, 17, 056303 (2010)) uses a pulsed-power-driven metallic liner to compress and inertially confine preheated and premagnetized fusion fuel. The fuel is premagnetized with a uniform axial seed field $B_{z,0}$ of 10 to 30 T, which is then compressed by the liner to nearly 1000~T. In the fuel, the ultra-high field reduces thermal conduction and enhances alpha-particle heating. Preheating the fuel to 100-300 eV eases requirements on liner-convergence; nonetheless, convergence ratios at stagnation of 20 or more may be necessary. The ability to maintain liner stability and uniformity through stagnation may ultimately determine the success of the MagLIF concept. The integrity of magnetically imploded liners is compromised both by electrode instabilities and by the magneto-Rayleigh Taylor (MRT) instability. Electrode instabilities form local perturbations that can mix liner material into the fuel prior to bulk compression. Recent experiments on the Z facility have shown that this instability is mitigated when the liner's ends implode onto a nylon ``cushion,'' which impedes local perturbation growth. Other recent experiments have, for the first time, studied the implosion dynamics of \textit{premagnetized} ($B_{z,0}$ \textgreater\ 0) MagLIF-type liners. When seeded with a 7 or 10 T axial field, these liners developed 3D-helix-like surface instabilities; such instabilities starkly contrast with the azimuthally-correlated MRT instabilities that have been consistently observed in many earlier \textit{unmagnetized} ($B_{z,0} =$ 0~T) experiments. Quite unexpectedly, the helical structure persisted throughout the implosion, even though the azimuthal drive field greatly exceeded the expected axial field at the liner surface for all but the earliest stages of the experiment. Thus far, no self-consistent model has reproduced this fundamentally 3D experimental result. [Preview Abstract] |
Monday, November 11, 2013 3:00PM - 3:30PM |
CI2.00003: Contrasting physics in sources of 1-20keV emission on the Z facility Invited Speaker: David Ampleford Imploding wire arrays on the 20 MA Z generator have recently provided some of the brightest laboratory sources of multi-keV photons, including $\sim$ 400kJ of Al K-shell radiation ($h\nu\sim 1-2keV$), 80kJ of Stainless Steel K-shell ($h\nu \sim 5-9$keV) and a few kJ of Kr and Mo emission ($h\nu \sim 13keV$ and $\sim 17keV$, respectively). The x-ray line emission in these sources originates from highly ionized charge states that are produced by thermalization of the high kinetic energies imparted to the ions by the jxB force. Spectroscopy demonstrates that pinch pressures can approach $\sim$ 40 Mbar. Here we discuss how the physics of these x-ray sources fall into three categories. Al wire arrays produce a column of plasma with densities up to $\sim 3.10^{21}$ions/cm$^3$. In this regime opacity limits the radiation from increasing linearly with the emitter density. Significant structure from instabilities can reduce the density and increasing the surface area, therefore increasing the total emission. The opacity of the column can be experimentally assessed using a Mg dopant. In contrast, Stainless Steel wire arrays operate in the traditional regime where implosion velocity is critical and, while opacity is present, it has less impact on the pinch emissivity. We have recently developed a technique for determining the implosion velocity based on the radiation pulse shape, demonstrating direct correlation between implosion velocity (up to $130$cm/$\mu$s), electron temperature in the stagnated pinch (up to 5keV) and the emissivity of K-shell photons (up to 80kJ). At higher photon energies, the velocities required for traditional thermal K-shell emission become prohibitive. Instead, recent experiments aim to optimize the production of hot electrons; these hot electrons cause inner-shell ionization leading to the production of non-thermal K-alpha emission. We contrast experimental data indicative of these different effects and discuss how they affect the radiative output of pinch plasmas, and how this insight can be used to better optimize these radiating pinches. [Preview Abstract] |
Monday, November 11, 2013 3:30PM - 4:00PM |
CI2.00004: Effective vs Thermal Ion Temperatures in the Weizmann Ne Z-Pinch: Modeling and Stagnation Physics Invited Speaker: John Giuliani Effective ion temperatures (Ti,eff), based on the widths of emission lines in Z-pinches, have been reported for over a decade to exceed the electron temperature by more than an order of magnitude. This is observed in mid-size current generators (3.5 MA) as well as on high current (\textgreater 15 MA) ones. Proposed explanations include turbulence, ion viscous heating, or 3D effects. Recent experiments with a Ne gas puff on a low current (0.5 MA) generator at the Weizmann Institute of Science also display this effect, but also provide extensive time and space resolved measurements of the plasma during stagnation [1]. The radiation-MHD code MACH2-TCRE at NRL has been used to model this Ne pinch in R-Z cylindrical geometry with a moving grid, and a non-LTE ionization kinetics coupled to a 3D radiation transport. The computed implosion dynamics depends on the initial density profile and shows flaring as seen in visible imaging. The computed electron temperature agrees with the data as does the peak K-shell power, but the pulse width is less. The calculated electron density varies strongly during stagnation, especially in the early phases, but is within the observed range during the radiation pulse. Ti,eff is computed analogously to the experimental technique: the simulation is post-processed for the emission profiles of the satellite lines, including the Doppler shifts due to the velocity structure in the K-shell emitting region. The resultant Ti,eff for the 2D model are significantly larger than the ion thermal temperatures early in the K-shell pulse, in agreement with the data. This implies that the broad line widths reflect strong radially velocity gradients near the axis. The underlying stagnation physics of thermalization and equilibration, and its relation to the detailed data, are examined for this pinch.\\[4pt] [1] E. Kroupp, et al., PRL, 98, 115001, 2007; PRL, 107, 105001 (2011). [Preview Abstract] |
Monday, November 11, 2013 4:00PM - 4:30PM |
CI2.00005: The Z Astrophysical Plasma Properties Collaboration Invited Speaker: Gregory Rochau The Z Facility at Sandia National Laboratories provides near-thermal, MJ-class x-ray sources that emit at powers up to 0.3 PW. This capability enables precise benchmark experiments of fundamental material properties in radiation heated matter at conditions previously unattainable in the laboratory. Experiments on Z can produce uniform, long-lived, and large plasmas at conditions that span volumes up to 100 cm$^3$, temperatures from 1-200 eV, and electron densities from 1E16-23 cm$^{-3}$. These unique characteristics and the ability to radiatively heat multiple experiments in a single shot have led to a new effort called the Z Astrophysical Plasma Properties (ZAPP) collaboration. This collaboration includes four national laboratories (SNL, LANL, LLNL, and CEA) and three universities (UT-Austin, UN-Reno, and Ohio State) and has been enabled by recent support from the NNSA in fundamental High Energy Density science. The focus of the ZAPP collaboration is to reproduce the radiation and material characteristics of astrophysical plasmas as closely as possible in the laboratory and use detailed spectral measurements to strengthen models for atoms in plasmas. Specific issues under investigation include the LTE opacity of iron at stellar-interior conditions, H-Balmer line shapes in white dwarf photospheres, photoionization around active galactic nuclei, and the efficiency of resonant Auger destruction in black-hole accretion disks. Each of these issues can be simultaneously studied with high precision by acquiring up to 59 individual spectra on a single Z shot. We present the challenges, opportunities, and initial results from ZAPP experiments on Z. [Preview Abstract] |
Monday, November 11, 2013 4:30PM - 5:00PM |
CI2.00006: Experimental Validation of Modeled Fe Opacities at Conditions Approaching the Base of the Solar Convection Zone Invited Speaker: Taisuke Nagayama Knowledge of the Sun is a foundation for other stars. However, after the solar abundance revision in 2005, standard solar models disagree with helioseismic measurements particularly at the solar convection zone base (CZB, $\mathrm{r\sim0.7\times R_{Sun}}$) [Basu, $\textit{et al.}$, Physics Reports $\textbf{457}$, 217 (2008)]. One possible explanation is an underestimate in the Fe opacity at the CZB [Bailey, $\textit{et al.}$, Phys. Plasmas $\textbf{16}$, 058101 (2009)]. Modeled opacities are important physics inputs for plasma simulations (e.g. standard solar models). However, modeled opacities are not experimentally validated at high temperatures because of three challenging criteria required for reliable opacity measurements: 1) smooth and strong backlighter, 2) plasma condition uniformity, and 3) simultaneous measurements of plasma condition and transmission. Fe opacity experiments are performed at the Sandia National Laboratories (SNL) Z-machine aiming at conditions close to those at the CZB (i.e. $\mathrm{T_{e}}$ =190 eV, $\mathrm{n_{e}=1\times10^{23}cm^{-3}}$). To verify the quality of the experiments, it is critical to investigate how well the three requirements are satisfied. The smooth and strong backlighter is provided by the SNL Z-pinch dynamic hohlraum. Fe plasma condition is measured by mixing Mg into the Fe sample and employing Mg K-shell line transmission spectroscopy. Also, an experiment is designed and performed to measure the level of non-uniformity in the Fe plasma by mixing Al and Mg dopants on the opposite side of the Fe sample and analyzing their spectra. We will present quantitative results on these investigations as well as the comparison of the measured opacity to modeled opacities. [Preview Abstract] |
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