Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session NI3: Z-pinches and ICF |
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Chair: Greg Rochau, Sandia National Laboratories Room: Ballroom AC |
Wednesday, November 16, 2011 9:30AM - 10:00AM |
NI3.00001: Investigating ICF target conditions through spectroscopy Invited Speaker: The fuel in inertial confinement fusion (ICF) targets is heated and compressed to extreme conditions, reaching multi-keV temperatures and higher-than-solid densities in a neutron- producing core with strong gradients and high velocities in the surrounding plasma. Measuring these conditions is an important step in understanding, accurately simulating, and, ultimately, controlling ICF target performance, whether the target is indirectly driven by laser-heated hohlraum emission or directly driven by lasers or magnetic fields. While neutron signals provide information about the core plasma, the emission and absorption spectra of high-energy x-rays can provide detailed information about core conditions, mix, gradients, velocities, and fields. We present modeled spectroscopic signatures of these quantities, demonstrate the importance of photon energy in diagnosing high-temperature core regions, and show how traditional atomic models must be modified to accurately describe x-ray emission from plasma at extreme densities. [Preview Abstract] |
Wednesday, November 16, 2011 10:00AM - 10:30AM |
NI3.00002: Investigation of Exploding Wire Plasmas Using High Resolution Point Projection X-ray Absorption Spectroscopy Invited Speaker: We have determined the properties of plasma around and between two exploding wires using high-resolution x-ray absorption spectroscopy. Plasma densities and temperatures ranging from $10^{20}cm^{-3}$ and a few $eV$ to $10^{17}cm^{-3}$ and $30 eV$ have been measured in experiments at Cornell University with two $25 \mu m$ aluminum (Al) wires spaced $1 mm$ apart driven by $\sim100 kA$ peak current pulses with $50-100 ns$ rise time [1]. The wire plasma was backlit by the $1.4-1.6 keV$ continuum radiation produced by a Mo wire X-pinch. The spectrometer employed two spherically bent quartz crystals to record the absorption and backlighter spectra simultaneously. The transition between the dense Al wire core and the coronal plasma is seen as a transition from cold K-edge absorption to Mg-, Na- and finally Ne-like absorption at the boundary. In the plasma that accumulates between the wires, ionization states up to Be-Like Al have been seen. The spectrometer geometry and $\sim2 \mu m$ X-pinch source size provide $0.3 eV$ spectral resolution and $20\mu m$ spatial resolution[2], enabling us to see $1\rightarrow2$ satellite transitions as separate lines as well as O-, F- and N-like $1\rightarrow3$ transitions that have not been seen before. A step wedge was used to calibrate the transmission, enabling density to be measured within $50\%$ and temperature to be measured within $25\%$. A genetic algorithm was developed to fit synthetic spectra calculated using the collisional-radiative code SCRAM[3] to the experimental spectra. In order to obtain agreement it was necessary to assume 3 plasma regions with variable thicknesses, thereby allowing the inferred plasma conditions to vary along the absorption path. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the National Nuclear Security Administration under DE-AC04-94AL85000\\[4pt] [1] P. F. Knapp, Ph.D. thesis, Cornell University, 2011\\[0pt] [2] Knapp {\it et al.}, Rev. Sci. Instrum., 82, 063501 (2011)\\[0pt] [3] Hansen {\it et al.}, High Energy Density Physics, 3(1-2):109 - 114 (2007) [Preview Abstract] |
Wednesday, November 16, 2011 10:30AM - 11:00AM |
NI3.00003: Thomson scattering measurements of cylindrical wire array parameters Invited Speaker: Wire-array z-pinch implosions are characterized by a long initial period in which the dense, stationary cores of the wires steadily ablate low-density plasma radially inward, where it can accumulate in an on-axis ``precursor'' plasma even before the main implosion begins. The plasma ablation stage is important, since up to about half of the initial wire array mass can be ablated during this stage. The ablation stage ends with the rapid implosion of the remaining mass to the array axis, a process subject to instabilities that can break up the imploding plasma and cause substantial amounts of mass to trail the main implosion and arrive only later (or not at all) on the axis. An optical Thomson scattering diagnostic has been developed and used for the first time to measure critical plasma parameters in wire array z-pinches, including 1) a measurement of the flow velocity of plasma ablated from the wires (at multiple radial positions inside the array), 2) the temperature of the precursor plasma accumulating on the array axis, and 3) the parameters of plasma stranded at large radius behind the implosion (trailing mass). These results allow detailed verification of 3-D MHD simulations of wire array z-pinch implosions. The data are also extremely important for designing and interpreting HEDP experiments based on ablation plasma flows, e.g. high Mach number radiatively cooled jets relevant to astrophysics. In collaboration with: S.V. Lebedev, S. Patankar, A. Colaitis, S.N. Bland, G. Burdiak, G.N. Hall, F. Suzuki-Vidal, G. Swadling, P. deGrouchy, L. Pickworth, E. Khoory, L. Suttle, J.P. Chittenden, R.A. Smith, H. Doyle. [Preview Abstract] |
Wednesday, November 16, 2011 11:00AM - 11:30AM |
NI3.00004: Solid Liner Implosions on Z for Producing Multi-Megabar, Shockless Compressions Invited Speaker: Recent experiments with cylindrical liners [1] on the Z-machine have utilized unshaped current drives where the early time drive pressure launches a shock into the initially solid liner. We explore the use of current pulse shaping techniques, originally developed for dynamic materials experiments on the Z-machine [2], to perform controlled cylindrical liner implosions. By driving the liner with a current pulse shape that prevents shock formation we avoid shock heating and melting the liner material and the corresponding decrease in electrical conductivity. This results in an imploding liner with a significant amount of its material in the solid phase and at multi-megabar pressures. Pressures in the solid region of a shaped pulse driven beryllium liner are expected to exceed 10 Mbar and have implosion velocities greater than 50 km/s. The solid liner experiments are diagnosed with multi-frame monochromatic X-ray backlighting which is used to infer the material density and pressure. These developments have lead to a new platform on the Z-machine that can be used to perform off-Hugoniot measurements at higher pressures than are accessible through magnetically driven planar geometries. This work was performed in collaboration with R.W. Lemke, R.D. McBride, M.D. Knudson, D.H. Dolan, and J P. Davis. \\[4pt] [1] Measurements of magneto-Rayleigh--Taylor instability growth during the implosion of initially solid metal liners, D. B. Sinars et al, Phys. Plasmas 18, 056301 (2011) \\[0pt] [2] Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments, R. W. Lemke et al, J. Appl. Phys. 98, 073530 (2005) [Preview Abstract] |
Wednesday, November 16, 2011 11:30AM - 12:00PM |
NI3.00005: Anomalous DD and TT yields relative to the DT yield in inertial-confinement-fusion implosions Invited Speaker: Measurements of the D(d,p)T (DD), T(t,2n)$^{4}$He (TT) and D(t,n)$^{4}$He (DT) reactions have been conducted using deuterium-tritium gas-filled inertial confinement fusion (ICF) implosions. In these experiments, which were carried out at the OMEGA laser facility, absolute spectral measurements of the DD protons and TT neutrons were conducted and compared to neutron-time-of-flight measured DT-neutron yields. From these measurements, it is concluded that the DD yield is anomalously low and the TT yield is anomalously high relative to the DT yield, an effect that is enhanced with increasing ion temperature. These results can be explained by an enrichment of tritium in the core of an ICF implosion, which may be present in ignition experiments planned on the National Ignition Facility. In addition, the spectral measurements of the TT-neutron spectrum were conducted for the first time at reactant central-mass energies in the range of 15-30 keV. The results from these measurements indicate that the TT reaction proceeds primarily through the direct three-body reaction channel, producing a continuous TT-neutron spectrum in the range 0 -- 9.5 MeV. This work was conducted in collaboration with J. A. Frenje, M. Gatu Johnson, M. J.-E. Manuel, H. G. Rinderknecht, N. Sinenian, F. H. Seguin, C. K. Li, R. D. Petrasso, P. B. Radha, J. A. Delettrez, V. Yu Glebov, D. D. Meyerhofer, T. C. Sangster, D. P. McNabb, P. A. Amendt, R. N. Boyd, J. R. Rygg, H. W. Herrmann, Y. H. Kim, G. P. Grim and A. D. Bacher. [Preview Abstract] |
Wednesday, November 16, 2011 12:00PM - 12:30PM |
NI3.00006: Recent Measurements of DT Gamma to Neutron Branching Ratio at ICF Conditions Invited Speaker: The total T(d,g)5He/T(d,n)4He branching ratio of (4.5 +/- 0.5)E-5 has been measured on Inertial Confinement Fusion (ICF) implosions at the OMEGA laser facility. Recent measurements have shown that the DT branching ratio at ICF is 2 - 3 times less than that of previously measured at particle accelerator facilities. Measurements were done at ion temperatures of (5 +/- 2) keV, which is quite low compared to previous measurements. Implication of the recent founding is that nuclear properties such as DT branching ratio might be reconsidered at low temperature ICF and stellar conditions. In practical sense, precise measurements of the branching ratio T(d,g)5He relative to T(d,n)4He are important in order to diagnose target areal density and resultant fusion yield of cryogenically-layered implosions at the National Ignition Facility (NIF). In this work, we have used LANL's Gas Cherenkov Detector (GCD), which provides a high bandwidth, energy thresholding capability for gamma-ray detection using gamma/electron/Cherenkov conversion. High-bandwidth aids the detection of D-T fusion gamma rays before the arrival of associated 14.1 MeV neutron-induced gammas; energy thresholding gives further protection against such undesirable backgrounds. In addition, to reduce systematic uncertainty, we have applied three independent calibration methods to characterize GCD response such as (1) D-3He gamma-rays generated at Omega laser where no absolute detector calibration was required because quite similar gamma spectrum from D3He and DT, (2) mono-energetic gamma rays generated at Duke University's High Intensity Gamma-ray Source (HIgS), and (3) 14-MeV neutron-induced inelastic gamma-rays generated at OMEGA using puck materials of known areal density placed near target center. In conjunction with an independent neutron yield measurements and ACCEPT and GEANT4 simulation codes, the resultant DT branching ratio was inferred. [Preview Abstract] |
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