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  • American Institute of Physics (AIP)  (56)
  • American Geophysical Union (AGU)
  • Nature Publishing Group
  • 1995-1999  (61)
  • 1980-1984
  • 1997  (61)
  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of a high-power aluminum-wire array Z-pinch implosion (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2188-2203 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Annular Al-wire Z-pinch implosions on the Saturn accelerator [D. D. Bloomquist et al., Proceedings, 6th Pulsed Power Conference (Institute of Electrical and Electronics Engineers, New York, 1987), p. 310] that have high azimuthal symmetry exhibit both a strong first and weaker second x-ray burst that correlate with strong and weaker radial compressions, respectively. Measurements suggest that the observed magnetic Rayleigh–Taylor (RT) instability prior to the first compression seeds an m=0 instability observed later. Analyses of axially averaged spectral data imply that, during the first compression, the plasma is composed of a hot core surrounded by a cooler plasma halo. Two-dimensional (2-D) radiation magnetohydrodynamic computer simulations show that a RT instability grows to the classic bubble and spike structure during the course of the implosion. The main radiation pulse begins when the bubble reaches the axis and ends when the spike finishes stagnating on axis and the first compression ends. These simulations agree qualitatively with the measured characteristics of the first x-ray pulse and the overall energetics, and they provide a 2-D view into the plasma hydrodynamics of the implosion.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872382
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  • 2
    Electronic Resource
    Electronic Resource
    New methods for diagnosing and controlling hohlraum drive asymmetry on Nova : The 38th annual meeting of the Division of Plasma Physics (DPP) of the American Physical Society (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 1862-1871 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A novel method to control lowest-order (P2) flux asymmetry in Nova cylindrical hohlraums [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] with fixed laser beams is to use a pair of axial gold disks of varying radii to partially block the capsule view of the laser-entrance holes. Some advantages in using axial disks include the prospect for added drive on target, the potential for P4 control when used in tandem with laser pointing, and possibly reduced time-dependent P2(t) flux asymmetry swings at early time. Neutron-based diagnostics have provided some suggestion of increased drive, but a more direct measure of drive enhancement is with the use of backlit, low-density (0.3 g/cc) foam surrogate targets. In this scheme, an ablatively driven, inwardly propagating shock is imaged in time using backlighting from an irradiated Ti disk placed outside of the hohlraum. The benefit in using low-density surrogate targets is an amplified shock speed that enables easier detection of both average shock motion (drive) and distortion (flux asymmetry). Experiments and calculations are in excellent agreement over a nearly 10% enhancement in peak drive temperature in the presence of axial gold disks. Measurements of lower-order distortion, P2(t) and P4(t), versus time for several laser pointings (without axial disks) using this technique have also been carried out and show good agreement between experiment and simulations. Efforts to further control time-dependent flux asymmetry using multiple ring, beam-phasing techniques on Nova, as will be required for the National Ignition Facility [J. Lindl, Phys. Plasmas 2, 3933 (1995)], are under development. Current designs indicate an appreciable reduction in P2(t) is possible. Significant control of time-integrated P4 flux asymmetry with appreciable inner and outer ring separation also appears possible. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872329
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  • 3
    Electronic Resource
    Electronic Resource
    Geometrical approach to fluid models (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 537-550 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Differential geometry based upon the Cartan calculus of differential forms is applied to investigate invariant properties of equations that describe the motion of continuous media. The main feature of this approach is that physical quantities are treated as geometrical objects. The geometrical notion of invariance is introduced in terms of Lie derivatives and a general procedure for the construction of local and integral fluid invariants is presented. The solutions of the equations for invariant fields can be written in terms of Lagrange variables. A generalization of the Hamiltonian formalism for finite-dimensional systems to continuous media is proposed. Analogously to finite-dimensional systems, Hamiltonian fluids are introduced as systems that annihilate an exact two-form. It is shown that Euler and ideal, charged fluids satisfy this local definition of a Hamiltonian structure. A new class of scalar invariants of Hamiltonian fluids is constructed that generalizes the invariants that are related with gauge transformations and with symmetries (Noether). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872576
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  • 4
    Electronic Resource
    Electronic Resource
    Beam dynamics studies with the heavy-ion linear induction accelerator MBE-4 (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 880-894 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Current amplification of heavy-ion beams is an integral feature of the induction linac approach to heavy-ion fusion (HIF). In this paper we report on amplification experiments conducted on a single beam of the Multiple Beam Experiment (MBE-4), a heavy-ion (Cs+) induction linac. Earlier MBE-4 experiments [H. Meuth et al., Nucl. Instrum. Methods Phys. Res. A 278, 153 (1989)] had demonstrated up-to-9× current amplification but had been accompanied by an up-to-2× increase of normalized transverse emittance. Experiments to pinpoint the causes of this emittance growth indicated various factors were responsible, including focusing aberrations and mismatch difficulties between the injector diode and the accelerator transport lattice, a localized quadrupole misalignment problem, and the interaction of transversely large beams with the nonlinear elements of the focusing lattice. Following ameliorative measures, new current amplification experiments, both with and without acceleration, showed that current amplifications of up to 3× and line charge density increases of up to (approximate)2× could be achieved without increasing the beam's normalized transverse emittance. Finally, both the transverse beam dynamics, and beam current and energy measurements were accurately modeled by numerical particle-in-cell simulations and longitudinal dynamics codes, respectively. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872147
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  • 5
    Electronic Resource
    Electronic Resource
    Spectroscopic diagnosis of foam z-pinch plasmas on SATURN : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1083-1086 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Solid and annular silicon aerogel and agar foams were imploded on the SATURN accelerator to study plasma initiation, acceleration, and stagnation. SATURN delivers 7 MA with a 50 ns rise time to these foam loads. We fielded several spectroscopic diagnostics to measure plasma parameters throughout the z-pinch discharge. A spatially resolved single frame time-gated extreme ultraviolet spectrometer measured the extent of plasma ablation off the surface of the foam. A time integrated crystal spectrometer showed that characteristic K shell radiation of silicon in the aerogel and of sulfur and sodium impurities in the agar were attenuated when the foam loads were coated with a conductive layer of gold. A time-resolved pinhole camera showed that in general the quality of the pinch implosions was poor but improved with increasing efforts to improve current continuity such as prepulse and conductive coatings. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147740
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  • 6
    Electronic Resource
    Electronic Resource
    Current initiation in low-density foam z-pinch plasmas : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 848-851 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Low density agar and aerogel foams were tested as z-pinch loads on the Saturn accelerator to study current flow initiation. In these first experiments, we studied the initial plasma conditions by measuring the visible emission at early times with a framing camera and a streaked one-dimensional imaging system. Later, near the stagnation when the plasma is hotter, x-ray imaging and spectral diagnostics were used to characterize the plasma. Filamentation and arcing at the current contacts were observed. Bright implosion features were also observed. Increasing the early time conductivity, by coating the target with a high-z layer and by providing a low-current prepulse, is the most important factor in obtaining good coupling to the machine. None of the pinches were uniform along the z axis. The prime causes of these problems are believed to be the electrode contacts and the current return configuration. Solutions are discussed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147706
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  • 7
    Electronic Resource
    Electronic Resource
    Interpretation of neutron time-of-flight signals from current-mode detectors : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 610-613 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Neutron time-of-flight (nToF) signals from current-mode neutron detectors are often used to determine burn-averaged ion temperature from ICF targets because the spread of the neutron energy distribution is a function of the temperature of the reacting ions. The measured signal will, however, be a convolution of the actual neutron signal, the detector response, and the response of the recording system. In addition, scattered neutrons will arrive at the detector later than unscattered neutrons, further broadening the signal. The ion temperature derived from nToF data depends strongly on the functions used to fit the data or the methods utilized to unfold the neutron energy spectrum. A functional form based on known and measured properties of the detectors is used to fit the integral of the time-of-flight signal, allowing ion temperature derivations from targets with lower neutron yield than previously possible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147763
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  • 8
    Electronic Resource
    Electronic Resource
    Measurements of neutron spectra from Nova targets : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 607-609 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have measured neutron spectra from targets on the Nova laser. The three basic classes of targets are (1) directly driven capsules with ablative or exploding pushers, (2) indirectly driven imploding capsules with various hohlraum drives, capsule thicknesses, capsule fill pressures, and hohlraum gas fills, and (3) gas-filled hohlraums containing deuterated gas mixtures. In many capsule implosions, the neutron spectra are significantly broader than predictions of fusion-reaction-weighted ion temperatures from simulations. For implosions of deuterated capsules filled with H2, the observed spectra are consistent with the predicted combination of thermal and motional broadening. For normal capsules filled with D2, we conjecture that implosion asymmetries may induce fluid motion within the compressed fuel which contributes to the measured spectral width, but not the fusion reactivity, and which is not included in the simulations. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147665
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  • 9
    Electronic Resource
    Electronic Resource
    Neutron time-of-flight signals from expanding or contracting spherical sources : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 614-617 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The width of the energy distribution of fusion-produced neutrons is often used as an indication of the temperature of the reacting ions. The Doppler broadening of the neutron energy is due to the center-of-mass velocity of reacting ion pairs and is characterized by the ion temperature for a Maxwellian distribution of ions with zero collective velocity. If there is bulk fluid motion or turbulence characterized by a velocity on the order of the ion thermal speed, a significant additional broadening may be introduced. Suggestions of this phenomenon have been observed for two classes of laser targets. The first is a "gas bag" target, in which a deuterated hydrocarbon gas is contained in a thin spherical membrane and illuminated uniformly. The second target is an ICF capsule with a deuterated plastic inner layer. In both cases, measured neutron energy distributions were wider than expected from theoretical ion temperatures and if interpreted as indicative of the ion temperature, are inconsistent with the neutron yields observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147666
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  • 10
    Electronic Resource
    Electronic Resource
    Time-dependent electron temperature diagnostics for high-power, aluminum z-pinch plasmas : Proceedings of the eleventh topical conference on high temperature plasma diagnostics (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 852-857 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Time-resolved x-ray pinhole photographs and time-integrated radially resolved x-ray crystal-spectrometer measurements of azimuthally symmetric aluminum-wire implosions suggest that the densest phase of the pinch is composed of a hot plasma core surrounded by a cooler plasma halo. The slope of the free-bound x-ray continuum, provides a time-resolved, model-independent diagnostic of the core electron temperature. A simultaneous measurement of the time-resolved K-shell line spectra provides the electron temperature of the spatially averaged plasma. Together, the two diagnostics support a one-dimensional radiation–hydrodynamic model prediction of a plasma whose thermalization on axis produces steep radial gradients in temperature, from temperatures in excess of 1 kV in the core to below 1 kV in the surrounding plasma halo. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147707
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