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  • 1
    Electronic Resource
    Electronic Resource
    Observation of reduced beam deflection using smoothed beams in gas-filled hohlraum symmetry experiments at Nova (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1609-1613 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Execution and modeling of drive symmetry experiments in gas-filled hohlraums have been pursued to provide both a better understanding of radiation symmetry in such hohlraums and to verify the accuracy of the design tools which are used to predict target performance for the National Ignition Facility (NIF) [J. Lindl, Phys. Plasmas 2, 3933 (1995)]. In this paper, the results of a series of drive symmetry experiments using gas-filled hohlraums at the Nova laser facility [C. Bibeau et al., Appl. Opt. 31, 5799 (1992)] at Lawrence Livermore National Laboratory are presented. A very important element of these experiments was the use of kineform phase plates (KPP) to smooth the Nova beams. The effect of smoothing the ten Nova beams with KPP phase plates is to remove most of the beam bending which had been observed previously, leaving a residual bending of only 1.5°, equivalent to a 35 μm pointing offset at the hohlraum wall. The results show that the symmetry variation with pointing of implosions in gas-filled hohlraums is consistent with time integrated modeling. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873982
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  • 2
    Electronic Resource
    Electronic Resource
    Long-implosion plasma radiation sources using "solid-fill" nozzles (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 533-541 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Solid-fill nozzles for long-implosion Z-pinch experiments to produce argon K-shell x rays (photon energy 〉3.1 keV) have been developed. With a 7 cm diam nozzle, which is appropriate for a 200 ns driver, stable implosions at 180 ns and 4 MA have produced peak argon K-shell yields exceeding 15 kJ. As previously seen with short (∼100 ns) implosion times, the K-shell yield scales as the fourth power of peak current, I4. Limited testing with a 10 cm nozzle, which is appropriate for a 〉250 ns driver, has also achieved a stable implosion. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1333696
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  • 3
    Electronic Resource
    Electronic Resource
    Efficient argon K-shell radiation from a Z pinch at currents 〉15 MA (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 3135-3138 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The first observations of gaseous load implosions with over 15 MA in 〉110 ns on the Z generator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] are reported. Starting from a diameter of over 8 cm, an argon double-shell Z pinch imploded to under 0.5 cm K-shell emission diameter. With a load mass of 0.8 mg/cm, K-shell x-ray output reached 274±24 kJ in a 15 TW peak power, 12 ns pulse. This record-high yield is consistent with the current-squared scaling predicted for the "efficient" emission regime. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1373418
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  • 4
    Electronic Resource
    Electronic Resource
    Radius and current scaling of argon K-shell radiation (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 75 (1994), S. 2781-2788 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Argon gas puff experiments on a 3–4 MA generator show that the K-shell x-ray yield is optimized for a 2.5 cm initial diameter and 110 ns implosion time. By varying the driver current, the K-shell yield varies from 13 to 18 kJ following a current to the fourth power scaling. Comparisons of these experimental data to one-dimensional radiation-hydrodynamic simulations with and without enhanced transport coefficients show that the measured K-shell yields exceed those predicted based solely on kinetic energy input.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.356217
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  • 5
    Electronic Resource
    Electronic Resource
    Gas-filled targets for large scale-length plasma interaction experiments on Nova (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2473-2479 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stimulated Brillouin backscatter from large scale-length gas-filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te(approximately-greater-than)3 keV), and gradient scale lengths (Ln∼2 mm, Lv(approximately-greater-than)6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas-filled balloons with polyimide walls (gasbags) and gas-filled hohlraums. Detailed characterization using x-ray imaging and x-ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time-resolved SBS backscatter from these targets is 〈3% for conditions similar to ignition target designs. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871208
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  • 6
    Electronic Resource
    Electronic Resource
    Optimization of x-ray emission in Z-pinch implosions: Theory versus experiment (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2590-2596 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Three sets of Z-pinch experiments were performed in recent years [Phys. Rev. E 50, 2166 (1994)] that confirmed the existence of a theoretically predicted region in (load mass)-(implosion velocity) space where efficient conversion of implosion energy to kilovolt x rays was possible [J. Appl. Phys. 67, 1725 (1990)]; but they also raised questions about the validity of the theoretical models [Phys. Plasmas 1, 321 (1994)] and about the influence of the pulse-power generator and load design on the detailed behavior of the measured x-ray yields. Newly completed experiments suggest that some of these influences can be overcome and that K-shell yields can be increased in greater accord with theoretical expectations. In this paper, a brief description of these experiments is given, and some comparisons with recent theoretical findings are made along with comparisons with the above-mentioned earlier work. These new comparisons demonstrate the need for further improvements in the theoretical models and in the way experiments are designed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871221
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  • 7
    Electronic Resource
    Electronic Resource
    Production and characterization of large plasmas from gas bag targets on Nova (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3161-3168 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Large plasmas are created by illuminating gas-filled thin-walled balloon-like targets using the Nova laser [E. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. The targets consist of a 5000–6000 A(ring) skin surrounding 1 atm of neopentane, which, when ionized, becomes a plasma with an electron density of 1021 electrons/cm3. X-ray images of the gas bag target are used to evaluate the size and uniformity of the plasma by comparison with LASNEX [R. M. More, J. Quant. Spectrosc. Radiat. Transfer 27, 345 (1982)] simulations. The gas bags are heated with converging and diverging beam spots. The most uniform plasmas are created by illuminating the target with large converging beam spots that overlap to cover most of the surface of the gas bag. The gas bag plasma is heated to a peak temperature of approximately 3.5 keV, with 25 kJ of 3ω laser light in a 1 ns square pulse. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871148
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  • 8
    Electronic Resource
    Electronic Resource
    Evidence for precursor plasma formation resulting from heterogeneous current channels in wire array loads (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1745-1753 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The initial current flow in single and double wire loads exploded by current generators with dI/dt in the range of 1010–1012 A/s has been investigated using laser absorption and interferometric techniques. The results clearly show heterogeneous current channel formation with a plasma corona surrounding a predominantly neutral core. In the two wire load case, the corona is observed to separate from the core and prematurely implode on axis. Parasitic current channels were observed when material, outgassed by a low level prepulse current, is ionized to form a current carrying plasma channel that temporarily shields the wire from the main current pulse. In a complementary study on a terawatt generator, the radiation spectra from Mg coated Al wire array loads contained Mg K-shell lines 10 ns before similar Al lines were observed, confirming the load straggling effects implied by the laser imaging measurements. The results are consistent with theoretical predictions of heterogeneous current channels and precursor plasma formation that lead to a softer implosion, reduced ion density, and reduced x-ray yield. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871693
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  • 9
    Electronic Resource
    Electronic Resource
    Initial results for an argon Z pinch using a double-shell gas puff (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 4223-4226 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent observations are given for an argon double-shell gas puff imploded with up to 4 MA in 200 ns on the Double Eagle generator [G. B. Frazier et al., Digest of Technical Papers, Fourth IEEE Pulsed Power Conference (IEEE, Piscataway, NJ, 1983), p. 583]. Good K-shell x-ray output with good pinch quality was observed. A novel experimental procedure was used to selectively seed the inner or outer gas plenums with a chlorine tracer. The tracer data provide the first direct experimental evidence that the mass initially closest to the axis is the dominant contributor to the hot core of the radiating pinch. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288491
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  • 10
    Electronic Resource
    Electronic Resource
    Plasma evolution from laser-driven gold disks. II. Computational design and analysis (1990)
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2448-2455 
    Details
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The lasnex computer code [Comments Plasma Phys. Controlled Fusion 2, 51 (1975)] was used in the design and analysis of an experimental study of laser-driven plasma blowoff from gold disks. In the study, several temporal profiles of 0.53 mm laser illumination were used, including square pulses, picket pulse trains, and pulses with graduated leading edges. Preliminary modeling suggested diagnostic techniques [time- and space-resolved imaging of M-band x-ray emission and time- and space-averaged measurements of high-energy (3.5–20 keV) x-ray spectra] that complemented diagnostics already used in such experiments (four-frame holographic imaging to determine the electron-density profile in the underdense corona plasma). In this article, the lasnex results are analyzed and are compared with the measured plasma electron-density profiles and with time- and space-averaged measurements of the corona temperature. The simulation tracks the observed electron-density profiles fairly well during the early portions of the laser drive, during which the spatial profiles are approximately self-similar, but overestimates the electron density in the later, steady-state segment of the profile. For the corona electron temperature, simulation and experiment agree to within the experimental accuracy of ±20%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.859510
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