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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Research on the formation of a hot hydrogen working fluid, which may be used in multiple concentric solid-density liner implosions, is reported. In such implosions, an axisymmetric outer liner is driven by a multi-megamp axial discharge, and a coaxial inner liner is driven by a working fluid contained between the liners. The fluid is shocklessly compressed to high pressure as the outer liner implodes around it. In the work reported here a 10 to 100 Torr pressure, hydrogen filled coaxial gun discharge was used to inject plasma into a diagnostic chamber simulating an interliner volume. Spectroscopically determined electron densities of between 1017 and 1018 cm−3 and electron temperatures in the 0.5–2.0 eV range were obtained with a fair degree of reproducibility and symmetry. Two-dimensional, time-dependent magnetohydrodyna- mic computer simulations of the working fluid formation experiment have been performed, and the computations suggest that the present experiment achieves electron number densities and temperatures at the lower extreme of these limits, and neutral densities ∼ 0.3–1.0 ×1019 cm−3. The simulations further suggest that the upper range, and beyond, can be achieved in a more energetic version of the present experiment.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments with coaxial plasma guns at currents in excess of ten megamperes have resulted in the production of high-voltage pulses (0.5 MV) and hard x radiation (10–200 keV). The x-radiation pulse occurs substantially after the high-voltage pulse suggesting that high-energy electrons are generated by dynamic processes in a very high speed ((approximately-greater-than)106 m/s), magnetized plasma flow. Such flows, which result from acceleration of relatively low-density plasma (10−4 vs 1.0 kg/m3) by magnetic fields of 20–30 T, support high voltages by the back electromotive force-u×B during the opening switch phase of the plasma flow switch. A simple model of classical ion slowing down and subsequent heating of background electrons can explain spectral evidence of 30-keV electron temperatures in fully stripped aluminum plasma formed from plasma flows of 1–2 × 106 m/s. Similar modeling and spectral evidence indicates tungsten ion kinetic energies of 4.5 MeV and 46 keV electron temperatures of a highly stripped tungsten plasma.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: High wire number, 25-mm-diameter tungsten wire arrays have been imploded on the 8-MA Saturn generator [R. B. Spielman et al., AIP Conference Proceeding 195, 3 (American Institute of Physics, Woodbury, NY 1989)], operating in a long-pulse mode. By varying the mass of the arrays from 710 to 6140 μg/cm, implosion times of 130–250 ns have been obtained with implosion velocities of 50–25 cm/μs, respectively. These Z-pinch implosions produced plasmas with millimeter diameters that radiated 600–800 kJ of x-rays, with powers of 20–49 TW; the corresponding pulsewidths were 19–7.5 ns, with risetimes ranging from 6.5 to 4.0 ns. These powers and pulsewidths are similar to those achieved with 50-ns implosion times on Saturn. Two-dimensional, radiation-magnetohydrodynamic calculations indicate that the imploding shells in these long implosion time experiments are comparable in width to those in the short-pulse cases. This can be due to lower initial perturbations. A heuristic wire array model suggests that the reduced perturbations, in the long-pulse cases, may be due to the individual wire merger occurring well before the acceleration of the shell. The experiments and modeling suggest that 150–200 ns implosion time Z-pinches could be employed for high-power, x-ray source applications. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 873-879 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Computational simulations aimed at optimizing the high-energy, high-power, multikilovolt electromagnetic radiation emitted by a rapidly moving compact toroidal (CT) plasma which stagnates against a stationary "wall" are performed for argon, krypton, and xenon plasmas over a range of CT parameters. CT kinetic energies vary from 2–10 MJ, impact speeds vary from 50–200 cm/μs, and CT masses vary from 5–11 mg. It is found that a 2 MJ Ar CT optimally emits 1–1.5 MJ of essentially K-line radiation (〉3 keV) for impact speeds of about 60–90 cm/μs; a 10 MJ Kr CT optimally emits about 1 MJ of essentially K-line radiation (〉12.5 keV) for impact speed of about 135 cm/μs; and a 10 MJ Xe CT optimally emits about 3 MJ of essentially L-line radiation (〉5 keV), about 0.5 MJ of continuum radiation above 10 keV, and about 0.1 MJ of continuum radiation above 20 keV, all also for impact speed of about 135 cm/μs. Pulse widths vary for the above optima from 7 ns at 135 cm/μs to 30 ns at 60 cm/μs. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 3448-3468 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of two-dimensional radiation magnetohydrodynamic calculations are presented of a Z-pinch implosion using a plasma flow switch. Results from a recent experiment using the high explosive driven generator Procyon, which delivered 16.5 MA to a plasma flow switch and switched about 15 MA into a static load, are used to study the implosion of a 29 mg load foil [J. H. Goforth et al., "Review of the Procyon Explosive Pulsed Power System,'' in Ninth IEEE Pulsed Power Conference, June 1993, Albuquerque, edited by K. R. Prestwich and W. L. Baker (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1993), p. 36]. The interaction of the switch with the load plasma and the effects of background plasma on the total radiation output is examined. Models which assume ideal switching are also included. Also included are the effects of perturbations in the load plasma which may be associated with initial vaporization of the load foil. If the background plasma density in the switch region and in the load region does not affect the dynamics, the pinch is predicted to produce a total radiation output of about 4 MJ. Including perturbations of the load plasma associated with switching and assuming a background plasma density after switching in excess of 10−7 g/cm3 results in a total output from the pinch of about 0.6 MJ. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2384-2389 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The stabilizing effect of sheared axial flows on the Rayleigh–Taylor instability in Z-pinch implosions is investigated. Dispersion relations are derived that include the effects of a density discontinuity and a sheared flow which produce the Rayleigh–Taylor and Kelvin–Helmholtz instabilities. The dispersion relations are examined in two limiting cases of practical interest, a uniform plasma implosion and an annular plasma implosion. The analytical results show a region of parameter space which provides complete stability. The analytical model is verified with nonlinear calculations using experimental implosion parameters and geometries. The results indicate that mitigation of the Rayleigh–Taylor instability is possible with a sufficient sheared axial flow. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 1415-1429 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Two-dimensional radiation magnetohydrodynamic simulations are presented that demonstrate the effects of magnetically driven Rayleigh–Taylor instabilities on the soft x-ray output from Z pinches. Instability models, which reproduce the current drive wave form and match visible framing camera data for instability wavelength and amplitude for implosions on capacitively driven inductive store systems, are used to study the structure of the x-ray output and the spectrum of radiation emitted from the pinch. The results indicate that standard magnetohydrodynamics is capable of reproducing much of the observed data when two-dimensional effects associated with Rayleigh–Taylor instabilities are included. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional computational methodology has been developed that uses a phenomenological representation of initial perturbations to model the evolution of magnetically driven Rayleigh–Taylor instabilities in a hollow Z pinch. The perturbed drive current waveform and x-ray output obtained from the two-dimensional models differ qualitatively from the results of unperturbed (one-dimensional) models. Furthermore, the perturbed results reproduce the principle features measured in a series of capacitor bank-driven pulsed power experiments. In this paper we discuss the computational approach and the computational sensitivity to initial conditions (including the initial perturbations). Representative examples are also presented of instability evolution during implosions, and the results are compared with experimentally measured current waveforms and visible framing camera images of perturbed implosions. Standard magnetohydrodynamic modeling, which includes instability growth in two dimensions, is found to reproduce the features seen in experiments. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 1477-1484 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Experiments using the Saturn pulsed power generator have produced high-velocity z-pinch plasma implosions with velocities over 100 cm/μs using both annular and uniform-fill gas injection initial conditions. Both types of implosion show evidence of the hydromagnetic Rayleigh–Taylor instability with the uniform-fill plasmas producing a more spatially uniform pinch. Two-dimensional magnetohydrodynamic simulations including unsteady flow of gas from a nozzle into the diode region have been used to investigate these implosions. The instability develops from the nonuniform gas flow field that forms as the gas expands from the injection nozzle. Instability growth is limited to the narrow unstable region of the current sheath. For the annular puff the unstable region breaks through the inner edge of the annulus increasing nonlinear growth as mass ejected from the bubble regions is not replenished by accretion. This higher growth leads to bubble thinning and disruption producing greater nonuniformity at pinch for the annular puff. The uniform puff provides gas to replenish bubble mass loss until just before pinch resulting in less bubble thinning and a more uniform pinch. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 2055-2062 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A simple, heuristic model for the early nonlinear phase of the Rayleigh–Taylor instability (RTI) in thin-cylindrical-shell Z-pinch implosions has been developed. This model is based on the fact that, as the field–plasma interface is deformed, there is a component of the applied force that acts to move mass from the low mass per unit area bubble region into the higher mass per unit area spike region. The resulting reduced mass per unit area of the bubble causes it to be preferentially accelerated ahead of the spike. The pinch begins to radiate as the bubble mass first reaches the axis, and it continues to radiate while the mass that is entrained within the spikes and within unperturbed parts of the shell also arrives on axis. This model relates the time at which the bubble arrives on axis to an initial wavelength and amplitude of a single mode of the RTI. Then, by comparing this to the time at which the unperturbed mass reaches the axis, one estimates pinch thermalization time, a quantity that is determined experimentally. Experimental data, together with analytic models, have been used to choose appropriate initial wavelength and amplitude both for foils and for certain gas puff implosions. By noting that thermalization time is a weak function of these parameters, it is argued that one may use the same values for an extrapolative study of qualitatively similar implosions. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
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