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  • 1
    Electronic Resource
    Electronic Resource
    X-ray backlighting of wire array Z-pinch implosions using X pinch : Papers from the thirteenth topical conference on high temperature plasma diagnostics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 72 (2001), S. 671-673 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The dynamics of wire arrays have been studied using a point-projection X-pinch x-ray backlighter installed in one of the return posts of the MAGPIE generator. Variations of diameter (15–50 μm aluminum) and number of wires (two or four) in the X-pinch enabled backlit images in the range of 140–200 ns after the current start. A temporal and spatial resolution of 〈1 ns and 5 μm is achieved. The radiographic images of aluminum wire array show that the wire cores are present at the original position until 80% of the implosion time and the size of the wire cores is 0.25 mm for aluminum and 0.1 mm for tungsten. A very fine structure of the order of 10 μm has been observed in titanium wire arrays. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1315647
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  • 2
    Electronic Resource
    Electronic Resource
    A high impedance mega-ampere generator for fiber z-pinch experiments (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 1533-1541 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: At Imperial College a mega-ampere generator for plasma implosion experiments has been designed, built, and commissioned. With a final line impedance of 1.25 Ω this terawatt class generator has been designed primarily to drive a maximum current of 1.8 MA with a rise time of 150 ns into high inductance z-pinch loads of interest to radiative collapse studies. This article describes the design and tests of the generator which has a novel configuration of lines and a new design of a magnetically insulated transmission line (MITL). In summary, the generator consists of four Marx generators each of the Hermes III type (2.4 MV, 84 kJ), each connected to 5 Ω pulse forming lines and trigatron gas switches. The power is fed into the matched 1.25 Ω vertical transfer line which feeds a diode stack and a short conical MITL in vacuum which concentrates the power into the z-pinch load. At 80% charge a current rising to 1.4 MA in 150 ns has been measured in a 15 nH inductive short. Similar results are obtained when using a plasma load. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1146884
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  • 3
    Electronic Resource
    Electronic Resource
    The effect of current prepulse on wire array Z-pinch implosions (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 375-377 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of a prepulse current on the behavior of wire array Z pinches is investigated. The experiments were performed on the MAGPIE generator (1 MA peak current at 240 ns) [I. Mitchell, J. M. Bayley, J. P. Chittenden et al., Rev. Sci. Instrum. 67, 1533 (1996)]. A linear ramp current, ∼500-ns long, was used as a prepulse. The array consisted of 32 15-μm aluminum wires, 23-mm long arranged in a 16-mm diameter circle. With a prepulse of 1 kA/wire, a low density precursor plasma column (ne∼2×1017 cm−3) is formed on the array axis before the start of the main current. Later, the soft x-ray emission shows the growth of an m=1 helical instability in the precursor plasma, which indicates the presence of a current. Without a prepulse current, the precursor plasma on axis is uniform and does not show any instability. The x-ray pulse at stagnation on axis is at least 30 times smaller with a prepulse current than without. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1417512
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  • 4
    Electronic Resource
    Electronic Resource
    One-, two-, and three-dimensional modeling of the different phases of wire array Z-pinch evolution : The 42nd annual meeting of the division of plasma physics of the American Physical Society and the 10th international congress on plasma physics (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 2305-2314 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of one-, two-, and three-dimensional (1-D, 2-D, and 3-D) resistive magnetohydrodynamic models are used to build up a composite model of the different phases of wire array Z-pinch implosions. 1-D(r) and 2-D(r,z) "cold-start" simulations of single wire experiments are used to illustrate some of the important processes in the plasma formation phase of wire arrays. Detailed comparison of the simulation results with data from single wire experiments provides an excellent method of code verification. 2-D simulations in the r–θ or x–y plane show how the combination of the core–corona structure of the wire plasmas and the magnetic field topology result in the formation of radial plasma streams and a precursor plasma on axis well before the implosion phase commences. The same 2-D(x–y) model is also used to illustrate how the implosion trajectories of nested wire arrays are controlled by the levels of momentum, energy, and magnetic flux which are transferred during their collision. Preliminary results showing the evolution of a single wire in the array in 3-D are presented. These results suggest that the dynamics and structure of imploding wire arrays at Imperial College can potentially be explained in terms of the current breaking through the wire cores rather than in terms of the Rayleigh–Taylor instability. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1343883
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  • 5
    Electronic Resource
    Electronic Resource
    Fast reconnection due to localized anomalous resistivity (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3180-3186 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A three-dimensional resistive magnetohydrodynamic code has been used to model the reconnection process at the m=1, n=1 surface, in periodic cylindrical geometry. Large current densities are expected at this reconnection layer and an enhancement of the transport properties is expected if the local drift speed exceeds a critical velocity, such as some multiple of the local sound speed. This effect is modeled in these simulations by the local enhancement of the resistivity coefficient where the criterion for micro-turbulence is satisfied. It is found that the reconnection times for this type of simulation are comparable to the reconnection times for a plasma where the resistivity is enhanced everywhere, implying that the reconnection is dominated by the local resistivity value and not its gradient. An analytic scaling law of the reconnection rate for the case when the local electron drift velocity is limited to a multiple of the sound speed is presented. This model predicts that when this multiple is (mi/me)1/2, reconnection times are close to experimental values in large tokamaks. Under these conditions, electron inertia and electron viscosity can be shown to be unimportant. The onset of micro-turbulence acts as a trigger for the reconnection process, and partial reconnection can occur if the conditions for micro-turbulence cease. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873046
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  • 6
    Electronic Resource
    Electronic Resource
    Coronal plasma behavior of the Z pinch produced from carbon and cryogenic deuterium fibers (1998)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 3366-3372 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A series of fiber pinch experiments has been carried out on the MAGPIE (mega-ampere generator for plasma implosion experiments) generator (1.8 MA, 150 ns) [Mithell et al., Rev. Sci. Instrum. 67, 1533 (1996)] to study the temporal evolution of the coronal plasma. Analysis of schlieren photographs, axial streak images and gated x-ray photographs gives the radial and axial motion of the coronal plasma. The influence of a current pulse (prepulse) of 30 kA applied 200 ns before the main discharge was also studied. Radial expansion velocities of 5.5×106 cm/s for carbon fiber shots without prepulse and 3.6×106 cm/s for carbon fibers with prepulse were measured. Axial wavelengths (λz) of dominant instabilities in the corona were between 0.05 and 0.2 cm corresponding to ka∼10–20. Comparison of the results obtained with carbon fibers with and without current prepulse and cryogenic deuterium fibers are presented. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873050
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  • 7
    Electronic Resource
    Electronic Resource
    Two-dimensional magneto-hydrodynamic modeling of carbon fiber Z-pinch experiments (1997)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 4309-4317 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional magneto-hydrodynamic simulation incorporating cold start conditions is used to explain the early phase of carbon fiber Z-pinch experiments. The rapid development of large scale, nonlinear m=0 perturbations in the plasma corona is reproduced. X-ray bright spot formation in the necks of the instability is followed by bright spot bifurcation and fast axial motion. Bright spot bifurcation is found to be due to axial components of the j×B force and occurs off-axis due to the presence of a residual core of unionized carbon. Artificial diagnostic images are generated from the simulations data to allow direct comparison with experimental x-ray imaging and laser probing diagnostics. The accurate reproduction of the experimental images provides confirmation that the experimentally observed features are a repercussion of the non-linear development of the m=0 instability in an ionizing medium. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872593
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  • 8
    Electronic Resource
    Electronic Resource
    The neoclassical theory of poloidal flow damping in a tokamak (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4513-4520 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The damping rate of the poloidal flow uθ in a tokamak is determined in the banana regime as an initial value problem. The bounce averaged drift kinetic equation is solved analytically for early times and numerically for longer time scales of the order of the ion–ion collision time τii. Initial conditions are chosen for the ion distribution function fi(t=0) describing states with similar flows uθ(t=0), but varying structures in pitch angle velocity space. At early times an analytical treatment shows that the damping characteristics of uθ(t) depend sensitively on whether or not the ions resposible for the flow are close to the trapped–passing boundary. Initial decay is shown to be of the form duθ/dt∼(νiiε/t)1/2. A numerical treatment then confirms this early time result and extends the solution to the long term asymptotic decay, which is found to be independent of the initial preparation of the system. This long term evolution is also found to tend to independence of inverse aspect ratio ε as t→0. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.872068
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  • 9
    Electronic Resource
    Electronic Resource
    The past, present, and future of Z pinches (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 1672-1680 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Z pinch is enjoying a renaissance as the world's most powerful yet efficient soft x-ray source which can energize large volume hohlraums for indirectly driven inertial confinement fusion. It has the advantages of being efficient and having high energy and power density. Its early history will be traced from the 18th century to the present day. The most notable feature of the Z pinch is its instability. The various regimes of stability analysis will be reviewed, including resistive and finite ion Larmor radius effects. Work in the last 10 years on single fibres, especially of cryogenic deuterium, gave neutrons that were of the same origin, namely, beam–plasma interactions, as reported by Kurchatov. The renaissance has come about through the implosion of arrays of fine wires. Research at Sandia National Laboratory has shown that by using more and finer wires, the x-ray radiation emitted at stagnation increased in power and decreased in pulse width. The understanding of these results has been advanced considerably by theory, simulation and smaller-scale, well diagnosed experiments showing the early uncorrelated m=0 instabilities on each wire, the inward jetting of plasma to the axis, the global Rayleigh–Taylor instability and the mitigating effect of nested arrays. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874047
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  • 10
    Electronic Resource
    Electronic Resource
    The dynamics of wire array Z-pinch implosions : The 40th annual meeting of the division of plasma physics of the american physical society (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 2016-2022 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Wire array Z-pinch dynamics are studied in experiments with 16-mm diameter arrays of between 8 and 64, 15-μm diameter aluminum wires, imploded in 200–260 ns by a 1.4-MA current pulse. Side-on laser probing shows early development of noncorrelated m=0-like instabilities with an axial wavelength ∼0.5 mm in individual wires. End-on interferometry (r-θ plane) shows azimuthal merging of the plasma with a density of 1017 cm−3 in 90–65 ns for 8–64 wires, respectively. At the same time low-density plasma reaches the array axis and forms a precursor pinch by 120–140 ns. At 0.7–0.85 of the implosion time a global m=0 instability with a wavelength of 1.7–2.3 mm was detected in soft x-ray gated images, laser probing, and optical streaks. The time when the instability reaches the observable level corresponds to the number of e-foldings for the growth of the classical Rayleigh–Taylor instability of ∫γ dt∼5.6–7. The scaling of this number with the number of wires is consistent with the instability growth from the seed level determined by the averaging of uncorrelated density perturbations in individual wires. Preliminary results from a 4×4 array permit the simultaneous observation by laser probing of the characteristic bubble and spike structure of the magneto Rayleigh–Taylor instability. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873456
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