S. C. Bott

Magnetic tower outflows from a radial wire array Z-pinch

We present the first results of high energy density laboratory astrophysics experiments which explore the evolution of collimated outflows and jets driven by a toroidal magnetic field. The experiments are scalable to astrophysical flows in that critical dimensionless numbers such as the Mach number, the plasma β and the magnetic Reynolds number are all in the astrophysically appropriate ranges. Our experiments use the MAGPIE pulsed power machine and allow us to explore the role of magnetic pressure in creating and collimating the outflow as well as showing the creation of a central jet within the broader outflow cavity. We show that currents flow along this jet and we observe its collimation to be enhanced by the additional hoop stresses associated with the generated toroidal field. Although at later times the jet column is observed to go unstable, the jet retains its collimation. We also present simulations of the magnetic jet evolution using our two-dimensional resistive magnetohydrodynamic laboratory code. We conclude with a discussion of the astrophysical relevance of the experiments and of the stability properties of the jet.

Production of radiatively cooled hypersonic plasma jets and links to astrophysical jets

We present results of high energy density laboratory experiments on the production of supersonic radiatively cooled plasma jets with dimensionless parameters (Mach number ∼30, cooling parameter ∼1 and density contrast ρj/ρa ∼ 10) similar to those in young stellar objects jets. The jets are produced using two modifications of wire array Z-pinch driven by 1 MA, 250 ns current pulse of MAGPIE facility at Imperial College, London. In the first set of experiments the produced jets are purely hydrodynamic and are used to study deflection of the jets by the plasma cross-wind, including the structure of internal oblique shocks in the jets. In the second configuration the jets are driven by the pressure of the toroidal magnetic field and this configuration is relevant to the astrophysical models of jet launching mechanisms. Modifications of the experimental configuration allowing the addition of the poloidal magnetic field and angular momentum to the jets are also discussed. We also present three-dimensional resistive magneto-hydrodynamic simulations of the experiments and discuss the scaling of the experiments to the astrophysical systems. (Some figures in this article are in colour only in the electronic version)

Structure of the dense cores and ablation plasmas in the initiation phase of tungsten wire-array Z pinches

The early stages of tungsten (W) wire-array Z-pinch implosions have been studied using two-frame point projection x-ray backlighting on the 1MA COBRA pulsed power generator [J. D. Douglass, J. B. Greenly, D. A. Hammer, and B. R. Kusse, in Proceedings of the 15th IEEE International Pulsed Power Conference, Monterey, 2005 (to be published)]. X-pinch backlighter images with subnanosecond time resolution and 4–10μm spatial resolution have been obtained of individual W exploding wires in 8-wire arrays that show evolution of wire-core and coronal plasma structures. The timing of the X-pinch x-ray bursts relative to the Z-pinch initiation time was adjusted over a 50ns time interval by varying the X-pinch mass per unit length. Wire-cores seen in two images separated in view by 120° show that the expansion is remarkably azimuthally symmetric. A strong correlation is observed between the structure on the dense exploding wire-cores and the structure of the ⩾1018∕cm3 ablation plasma being drawn from radial prominence...

Generation of episodic magnetically driven plasma jets in a radial foil Z-pinch

We present experimental results of the formation of magnetically driven plasma jets, showing for the first time a way of producing episodic jet/ouflows in the laboratory. The jets are produced using a 6.5 μm thick aluminum disk (a radial foil), which is subjected to the 1 MA, 250 ns current pulse from the MAGPIE generator [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. The early time motion of the foil is characterized by the bulk motion of the mass due to the magnetic pressure, together with the formation of a surface plasma following the direction of the J×B force. A low density plasma fills the region above the foil preceding the formation of subsequent magnetically driven jets on the axis of expanding magnetic bubbles. The outflows emerge in timescales of ∼30–40 ns and their episodic nature is the result of current reconnection in the foil, aided by the formation of current-driven instabilities in the jet and the distribution of mass available from the foil. The additional inductance due ...

Dynamics of conical wire array z-pinch implosions.

A modification of the wire array Z pinch, the conical wire array, has applications to the understanding of wire array implosions and potentially to pulse shaping relevant to inertial confinement fusion. Results are presented from imploding conical wire array experiments performed on university scale 1 MA generators—the MAGPIE generator (1 MA, 240 ns) at Imperial College London [I. H. Mitchell et al., Rev. Sci Instrum. 67, 1533 (1996)] and the Nevada Terawatt Facility’s Zebra generator (1 MA, 100 ns) at the University of Nevada, Reno [B. Bauer et al., in Dense Z-Pinches, edited by N. Pereira, J. Davis, and P. Pulsifer (AIP, New York, 1997), Vol. 409, p. 153]. This paper will discuss the implosion dynamics of conical wire arrays. Data indicate that mass ablation from the wires in this complex system can be reproduced with a rocket model with fixed ablation velocity. Modulations in the ablated plasma are present, the wavelength of which is invariant to a threefold variation in magnetic field strength. The ax...

Extreme ultraviolet imaging of wire array z-pinch experiments

A multi-frame extreme ultraviolet imaging system based on four pinhole cameras, each backed by a gated microchannel plate (MCP) detector, was used to analyze plasma formation and dynamics in wire array z-pinch experiments on the MAGPIE generator (1 MA, 240 ns) at Imperial College (London). The use of pinhole size, object, and image distances, and MCP sensitivity to determine the spatial and spectral response of the diagnostic is discussed. A high magnification, high resolution version of the diagnostic has produced important information on wire initiation and plasma ablation in various materials. A low magnification version has allowed direct study of the snowplough sheath during array implosion, and of plasma instabilities during stagnation.

Quantitative analysis of plasma ablation using inverse wire array Z pinches

An inverse (exploding) wire array configuration, in which the wires form a cylinder around a current carrying electrode on axis, was used to study the ablation phase of the wires. This configuration allows the parameters of the plasma from individual wires of the array to be measured as the ablated plasma streams propagate in the outward radial direction. The density distribution and the evolution of the natural mode of modulation of the ablation flow was measured with interferometry and soft x-ray imaging. Measurements of the voltage across the array, which in this configuration is determined by the private magnetic flux around the individual wires, allow information on the localization of the current to be obtained. Results are compared to three-dimensional magnetohydrodynamics simulations.

Supersonic Radiatively Cooled Rotating Flows and Jets in the Laboratory

The first laboratory astrophysics experiments to produce a radiatively cooled plasma jet with dynamically significant angular momentum are discussed. A new configuration of wire array z pinch, the twisted conical wire array, is used to produce convergent plasma flows each rotating about the central axis. Collision of the flows produces a standing shock and jet that each have supersonic azimuthal velocities. By varying the twist angle of the array, the rotation velocity of the system can be controlled, with jet rotation velocities reaching approximately 18% of the propagation velocity.

Implosion and stagnation of wire array Z pinches

Detailed measurements of the dynamics of aluminum wire array Z pinches from immediately prior to implosion until stagnation and dissipation on axis are presented. Before implosion, the ∼0.5mm axial modulation seen in earlier laser probing images is observed as ablation on the surface of the wire cores facing away from the array axis. This results in the complete ablation of sections of the wire cores and a redistribution of current at the start of implosion. The dynamics of implosion are then strongly influenced by the number of wires in the array. With only eight wires, discrete snowplough bubbles expand from each wire toward the precursor. There is little, if any, correlation between the bubbles from adjacent wires, and a large temporal spread over which the bubbles arrive at the precursor is observed, along with a long rise time, low power soft x-ray pulse. With 32 or more wires, bubbles from adjacent wires merge close to the array edge to form an imploding sheath. The front edge of the sheath is well ...

Structure of stagnated plasma in aluminum wire array Z pinches

Experiments with aluminum wire array Z pinches have been carried out on the mega-ampere generator for plasma implosion experiments (MAGPIE) at Imperial College London [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1533 (1996)]. It has been shown that in these arrays, there are two intense sources of radiation during stagnation; Al XII line emission from a precursor-sized object, and both continuum and Al XIII radiation from bright spots of either significantly higher temperature or density randomly distributed around this object so as to produce a hollow emission profile. Spatially resolved spectra produced by spherically bent crystals were recorded, both time-integrated and time-resolved, and were used to show that these two sources of radiation peak at the same time.