D. J. Ampleford

Laboratory Astrophysics and Collimated Stellar Outflows: The Production of Radiatively Cooled Hypersonic Plasma Jets

We present the first results of astrophysically relevant experiments where highly supersonic plasma jets are generated via conically convergent flows. The convergent flows are created by electrodynamic acceleration of plasma in a conical array of fine metallic wires (a modification of the wire array Z-pinch). Stagnation of plasma flow on the axis of symmetry forms a standing conical shock effectively collimating the flow in the axial direction. This scenario is essentially similar to that discussed by Canto and collaborators as a purely hydrodynamic mechanism for jet formation in astrophysical systems. Experiments using different materials (Al, Fe, and W) show that a highly supersonic (M ~ 20), well-collimated jet is generated when the radiative cooling rate of the plasma is significant. We discuss scaling issues for the experiments and their potential use for numerical code verification. The experiments also may allow direct exploration of astrophysically relevant issues such as collimation, stability, and jet-cloud interactions.

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.

Jet Deflection via Crosswinds: Laboratory Astrophysical Studies

We present new data from high energy density laboratory experiments designed to explore the interaction of a heavy hypersonic radiative jet with a crosswind. The jets are generated with the MAGPIE pulsed power machine, where converging conical plasma flows are produced from a cylindrically symmetric array of inclined wires. Radiative hypersonic jets emerge from the convergence point. The crosswind is generated by ablation of a plastic foil via soft X-rays from the plasma convergence region. Our experiments show that the jets are deflected by the action of the crosswind, with the angle of deflection dependent on the proximity of the foil. Shocks within the jet beam are apparent in the data. Analysis of the data shows that the interaction of the jet and crosswind is collisional and therefore in the hydrodynamic regime. MHD plasma code simulations of the experiments are able to recover the deflection behavior seen in the experiments. We consider the astrophysical relevance of these experiments, applying published models of jet deflection developed for active galactic nuclei and young stellar objects. Fitting the observed jet deflections to quadratic trajectories predicted by these models allows us to recover a set of plasma parameters consistent with the data. We also present results of three-dimensional numerical simulations of jet deflection using a new astrophysical adaptive mesh refinement code. These simulations show highly structured shocks occurring within the beam similar to what was observed in the experiments.

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)

X-ray power and yield measurements at the refurbished Z machine

Advancements have been made in the diagnostic techniques to measure accurately the total radiated x-ray yield and power from z-pinch implosion experiments at the Z machine with high accuracy. The Z machine is capable of outputting 2 MJ and 330 TW of x-ray yield and power, and accurately measuring these quantities is imperative. We will describe work over the past several years which include the development of new diagnostics, improvements to existing diagnostics, and implementation of automated data analysis routines. A set of experiments on the Z machine were conducted in which the load and machine configuration were held constant. During this shot series, it was observed that the total z-pinch x-ray emission power determined from the two common techniques for inferring the x-ray power, a Kimfol filtered x-ray diode diagnostic and the total power and energy diagnostic, gave 449 TW and 323 TW, respectively. Our analysis shows the latter to be the more accurate interpretation. More broadly, the comparison demonstrates the necessity to consider spectral response and field of view when inferring x-ray powers from z-pinch sources.

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.

Use of linear wire array Z pinches to examine plasma dynamics in high magnetic fields

Planar arrangements of tungsten wires, with a flat, closely positioned current return surface, were driven by the Magpie generator (1 MA,240 ns) [I. H. Mitchell et al., Rev. Sci. Instrum. 67, 1553 (1996)]. This “linear array” configuration was expected to increase the magnetic field and forces experienced by the wires to the same level as created in experiments at the 20 MA Z facility. The wires in the linear array ablated, on average, (5–6)× faster than in 1 MA cylindrical array experiments—consistent with the expected increase in force at the wires. Streams of coronal plasma flowed from the wires to form a precursor column offset from the plane of the linear array. The dynamics of the column suggested that a much larger fraction of current flowed through it than in cylindrical array experiments (∼20%–30% cf. <7%). Inductance determined the division of current between the wires of the array; the lower inductance edge wires experienced higher currents and magnetic fields than the center wires, further inc...

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.

Opacity and gradients in aluminum wire array z-pinch implosions on the Z pulsed power facility

Aluminum wire array z pinches imploded on the Z generator are an extremely bright source of 1–2 keV radiation, with close to 400 kJ radiated at photon energies >1 keV and more than 50 kJ radiated in a single line (Al Ly-α). Opacity plays a critical role in the dynamics and K-shell radiation efficiency of these pinches. Where significant structure is present in the stagnated pinch this acts to reduce the effective opacity of the system as demonstrated by direct analysis of spectra. Analysis of time-integrated broadband spectra (0.8–25 keV) indicates electron temperatures ranging from a few 100 eV to a few keV are present, indicative of substantial temperature gradients.

Formation of Working Surfaces in Radiatively Cooled Laboratory Jets

Whilst observations provide many examples of collimated outflows or jets from astrophysical bodies, there remain unresolved questions relating to their formation, propagation and stability. The ability to form scaled jets in the laboratory has provided many useful insights. Experiments (Lebedev et al.: 2002, ApJ 564, 113) using conical arrays of fine metallic wires on the MAGPIE generator (1MA in 240 ns) have produced radiatively cooled collimated jets in vacuum using the redirection of convergent flows by a conical shock. Here we present results of a jet produced by this method propagating through a photo-ionized, quasi-stationary gas cloud. A working surface is observed at the head of the jet. The velocity of this working surface is lower than the velocity of a jet tip in vacuum.