J. Rapley

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 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.

Modeling Magnetic Tower Jets in the Laboratory

The twisting of magnetic fields threading an accretion system can lead to the generation on axis of toroidal field loops. As the magnetic pressure increases, the toroidal field inflates, producing a flow. Collimation is due to a background corona, which radially confines this axially growing “magnetic tower”. We investigate the possibility of studying in the laboratory the dynamics, confinement and stability of magnetic tower jets. We present two-dimensional resistive magnetohydrodynamic simulations of radial arrays, which consist of two concentric electrodes connected radially by thin metallic wires. In the laboratory, a radial wire array is driven by a 1 MA current which produces a hot, low density background plasma. During the current discharge a low plasma beta (β < 1), magnetic cavity develops in the background plasma (β is the ratio of thermal to magnetic pressure). This laboratory magnetic tower is driven by the magnetic pressure of the toroidal field and it is surrounded by a shock envelope. On axis, a high density column is produced by the pinch effect. The background plasma has ≳1, and in the radial direction the magnetic tower is confined mostly by the thermal pressure. In contrast, in the axial direction the pressure rapidly decays and an elongated, well collimated magnetic-jet develops. This is later disrupted by the development of m = 0 instabilities arising in the axial column.

3D MHD Simulations of Laboratory Plasma Jets

Jets and outflows are thought to be an integral part of accretion phenomena and are associated with a large variety of objects. In these systems, the interaction of magnetic fields with an accretion disk and/or a magnetized central object is thought to be responsible for the acceleration and collimation of plasma into jets and wider angle flows. In this paper we present three-dimensional MHD simulations of magnetically driven, radiatively cooled laboratory jets that are produced on the MAGPIE experimental facility. The general outflow structure comprises an expanding magnetic cavity which is collimated by the pressure of an extended plasma background medium, and a magnetically confined jet which develops within the magnetic cavity. Although this structure is intrinsically transient and instabilities in the jet and disruption of the magnetic cavity ultimately lead to its break-up, a well collimated, “knotty” jet still emerges from the system; such clumpy morphology is reminiscent of that observed in many astrophysical jets. The possible introduction in the experiments of angular momentum and axial magnetic field will also be discussed.

Chemically etched modulation in wire radius for wire array Z-pinch perturbation studies

A technique for manufacturing wires with imposed modulation in radius with axial wavelengths as short as 1 mm is presented. Extruded aluminum 5056 with 15 μm diameter was masked and chemically etched to reduce the radius by ∼20% in selected regions. Characterized by scanning electron microscopy, the modulation in radius is a step function with a ∼10 μm wide conical transition between thick and thin segments, with some pitting in etched regions. Techniques for mounting and aligning these wires in arrays for fast z-pinch experiments will be discussed. Axially mass-modulated wire arrays of this type will allow the study of seeded Rayleigh-Taylor instabilities in z pinches, corona formation, wire initiation with varying current density in the wire core, and correlation of perturbations between adjacent wires. This tool will support magnetohydrodynamics code validation in complex three-dimensional geometries, and perhaps x-ray pulse shaping.

Laboratory Modeling of Radiatively Cooled Jets Using Conical Wire Array Z‐pinches

We present the results of astrophysically relevant laboratory experiments carried out on the MAGPIE pulsed power facility. Collimated, radiatively cooled outflows are observed in a number of astrophysical situations including Young Stellar Objects and Planetary Nebulae. To model these jets, highly supersonic (Mach number 20–30), radiatively cooled plasma jets are produced using conically convergent flows obtained by applying a fast rising current to a conical arrangement of fine metallic wires. Methods of varying the jet cooling length, the density contrast between jet and surrounding material and the angular momentum of the jet have been developed. We have also been able to model other observed jet features such as the deflection of jets by a side wind. Such a mechanism has been proposed to explain observations where bipolar jets are both curved in the same direction, producing a C‐shaped symmetry. The laboratory jets are significantly deflected without loss of collimation by the ram pressure from a phot...

Laboratory astrophysics: 2D and 3D numerical modeling of jets and flows produced in wire array experiments

Numerical modeling of jets formed in conical wire array Z‐pinch experiments shows that scaled, astrophysically relevant flows can be obtained in the laboratory. These jets are hypersonic, with Mach number in excess of 20, are radiatively cooled and have a length to width ratio of ∼ 1:10. Furthermore the jet formation mechanism is due to the hydrodynamic confinement of a standing conical shock, which redirects and collimates the converging plasma flow. The jets produced are also characterized by large Reynolds and Peclet numbers. Jet‐wind interactions are modeled assuming a supersonically expanding radiatively ablated plasma wind. The jet bends away from the wind and it remains well collimated during and after the interaction, with the bending determined by the ram pressure of the impinging wind. A further development in wire array laboratory astrophysics experiments is the use of radial arrays, where the interaction of a “freely” expanding toroidal‐like plasma cloud produces an axial collimated flow; subs...

Laboratory Experiments with Supersonic Radiatively Cooled Jets: Jet Deflection via Crosswinds and Magnetic Tower Outflows

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 YSO jets. The jets are produced using two modifications of wire array Z‐pinch driven by 1MA, 250ns 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 addition of the poloidal magnetic field and angular momentum to the jets are also discussed. We also present three‐dimensional resistive magneto‐hydrodynamic ...

Plasma Ablation and Precursor Column Formation in Wire‐Array Z‐Pinches

This paper summarises the present understanding of the processes leading to precursor column formation in cylindrical wire arrays on the 1MA MAGPIE generator at Imperial College London. The presence of precursor plasma affects the interaction of the wire array with on-axis targets, such as in the dynamic hohlraum system, and so good characterisation is essential for such experiments. Column formation parameters are directly determined by the collisionality of the plasma streams ablated from the wires as they arrive at the array axis, which depends on the array material, drive current and velocity. Direct experimental measurements of the diameter variation during the collapse and formation phase of the precursor column will be presented, along with soft x-ray emission, and quantitative radiography. The correlation of emission to column contraction indicates a non-linear collapse as a result of increasing on-axis density and radiative cooling. Differences in the formation and late-time behaviour for tungsten and aluminium arrays are observed, and characteristic values for several common materials are seen to vary according to atomic mass. Data is in good agreement with hydrodynamic code, and a recently published kinetic description of the precursor column, which predict many of the observed features.