32 research outputs found
Numerical study of jets produced by conical wire arrays on the Magpie pulsed power generator
The aim of this work is to model the jets produced by conical wire arrays on
the MAGPIE generator, and to design and test new setups to strengthen the link
between laboratory and astrophysical jets. We performed the modelling with
direct three-dimensional magneto-hydro-dynamic numerical simulations using the
code GORGON. We applied our code to the typical MAGPIE setup and we
successfully reproduced the experiments. We found that a minimum resolution of
approximately 100 is required to retrieve the unstable character of the jet. We
investigated the effect of changing the number of wires and found that arrays
with less wires produce more unstable jets, and that this effect has magnetic
origin. Finally, we studied the behaviour of the conical array together with a
conical shield on top of it to reduce the presence of unwanted low density
plasma flows. The resulting jet is shorter and less dense.Comment: Accepted for publication in Astrophysics & Space Science. HEDLA 2010
conference procedings. Final pubblication will be available on Springe
Comparing Poynting flux dominated magnetic tower jets with kinetic-energy dominated jets
Magnetic Towers represent one of two fundamental forms of MHD outflows.
Driven by magnetic pressure gradients, these flows have been less well studied
than magneto-centrifugally launched jets even though magnetic towers may well
be as common. Here we present new results exploring the behavior and evolution
of magnetic tower outflows and demonstrate their connection with pulsed power
experimental studies and purely hydrodynamic jets which might represent the
asymptotic propagation regimes of magneto-centrifugally launched jets.
High-resolution AMR MHD simulations (using the AstroBEAR code) provide insights
into the underlying physics of magnetic towers and help us constrain models of
their propagation. Our simulations have been designed to explore the effects of
thermal energy losses and rotation on both tower flows and their hydro
counterparts. We find these parameters have significant effects on the
stability of magnetic towers, but mild effects on the stability of hydro jets.
Current-driven perturbations in the Poynting Flux Dominated (PDF) towers are
shown to be amplified in both the cooling and rotating cases. Our studies of
the long term evolution of the towers show that the formation of weakly
magnetized central jets within the tower are broken up by these instabilities
becoming a series of collimated clumps which magnetization properties vary over
time. In addition to discussing these results in light of laboratory
experiments, we address their relevance to astrophysical observations of young
star jets and outflow from highly evolved solar type stars.Comment: 11 pages, 4 figures, accepted for publication in the High Energy
Density Physics Journal corresponding to the proceedings of the 9th
International Conference on High Energy Density Laboratory Astrophysics, May
4, 2012, Tallahassee Florid
Experimental Studies of Magnetically Driven Plasma Jets
We present experimental results on the formation of supersonic, radiatively
cooled jets driven by pressure due to the toroidal magnetic field generated by
the 1.5 MA, 250 ns current from the MAGPIE generator. The morphology of the jet
produced in the experiments is relevant to astrophysical jet scenarios in which
a jet on the axis of a magnetic cavity is collimated by a toroidal magnetic
field as it expands into the ambient medium. The jets in the experiments have
similar Mach number, plasma beta and cooling parameter to those in protostellar
jets. Additionally the Reynolds, magnetic Reynolds and Peclet numbers are much
larger than unity, allowing the experiments to be scaled to astrophysical
flows. The experimental configuration allows for the generation of episodic
magnetic cavities, suggesting that periodic fluctuations near the source may be
responsible for some of the variability observed in astrophysical jets.
Preliminary measurements of kinetic, magnetic and Poynting energy of the jets
in our experiments are presented and discussed, together with estimates of
their temperature and trapped toroidal magnetic field.Comment: 7 pages, 6 figures, accepted for publication in Astrophysics & Space
Scienc
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.Comment: 15 pages, 4 figures, accepted by Astrophysics and Space Science for
Special Issue High Energy Density Laboratory Astrophysics Conferenc
Modelling high density phenomena in hydrogen fibre Z-pinches
The application of hydrogen fibre Z-pinches to the study of the radiative collapse phenomenon is studied computationally. Two areas of difficulty, the formation of a fully ionized pinch from a cryogenic fibre and the processes leading to collapse termination, are addressed in detail. A zero-D model based on the energy equation highlights the importance of particle end losses and changes in the Coulomb logarithm upon collapse initiation and termination. A 1-D Lagrangian resistive MHD code shows the importance of the changing radial profile shapes, particularly in delaying collapse termination. A 1-D, three fluid MHD code is developed to model the ionization of the fibre by thermal conduction from a high temperature surface corona to the cold core. Rate equations for collisional ionization, 3-body recombination and equilibration are solved in tandem with fluid equations for the electrons, ions and neutrals. Continuum lowering is found to assist ionization at the corona-core interface. The high density plasma phenomena responsible for radiative collapse termination are identified as the self-trapping of radiation and free electron degeneracy. A radiation transport model and computational analogues for the effects of degeneracy upon the equation of state, transport coefficients and opacity are implemented in the 1-D, single fluid model. As opacity increases the emergent spectrum is observed to become increasingly Planckian and a fall off in radiative cooling at small radii and low frequencies occurs giving rise to collapse termination. Electron degeneracy terminates radiative collapse by supplementing the radial pressure gradient until the electromagnetic pinch force is balanced. Collapse termination is found to be a hybrid process of opacity and degeneracy effects across a wide range of line densities with opacity dominant at large line densities but with electron degeneracy becoming increasingly important at lower line densities. (author)Available from British Library Document Supply Centre-DSC:DX205768 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo
Numerical simulations of Z-Pinch experiments to create supersonic differentially-rotating plasma flows
In the context of high energy density laboratory astrophysics, we aim to produce and
study a rotating plasma relevant to accretion discs physics. We devised an experimental
setup based on a modified cylindrical wire array and we studied it numerically with the
three-dimensional, resistive magneto-hydrodynamic code GORGON. The simulations show that a
rotating plasma cylinder is formed, with typical rotation velocity ~35 km/s and
Mach number ~5. In addition, the plasma ring is differentially rotating and
strongly radiatively cooled. The introduction of external magnetic fields is
discussed
Effect of perturbations on yield in ICF targets – 4
ICF simulations were carried out with the amplitude of perturbations from spherical symmetry treated as a free parameter with the aim of reproducing experimentally observed yields. The simulations began at peak velocity and multi-wavelength perturbations were imposed in the velocity field. It was found that increasing the perturbation caused the gamma bang time to lag behind the xray bang time. Fluid motion broadening of the neutron spectrum was also examined. The effect of perturbation amplitude on alpha particle losses was investigated