623 research outputs found
Instability growth for magnetized liner inertial fusion seeded by electro-thermal, electro-choric, and material strength effects
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
Modification of classical electron transport due to collisions between electrons and fast ions
A Fokker-Planck model for the interaction of fast ions with the thermal
electrons in a quasi-neutral plasma is developed. When the fast ion population
has a net flux (i.e. the distribution of the fast ions is anisotropic in
velocity space) the electron distribution function is significantly perturbed
from Maxwellian by collisions with the fast ions, even if the fast ion density
is orders of magnitude smaller than the electron density. The Fokker-Planck
model is used to derive classical electron transport equations (a generalized
Ohm's law and a heat flow equation) that include the effects of the
electron-fast ion collisions. It is found that these collisions result in a
current term in the transport equations which can be significant even when
total current is zero. The new transport equations are analyzed in the context
of a number of scenarios including particle heating in ICF and MIF
plasmas and ion beam heating of dense plasmas
Jet Deflection via Cross winds: Laboratory Astrophysical Studies
We present new data from High Energy Density (HED) laboratory experiments
designed to explore the interaction of a heavy hypersonic radiative jet with a
cross wind. 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 cross wind 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 cross wind 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
cross wind is collisional and therefore in the hydro-dynamic regime. MHD plasma
code simulations of the experiments are able to recover the deflection
behaviour seen in the experiments. We consider the astrophysical relevance of
these experiments applying published models of jet deflection developed for AGN
and YSOs. 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 3-D 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 experimentsComment: Submitted to ApJ. For a version with figures go to
http://web.pas.rochester.edu/~afrank/labastro/CW/Jet-Wind-Frank.pd
An Experimental Platform for Pulsed-Power Driven Magnetic Reconnection
We describe a versatile pulsed-power driven platform for magnetic
reconnection experiments, based on exploding wire arrays driven in parallel
[Suttle, L. G. et al. PRL, 116, 225001]. This platform produces inherently
magnetised plasma flows for the duration of the generator current pulse (250
ns), resulting in a long-lasting reconnection layer. The layer exists for long
enough to allow evolution of complex processes such as plasmoid formation and
movement to be diagnosed by a suite of high spatial and temporal resolution
laser-based diagnostics. We can access a wide range of magnetic reconnection
regimes by changing the wire material or moving the electrodes inside the wire
arrays. We present results with aluminium and carbon wires, in which the
parameters of the inflows and the layer which forms are significantly
different. By moving the electrodes inside the wire arrays, we change how
strongly the inflows are driven. This enables us to study both symmetric
reconnection in a range of different regimes, and asymmetric reconnection.Comment: 14 pages, 9 figures. Version revised to include referee's comments.
Submitted to Physics of Plasma
Formation and Structure of a Current Sheet in Pulsed-Power Driven Magnetic Reconnection Experiments
We describe magnetic reconnection experiments using a new, pulsed-power
driven experimental platform in which the inflows are super-sonic but
sub-Alfv\'enic.The intrinsically magnetised plasma flows are long lasting,
producing a well-defined reconnection layer that persists over many
hydrodynamic time scales.The layer is diagnosed using a suite of high
resolution laser based diagnostics which provide measurements of the electron
density, reconnecting magnetic field, inflow and outflow velocities and the
electron and ion temperatures.Using these measurements we observe a balance
between the power flow into and out of the layer, and we find that the heating
rates for the electrons and ions are significantly in excess of the classical
predictions. The formation of plasmoids is observed in laser interferometry and
optical self-emission, and the magnetic O-point structure of these plasmoids is
confirmed using magnetic probes.Comment: 14 pages, 12 figures. Accepted for publication in Physics of Plasma
Formation of Episodic Magnetically Driven Radiatively Cooled Plasma Jets in the Laboratory
We report on experiments in which magnetically driven radiatively cooled
plasma jets were produced by a 1 MA, 250 ns current pulse on the MAGPIE pulsed
power facility. The jets were driven by the pressure of a toroidal magnetic
field in a ''magnetic tower'' jet configuration. This scenario is characterized
by the formation of a magnetically collimated plasma jet on the axis of a
magnetic ''bubble'', confined by the ambient medium. The use of a radial
metallic foil instead of the radial wire arrays employed in our previous work
allows for the generation of episodic magnetic tower outflows which emerge
periodically on timescales of ~30 ns. The subsequent magnetic bubbles propagate
with velocities reaching ~300 km/s and interact with previous eruptions leading
to the formation of shocks.Comment: 6 pages, 5 figures. Accepted for publication in Astrophysics & Space
Scienc
Instability growth for magnetized liner inertial fusion seeded by electro-thermal, electro-choric, and material strength effects
Extended-magnetohydrodynamics in under-dense plasmas
Extended-magnetohydrodynamics (MHD) transports magnetic flux and electron energy in high-energy-density experiments, but individual transport effects remain unobserved experimentally. Two factors are responsible in defining the transport: electron temperature and electron current. Each electron energy transport term has a direct analog in magnetic flux transport. To measure the thermally driven transport of magnetic flux and electron energy, a simple experimental configuration is explored computationally using a laser-heated pre-magnetized under-dense plasma. Changes to the laser heating profile precipitate clear diagnostic signatures from the Nernst, cross-gradient-Nernst, anisotropic conduction, and Righi-Leduc heat-flow. With a wide operating parameter range, this configuration can be used in both small and large scale facilities to benchmark MHD and kinetic transport in collisional/semi-collisional, local/non-local, and magnetized/unmagnetized regimes
On the structure and stability of magnetic tower jets
Modern theoretical models of astrophysical jets combine accretion, rotation,
and magnetic fields to launch and collimate supersonic flows from a central
source. Near the source, magnetic field strengths must be large enough to
collimate the jet requiring that the Poynting flux exceeds the kinetic-energy
flux. The extent to which the Poynting flux dominates kinetic energy flux at
large distances from the engine distinguishes two classes of models. In
magneto-centrifugal launch (MCL) models, magnetic fields dominate only at
scales engine radii, after which the jets become
hydrodynamically dominated (HD). By contrast, in Poynting flux dominated (PFD)
magnetic tower models, the field dominates even out to much larger scales. To
compare the large distance propagation differences of these two paradigms, we
perform 3-D ideal MHD AMR simulations of both HD and PFD stellar jets formed
via the same energy flux. We also compare how thermal energy losses and
rotation of the jet base affects the stability in these jets. For the
conditions described, we show that PFD and HD exhibit observationally
distinguishable features: PFD jets are lighter, slower, and less stable than HD
jets. Unlike HD jets, PFD jets develop current-driven instabilities that are
exacerbated as cooling and rotation increase, resulting in jets that are
clumpier than those in the HD limit. Our PFD jet simulations also resemble the
magnetic towers that have been recently created in laboratory astrophysical jet
experiments.Comment: 16 pages, 11 figures, published in ApJ: ApJ, 757, 6
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