Participants: Dr D A Diver, Dr H E Potts, Prof J C Brown, Dr A MacKinnon, Dr R K Barrett
Lattice-Boltzmann (LB) simulations are the basis of a novel 3-D gas-plasma
simulation code, based on a hybrid square-lattice LB method for simulating the
neutral component, and an extended ambipolar treatment of the plasma dynamics.
Mutual interaction is achieved through collisions, and surface interactions,
including self-consistent sheath modelling, will also feature, particularly
for the high-pressure, collisional regime. The interaction between unsteady
flows and sheath evolution will be given particular emphasis, given the implications
for ion energies and the particle flux through the sheath.
Pulsar plasmas are notoriously difficult to simulate, given the ultra-high energies involved, and the complexity of the pulsar geometry. In collaboration with Prof A A da Costa (CEIST, Lisbon) numerical simulations of electron-positron plasma oscillations have been undertaken, using a specialised finite-difference Lax-Wendroff integrator. The results confirm that sharp density gradients evolve inevitably for all but trivial oscillation amplitudes. Moreover, the electric field associated with the evolving oscillation is significant in comparison with the electric field of the pulsar rotation when transformed to the observer's frame, showing that collective effects cannot be ignored when calculating the overall electrodynamical behaviour of the pulsar atmosphere. Investigations currently underway in this programme include the study of runaway particle scattering from such isolated field structures, and the direct coupling between plasma electrostatic modes and plasma electromagnetic waves once the density gradient exceeds a critical threshold.
Practical investigations are underway into the bulk magnetised fluid properties
of magnetic liquids such as ferrofluids. The evolution of unsteady surface waves
under frequency and amplitude dependent magnetic drivers shows not only wave
breaking, but also rapid, energetic jet formation; theoretical and numerical
modelling agree closely, and allow new insight into the physical processes,
particularly the role of the surface tension. Further experiments are planned
with magnetic fields closer to saturation. Large, discrete drops of ferrofluid
have been suspended against gravity by specially tailored magnetic fields, in
a micro-gravity type simulation These large drops have been excited magnetically,
responding in non-linear spheroidal modes. Under extreme conditions the drops
can be forced to split apart. A new simplified model of drop dynamics including
the magnetic pressure and surface tension effects has been developed, and produced
excellent agreement with experimental results. Future experiments include inducing
drop rotation and shear by applying a rotational magnetic field structure in
the
horizontal plane.