780 research outputs found
Modeling the Parker instability in a rotating plasma screw pinch
We analytically and numerically study the analogue of the Parker (magnetic
buoyancy) instability in a uniformly rotating plasma screw pinch confined in a
cylinder. Uniform plasma rotation is imposed to create a centrifugal
acceleration, which mimics the gravity required for the classical Parker
instability. The goal of this study is to determine how the Parker instability
could be unambiguously identified in a weakly magnetized, rapidly rotating
screw pinch, in which the rotation provides an effective gravity and a radially
varying azimuthal field is controlled to give conditions for which the plasma
is magnetically buoyant to inward motion. We show that an axial magnetic field
is also required to circumvent conventional current driven magnetohydrodynamic
(MHD) instabilities such as the sausage and kink modes that would obscure the
Parker instability. These conditions can be realized in the Madison Plasma
Couette Experiment (MPCX). Simulations are performed using the extended MHD
code NIMROD for an isothermal compressible plasma model. Both linear and
nonlinear regimes of the instability are studied, and the results obtained for
the linear regime are compared with analytical results from a slab geometry.
Based on this comparison, it is found that in a cylindrical pinch the magnetic
buoyancy mechanism dominates at relatively large Mach numbers (M>5), while at
low Mach numbers (M<1) the instability is due to the curvature of magnetic
field lines. At intermediate values of Mach number (1<M<5) the Coriolis force
has a strong stabilizing effect on the plasma. A possible scenario for
experimental demonstration of the Parker instability in MPCX is discussed
The Wisconsin Plasma Astrophysics Laboratory
The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user
facility designed to study a range of astrophysically relevant plasma processes
as well as novel geometries that mimic astrophysical systems. A multi-cusp
magnetic bucket constructed from strong samarium cobalt permanent magnets now
confines a 10 m, fully ionized, magnetic-field free plasma in a spherical
geometry. Plasma parameters of to eV and
to cm provide an ideal testbed
for a range of astrophysical experiments including self-exciting dynamos,
collisionless magnetic reconnection, jet stability, stellar winds, and more.
This article describes the capabilities of WiPAL along with several
experiments, in both operating and planning stages, that illustrate the range
of possibilities for future users.Comment: 21 pages, 12 figures, 2 table
On Dispersive and Classical Shock Waves in Bose-Einstein Condensates and Gas Dynamics
A Bose-Einstein condensate (BEC) is a quantum fluid that gives rise to
interesting shock wave nonlinear dynamics. Experiments depict a BEC that
exhibits behavior similar to that of a shock wave in a compressible gas, eg.
traveling fronts with steep gradients. However, the governing Gross-Pitaevskii
(GP) equation that describes the mean field of a BEC admits no dissipation
hence classical dissipative shock solutions do not explain the phenomena.
Instead, wave dynamics with small dispersion is considered and it is shown that
this provides a mechanism for the generation of a dispersive shock wave (DSW).
Computations with the GP equation are compared to experiment with excellent
agreement. A comparison between a canonical 1D dissipative and dispersive shock
problem shows significant differences in shock structure and shock front speed.
Numerical results associated with the three dimensional experiment show that
three and two dimensional approximations are in excellent agreement and one
dimensional approximations are in good qualitative agreement. Using one
dimensional DSW theory it is argued that the experimentally observed blast
waves may be viewed as dispersive shock waves.Comment: 24 pages, 28 figures, submitted to Phys Rev
Spin-Isospin Structure and Pion Condensation in Nucleon Matter
We report variational calculations of symmetric nuclear matter and pure
neutron matter, using the new Argonne v18 two-nucleon and Urbana IX
three-nucleon interactions. At the equilibrium density of 0.16 fm^-3 the
two-nucleon densities in symmetric nuclear matter are found to exhibit a
short-range spin-isospin structure similar to that found in light nuclei. We
also find that both symmetric nuclear matter and pure neutron matter undergo
transitions to phases with pion condensation at densities of 0.32 fm^-3 and 0.2
fm^-3, respectively. Neither transtion occurs with the Urbana v14 two-nucleon
interaction, while only the transition in neutron matter occurs with the
Argonne v14 two-nucleon interaction. The three-nucleon interaction is required
for the transition to occur in symmetric nuclear matter, whereas the the
transition in pure neutron matter occurs even in its absence. The behavior of
the isovector spin-longitudinal response and the pion excess in the vicinity of
the transition, and the model dependence of the transition are discussed.Comment: 44 pages RevTeX, 15 postscript figures. Minor modifications to
original postin
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