275 research outputs found
Current driven rotating kink mode in a plasma column with a non-line-tied free end
First experimental measurements are presented for the kink instability in a
linear plasma column which is insulated from an axial boundary by finite sheath
resistivity. Instability threshold below the classical Kruskal-Shafranov
threshold, axially asymmetric mode structure and rotation are observed. These
are accurately reproduced by a recent kink theory, which includes axial plasma
flow and one end of the plasma column that is free to move due to a
non-line-tied boundary condition.Comment: 4 pages, 6 figure
A Two-Temperature Model of the Intracluster Medium
We investigate evolution of the intracluster medium (ICM), considering the
relaxation process between the ions and electrons. According to the standard
scenario of structure formation, ICM is heated by the shock in the accretion
flow to the gravitational potential well of the dark halo. The shock primarily
heats the ions because the kinetic energy of an ion entering the shock is
larger than that of an electron by the ratio of masses. Then the electrons and
ions exchange the energy through coulomb collisions and reach the equilibrium.
From simple order estimation we find that the region where the electron
temperature is considerably lower than the ion temperature spreads out on a Mpc
scale. We then calculate the ion and electron temperature profiles by combining
the adiabatic model of two-temperature plasma by Fox & Loeb (1997) with
spherically symmetric N-body and hydrodynamic simulations based on three
different cosmological models. It is found that the electron temperature is
about a half of the mean temperature at radii 1 Mpc. This could lead to
an about 50 % underestimation in the total mass contained within 1 Mpc
when the electron temperature profiles are used. The polytropic indices of the
electron temperature profiles are whereas those of mean
temperature for Mpc. This result is consistent both
with the X-ray observations on electron temperature profiles and with some
theoretical and numerical predictions about mean temperature profiles.Comment: 20 pages with 6 figures. Accepted for publication in Ap
Ideal magnetohydrodynamic simulation of magnetic bubble expansion as a model for extragalactic radio lobes
Nonlinear ideal magnetohydrodynamic (MHD) simulations of the propagation and
expansion of a magnetic "bubble" plasma into a lower density, weakly-magnetized
background plasma are presented. These simulations mimic the geometry and
parameters of the Plasma Bubble Expansion Experiment (PBEX) [A. G. Lynn, Y.
Zhang, S. C. Hsu, H. Li, W. Liu, M. Gilmore, and C. Watts, Bull. Amer. Phys.
Soc. {\bf 52}, 53 (2007)], which is studying magnetic bubble expansion as a
model for extra-galactic radio lobes. The simulations predict several key
features of the bubble evolution. First, the direction of bubble expansion
depends on the ratio of the bubble toroidal to poloidal magnetic field, with a
higher ratio leading to expansion predominantly in the direction of propagation
and a lower ratio leading to expansion predominantly normal to the direction of
propagation. Second, an MHD shock and a trailing slow-mode compressible MHD
wavefront are formed ahead of the bubble as it propagates into the background
plasma. Third, the bubble expansion and propagation develop asymmetries about
its propagation axis due to reconnection facilitated by numerical resistivity
and to inhomogeneous angular momentum transport mainly due to the background
magnetic field. These results will help guide the initial experiments and
diagnostic measurements on PBEX.Comment: 33 pages, 37 figures, submitted to Physics of Plasma
A Textbook Example of a Bow Shock in the Merging Galaxy Cluster 1E0657-56
The Chandra image of the merging, hot galaxy cluster 1E0657-56 reveals a bow
shock propagating in front of a bullet-like gas cloud just exiting the
disrupted cluster core. This is the first clear example of a shock front in a
cluster. From the jumps in the gas density and temperature at the shock, the
Mach number of the bullet-like cloud is 2-3. This corresponds to a velocity of
3000-4000 km/s relative to the main cluster, which means that the cloud
traversed the core just 0.1-0.2 Gyr ago. The 6-7 keV "bullet" appears to be a
remnant of a dense cooling flow region once located at the center of a merging
subcluster whose outer gas has been stripped by ram pressure. The bullet's
shape indicates that it is near the final stage of being destroyed by ram
pressure and gas dynamic instabilities, as the subcluster galaxies move well
ahead of the cool gas. The unique simplicity of the shock front and bullet
geometry in 1E0657-56 may allow a number of interesting future measurements.
The cluster's average temperature is 14-15 keV but shows large spatial
variations. The hottest gas (T>20 keV) lies in the region of the radio halo
enhancement and extensive merging activity involving subclusters other than the
bullet.Comment: Revision: minor clarifications, mention optical results. 5 pages,
uses emulateapj.sty, includes color figures (grayscale version available from
http://hea-www.harvard.edu/~maxim/papers). ApJ Letters in pres
Compressibility effect on magnetic-shear-localized ideal magnetohydrodynamic interchange instability
Ideal relaxation of the Hopf fibration
Ideal MHD relaxation is the topology-conserving reconfiguration of a magnetic
field into a lower energy state where the net force is zero. This is achieved
by modeling the plasma as perfectly conducting viscous fluid. It is an
important tool for investigating plasma equilibria and is often used to study
the magnetic configurations in fusion devices and astrophysical plasmas. We
study the equilibrium reached by a localized magnetic field through the
topology conserving relaxation of a magnetic field based on the Hopf fibration
in which magnetic field lines are closed circles that are all linked with one
another. Magnetic fields with this topology have recently been shown to occur
in non-ideal numerical simulations. Our results show that any localized field
can only attain equilibrium if there is a finite external pressure, and that
for such a field a Taylor state is unattainable. We find an equilibrium plasma
configuration that is characterized by a lowered pressure in a toroidal region,
with field lines lying on surfaces of constant pressure. Therefore, the field
is in a Grad-Shafranov equilibrium. Localized helical magnetic fields are found
when plasma is ejected from astrophysical bodies and subsequently relaxes
against the background plasma, as well as on earth in plasmoids generated by
e.g.\ a Marshall gun. This work shows under which conditions an equilibrium can
be reached and identifies a toroidal depression as the characteristic feature
of such a configuration
- …