495 research outputs found
On the Origin of Radial Magnetic Fields in Young Supernova Remnants
We study the radio emission from young supernova remnants by means of 3D
numerical MHD simulations of the Rayleigh-Taylor instability in the shell of
the remnant. The computation is carried out in spherical polar coordinates () by using a moving grid technique which allows us to finely
resolve the shell. Three-dimensional result shows more turbulent (complex)
structures in the mixing region than the two-dimensional result, and the
instability is found to deform the reverse shock front. Stokes parameters (I,Q,
and U) are computed to study the radio properties of the remnant. The total
intensity map shows two distinctive regions (inner and outer shells). The inner
shell appears to be complex and turbulent exhibiting loop structures and plumes
as a result of the Rayleigh-Taylor instability, while the outer shell is faint
and laminar due to the shocked uniform ambient magnetic fields. The inner shell
resembles the observed radio structure in the main shell of young SNRs, which
is evidence that the Rayleigh-Taylor instability is an ongoing process in young
SNRs. When only the peculiar components of the magnetic fields generated by the
instability are considered, the polarization B-vector in the inner radio shell
is preferentially radial with about of fractional polarization
which is higher than the observed value. The fractional polarization is lowest
in the turbulent inner shell and increases outward, which is attributed to the
geometric effect. The polarized intensity is found to be correlated with the
total intensity. We demonstrate that the polarized intensity from the turbulent
region can dominate over the polarized intensity from the shocked uniform
fields if the amplified field is sufficiently strong. Therefore, we conclude
that the Rayleigh-Taylor instability can explain the dominant radial magneticComment: 26 pages, Latex, 13 Postscript figures. Figures obtainable by email
to [email protected]. Accepted for publication in the Astrophysical Journa
Interaction of a Pulsar Wind with the Expanding Supernova Remnant
Recent HST observations of the Crab Nebula show filamentary structures that
appear to originate from the Rayleigh-Taylor (R-T) instability operating on the
supernova ejecta accelerated by the pulsar-driven wind. In order to understand
the origin and formation of the filaments in the Crab Nebula, we study the
interaction of a pulsar wind with the uniformly expanding supernova remnant by
means of numerical simulation. By performing two-dimensional numerical
simulations, we find three independent instabilities in the interaction region
between the pulsar wind and the expanding supernova remnant. The most important
instability develops as the shock driven by the pulsar bubble becomes
accelerated (). The instability produces pronounced
filamentary structures that resemble the observed filaments in the Crab Nebula.
Our numerical simulations can reproduce important observational features of the
Crab Nebula. The high density heads in the R-T finger tips are produced because
of the compressibility of the gas. The density of these heads is found to be
about 10 times higher than other regions in the fingers. The mass contained in
the R-T fingers is found to be of the total shocked mass and the
kinetic energy within the R-T fingers is of the total kinetic
energy of the shocked flow. The R-T fingers are found to accelerate with a
slower rate than the shock front, which is consistent with the observations. By
comparing our simulations and the observations, we infer that the some
finger-like filaments (region F or G in Hester's observation) started to
develop about 657 years ago.Comment: 16 pages, 9 figures, 1 table, accepted for Astrophysical Journa
Tolerance Sensitivity Analysis and Robust Optimal Design Method of a Surface-Mounted Permanent Magnet Motor by Using a Hybrid Response Surface Method Considering Manufacturing Tolerances
This paper presents a robust optimal design method using a hybrid response surface method (H-RSM) which directly finds an optimal point satisfying a target Z-value or a probability of failure. Through three steps, this paper achieves the goal that is to increase the open-circuit airgap flux (OCAF) in a surface-mounted permanent magnet motor and decrease its variation caused by variations of the airgap lengths including an additional one between permanent magnets and rotor back yoke. First, the OCAF equation is derived from the magnetic equivalent circuit (MEC) considering the additional airgap. Then, the equation is validated by comparing its results with those of the finite element method (FEM) modeled by the slotless stator. Next, the tolerance sensitivity analysis, using the partial derivative of the OCAF equation with respect to the airgap length, is performed to investigate the effects of design variables on the OCAF. It is shown that increasing the magnet thickness is effective for both increasing mean of the OCAF and reducing its variation. Finally, robust optimal design is performed using the H-RSM, in which all data are obtained from the FEM modeled by the slotted stator. The results of the robust optimal design are verified using the FEM
Temperature-dependent evolutions of excitonic superfluid plasma frequency in a srong excitonic insulator candidate, TaNiSe
We investigate an interesting anisotropic van der Waals material,
TaNiSe, using optical spectroscopy. TaNiSe has been
known as one of the few excitonic insulators proposed over 50 years ago.
TaNiSe has quasi-one dimensional chains along the -axis. We have
obtained anisotropic optical properties of a single crystal TaNiSe
along the - and -axes. The measured - and -axis optical
conductivities exhibit large anisotropic electronic and phononic properties.
With regard to the -axis optical conductivity, a sharp peak near 3050
cm at 9 K, with a well-defined optical gap ( 1800
cm) and a strong temperature-dependence, is observed. With an increase
in temperature, this peak broadens and the optical energy gap closes around
325 K(). The spectral weight redistribution with respect to the
frequency and temperature indicates that the normalized optical energy gap
() is . The
temperature-dependent superfluid plasma frequency of the excitonic condensation
in TaNiSe has been determined from measured optical data. Our
findings may be useful for future research on excitonic insulators.Comment: 17 pages, 5 figure
Sensitivity Comparison of Open-Circuit Airgap Flux Between Surface-Mounted Permanent Magnet and Spoke-Type Permanent Magnet Machines Considering Manufacturing Tolerances
The study compares the sensitivities of open-circuit airgap flux (OCAF) between a surface-mounted permanent magnet (SPM) machine and a spoke-type PM machine based on variations in airgap length including additional airgaps between permanent magnets and rotor core and between segmented stator cores to achieve high quality electric machines. Analytical equations deduced from magnetic equivalent circuits (MECs) are used to directly compare natural-born characteristics of the OCAF of the two machines. First, the MEC of each machine is modeled by considering two additional airgaps between the PMs and rotor core and between the segmented stator cores. Second, the OCAF equation of each machine is derived from the MEC to analyze the effects of the design variables on the OCAF. Subsequently, the partial derivative equation of the OCAF equation with respect to the airgap length is obtained for sensitivity analysis. A comparison of the equations of the two machines indicates that the spoke-type PM machine exhibits inherently higher sensitivity and average value of the OCAF when compared to that of the SPM machine. Finally, the results are validated via a two-dimensional finite element method (FEM) by considering the variations in airgap lengths
The Density Spike in Cosmic-Ray-Modified Shocks: Formation, Evolution, and Instability
We examine the formation and evolution of the density enhancement (density
spike) that appears downstream of strong, cosmic-ray-modified shocks. This
feature results from temporary overcompression of the flow by the combined
cosmic-ray shock precursor/gas subshock. Formation of the density spike is
expected whenever shock modification by cosmic-ray pressure increases strongly.
That occurence may be anticipated for newly generated strong shocks or for
cosmic-ray-modified shocks encountering a region of higher external density,
for example. The predicted mass density within the spike increases with the
shock Mach number and with shocks more dominated by cosmic-ray pressure. We
find this spike to be linearly unstable under a modified Rayleigh-Taylor
instability criterion at the early stage of its formation. We confirm this
instability numerically using two independent codes based on the two-fluid
model for cosmic-ray transport. These two-dimensional simulations show that the
instability grows impulsively at early stages and then slows down as the
gradients of total pressure and gas density decrease. Observational discovery
of this unstable density spike behind shocks, possibly through radio emission
enhanced by the amplified magnetic fields would provide evidence for the
existence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap
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