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 ($r, \theta, \phi$) 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 $20 \sim 50\%$ 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 ($r \propto t^{6/5}$). 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 $60$ of the total shocked mass and the kinetic energy within the R-T fingers is $55$ 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, Ta2_2NiSe5_5

We investigate an interesting anisotropic van der Waals material, Ta$_{2}$NiSe$_{5}$, using optical spectroscopy. Ta$_{2}$NiSe$_{5}$ has been known as one of the few excitonic insulators proposed over 50 years ago. Ta$_{2}$NiSe$_{5}$ has quasi-one dimensional chains along the $a$-axis. We have obtained anisotropic optical properties of a single crystal Ta$_{2}$NiSe$_{5}$ along the $a$- and $c$-axes. The measured $a$- and $c$-axis optical conductivities exhibit large anisotropic electronic and phononic properties. With regard to the $a$-axis optical conductivity, a sharp peak near 3050 cm$^{-1}$ at 9 K, with a well-defined optical gap ($\Delta^{EI} \simeq$ 1800 cm$^{-1}$) and a strong temperature-dependence, is observed. With an increase in temperature, this peak broadens and the optical energy gap closes around $\sim$325 K($T_c^{EI}$). The spectral weight redistribution with respect to the frequency and temperature indicates that the normalized optical energy gap ($\Delta^{EI}(T)/\Delta^{EI}(0)$) is $1-(T/T_c^{EI})^2$. The temperature-dependent superfluid plasma frequency of the excitonic condensation in Ta$_{2}$NiSe$_{5}$ 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