1,389 research outputs found
On Flux Rope Stability and Atmospheric Stratification in Models of Coronal Mass Ejections Triggered by Flux Emergence
Flux emergence is widely recognized to play an important role in the
initiation of coronal mass ejections. The Chen-Shibata (2000) model, which
addresses the connection between emerging flux and flux rope eruptions, can be
implemented numerically to study how emerging flux through the photosphere can
impact the eruption of a pre-existing coronal flux rope. The model's
sensitivity to the initial conditions and reconnection micro-physics is
investigated with a parameter study. In particular, we aim to understand the
stability of the coronal flux rope in the context of X-point collapse and the
effects of boundary driving in both unstratified and stratified atmospheres. In
the absence of driving, we assess the behavior of waves in the vicinity of the
X-point. With boundary driving applied, we study the effects of reconnection
micro-physics and atmospheric stratification on the eruption. We find that the
Chen-Shibata equilibrium can be unstable to an X-point collapse even in the
absence of driving due to wave accumulation at the X-point. However, the
equilibrium can be stabilized by reducing the compressibility of the plasma,
which allows small-amplitude waves to pass through the X-point without
accumulation. Simulations with the photospheric boundary driving evaluate the
impact of reconnection micro-physics and atmospheric stratification on the
resulting dynamics: we show the evolution of the system to be determined
primarily by the structure of the global magnetic fields with little
sensitivity to the micro-physics of magnetic reconnection; and in a stratified
atmosphere, we identify a novel mechanism for producing quasi-periodic behavior
at the reconnection site behind a rising flux rope as a possible explanation of
similar phenomena observed in solar and stellar flares.Comment: Submitted Feb 28, 2014 to, accepted Aug 14, 2014 by Astronomy &
Astrophysics. 13 pages, 10 figures, 2 table
СONCEPT AND CALCULATION OF THE LIMIT TRANSVERSE SIZE OF CAPILLARIES
Porous medium are products of processing in food, agricultural, chemical and many other industries. Calculations of processes with wet porous medium are based on capillary properties of the liquid in a pore space. The capillary properties of liquids in porous media are established in pore models in the form of thin tubes of circular or slit transverse sections. The intensity of the processes occurring in it depends on the nature of the filling of the pore space with liquid. Filling with liquid and the formation of a capillary layer is possible only in small pores. However, there is no analytical justification for the transverse pore size, more than which it cannot be filled with liquid by capillary forces. With this in mind, the concept of the limiting transverse size of a capillary for a liquid under conditions of complete wetting is introduced. The limiting size calculation is based on two conditions: the shape of the axial section of the meniscus surface has the appearance of a semicircle and its extremum point is located at the level of the free surface of the fluid supplying the capillary. A capillary column cannot form in larger pores. The absence of formulas for calculating capillaries of the limiting sizes can introduce a significant error into the analytical calculation of the moisture content in the capillary layer of a liquid in porous media and moisture transfer processes. The aim of the study was to obtain formulas for calculating the limiting (largest) sizes of capillaries of a circular, flat slit section and annular transverse sections with complete wetting of their walls. For the conditions above, it was identified that the limiting distance between the walls was independent from annular capillary diameter. The formulas for the limiting transverse sizes of the flat slit and annular capillaries turned out to be the same under the assumptions above. This indicates a weak dependence of the limiting size of a slit capillary on the curvature of its transverse section. Examples of calculations of capillaries of the limiting sizes are performed.Porous medium are products of processing in food, agricultural, chemical and many other industries. Calculations of processes with wet porous medium are based on capillary properties of the liquid in a pore space. The capillary properties of liquids in porous media are established in pore models in the form of thin tubes of circular or slit transverse sections. The intensity of the processes occurring in it depends on the nature of the filling of the pore space with liquid. Filling with liquid and the formation of a capillary layer is possible only in small pores. However, there is no analytical justification for the transverse pore size, more than which it cannot be filled with liquid by capillary forces. With this in mind, the concept of the limiting transverse size of a capillary for a liquid under conditions of complete wetting is introduced. The limiting size calculation is based on two conditions: the shape of the axial section of the meniscus surface has the appearance of a semicircle and its extremum point is located at the level of the free surface of the fluid supplying the capillary. A capillary column cannot form in larger pores. The absence of formulas for calculating capillaries of the limiting sizes can introduce a significant error into the analytical calculation of the moisture content in the capillary layer of a liquid in porous media and moisture transfer processes. The aim of the study was to obtain formulas for calculating the limiting (largest) sizes of capillaries of a circular, flat slit section and annular transverse sections with complete wetting of their walls. For the conditions above, it was identified that the limiting distance between the walls was independent from annular capillary diameter. The formulas for the limiting transverse sizes of the flat slit and annular capillaries turned out to be the same under the assumptions above. This indicates a weak dependence of the limiting size of a slit capillary on the curvature of its transverse section. Examples of calculations of capillaries of the limiting sizes are performed
Variational optimization of tensor-network states with the honeycomb-lattice corner transfer matrix
We develop a method of variational optimization of the infinite projected
entangled pair states on the honeycomb lattice. The method is based on the
automatic differentiation of the honeycomb lattice corner transfer matrix
renormalization group. We apply the approach to the antiferromagnetic
Heisenberg spin-1/2 model on the honeycomb lattice. The developed formalism
gives quantitatively accurate results for the main physical observables and has
a necessary potential for further extensions
Optical Superradiance from Nuclear Spin Environment of Single Photon Emitters
We show that superradiant optical emission can be observed from the polarized
nuclear spin ensemble surrounding a single photon emitter such as a single
quantum dot (QD) or Nitrogen-Vacancy (NV) center. The superradiant light is
emitted under optical pumping conditions and would be observable with realistic
experimental parameters.Comment: 4+ pages, 3 figures, considerably rewritten, conclusions unchanged,
accepted versio
Breakdown of the local density approximation in interacting systems of cold fermions in strongly anisotropic traps
We consider spin-polarized mixtures of cold fermionic atoms on the BEC side
of the Feshbach resonance. We demonstrate that a strongly anisotropic confining
potential can give rise to a double-peak structure in the axial distribution of
the density difference and a polarization-dependent aspect ratio of the
minority species. Both phenomena appear as a result of the breakdown of the
local density approximation for the phase-separated regime. We speculate on the
implications of our findings for the unitary regime.Comment: Final published versio
Ultraslow light in inhomogeneously broadened media
We calculate the characteristics of ultraslow light in an inhomogeneously
broadened medium. We present analytical and numerical results for the group
delay as a function of power of the propagating pulse. We apply these results
to explain the recently reported saturation behavior [Baldit {\it et al.}, \prl
{\bf 95}, 143601 (2005)] of ultraslow light in rare earth ion doped crystal.Comment: 4 pages, 5 figure
DEFORMATION FEATURES OF UNIAXIAL COMPRESSION OF SAMPLES FROM THE POTATO TUBERS FLESH
The article analyzes the deformation mechanism under uniaxial compression of cylindrical samples cut from the pulp of potato tuber. In its typical S-shaped stress-strain diagram, three characteristic zones of different types of dominant deformation can be distinguished. Clarified the boundary between the second and third zones. The rationale for changing the type of deformations in each zone by adding a new type to the previous one is presented. Grounded elastic deformation in the first zone, which is associated with stretching of the cell membranes. In the second zone, the destruction of individual cells distributed in the sample volume occurs and in the third zone — the predominant gradual unification of these destructions. An explanation is given of the characteristic brittle fracture of the sample with significant residual deformations at the end of the third zone. The erroneous determination of the deformations of the second and third zones as plastic (yield) was noted. In this regard, a new term was introduced — modulus of rigidity Z. The change in the modulus of stiffness Z with increasing irreversible deformations indicates structural changes in the flesh of the sample due to its gradually collapsing cell structure. The reason for straightening diagrams for samples from fresh, hard potato tubers is explained. Taking into account the type of deformations by zones allows us to reasonably proceed to the creation of criteria for quantitative assessments of the degree of flaccidity of potato tubers.The article analyzes the deformation mechanism under uniaxial compression of cylindrical samples cut from the pulp of potato tuber. In its typical S-shaped stress-strain diagram, three characteristic zones of different types of dominant deformation can be distinguished. Clarified the boundary between the second and third zones. The rationale for changing the type of deformations in each zone by adding a new type to the previous one is presented. Grounded elastic deformation in the first zone, which is associated with stretching of the cell membranes. In the second zone, the destruction of individual cells distributed in the sample volume occurs and in the third zone — the predominant gradual unification of these destructions. An explanation is given of the characteristic brittle fracture of the sample with significant residual deformations at the end of the third zone. The erroneous determination of the deformations of the second and third zones as plastic (yield) was noted. In this regard, a new term was introduced — modulus of rigidity Z. The change in the modulus of stiffness Z with increasing irreversible deformations indicates structural changes in the flesh of the sample due to its gradually collapsing cell structure. The reason for straightening diagrams for samples from fresh, hard potato tubers is explained. Taking into account the type of deformations by zones allows us to reasonably proceed to the creation of criteria for quantitative assessments of the degree of flaccidity of potato tubers
Two-fluid and magnetohydrodynamic modelling of magnetic reconnection in the MAST spherical tokamak and the solar corona
Twisted magnetic flux ropes are ubiquitous in space and laboratory plasmas,
and the merging of such flux ropes through magnetic reconnection is an
important mechanism for restructuring magnetic fields and releasing free
magnetic energy. The merging-compression scenario is one possible start up
scheme for spherical tokamaks, which has been used on the Mega Amp Spherical
Tokamak MAST. Two current-carrying plasma rings, or flux ropes, approach each
other through the mutual attraction of their like currents, and merge, through
magnetic reconnection, into a single plasma torus, with substantial plasma
heating. 2D resistive MHD and Hall MHD simulations of this process are
reported, and new results for the temperature distribution of ions and
electrons are presented. A model of the based on relaxation theory is also
described, which is now extended to tight aspect ratio geometry. This model
allows prediction of the final merged state and the heating. The implications
of the relaxation model for heating of the solar corona are also discussed, and
a model of the merger of two or more twisted coronal flux ropes is presented,
allowing for different senses of twist
Efficient Guiding of Cold Atoms though a Photonic Band Gap Fiber
We demonstrate the first guiding of cold atoms through a 88 mm long piece of
photonic band gap fiber. The guiding potential is created by a far-off
resonance dipole trap propagating inside the fiber with a hollow core of 12 mu
m. We load the fiber from a dark spot 85-Rb magneto optical trap and observe a
peak flux of more than 10^5 atoms/s at a velocity of 1.5 m/s. With an
additional reservoir optical dipole trap, a constant atomic flux of 1.5 10^4
atoms/s is sustained for more than 150\,ms. These results open up interesting
possibilities to study nonlinear light-matter interaction in a nearly
one-dimensional geometry and pave the way for guided matter wave
interferometry.Comment: 8 pages, 3 figure
Efficient fiber-optical interface for nanophotonic devices
We demonstrate a method for efficient coupling of guided light from a single
mode optical fiber to nanophotonic devices. Our approach makes use of
single-sided conical tapered optical fibers that are evanescently coupled over
the last ~10 um to a nanophotonic waveguide. By means of adiabatic mode
transfer using a properly chosen taper, single-mode fiber-waveguide coupling
efficiencies as high as 97(1)% are achieved. Efficient coupling is obtained for
a wide range of device geometries which are either singly-clamped on a chip or
attached to the fiber, demonstrating a promising approach for integrated
nanophotonic circuits, quantum optical and nanoscale sensing applications.Comment: 7 pages, 4 figures, includes supplementary informatio
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