1,774 research outputs found
Hydrodynamic states of phonons in insulators
The Chapman-Enskog method is generalized for accounting the effect of kinetic
modes on hydrodynamic evolution. Hydrodynamic states of phonon system of
insulators have been studied in a small drift velocity approximation. For
simplicity, the investigation was carried out for crystals of the cubic class
symmetry. It has been found that in phonon hydrodynamics, local equilibrium is
violated even in the approximation linear in velocity. This is due to the
absence of phonon momentum conservation law that leads to a drift velocity
relaxation. Phonon hydrodynamic equations which take dissipative processes into
account have been obtained. The results were compared with the standard theory
based on the local equilibrium validity. Integral equations have been obtained
for calculating the objects of the theory (including viscosity and heat
conductivity). It has been shown that in low temperature limit, these equations
are solvable by iterations. Steady states of the system have been considered
and an expression for steady state heat conductivity has been obtained. It
coincides with the famous result by Akhiezer in the leading low temperature
approximation. It has been established that temperature distribution in the
steady state of insulator satisfies a condition of heat source absence.Comment: 10 page
Experimental study of a transformer with superconducting elements for fault current limitation and energy redistribution
Numerous proposed and developed superconducting fault current limiters and
self-limiting transformers limit successfully fault currents but do not provide
uninterrupted supplying of consumers. A design investigated in the work
combines the functions of a conventional transformer with the functions of fast
energy redistribution and fault protection. The device constitutes a
transformer containing an additional high-temperature superconducting (HTS)
coil short-circuited by a thin film HTS switching element. Fault current
limitation and redistribution of the power flow to a standby line are achieved
as a result of a fast transition of the superconducting switching element from
the superconducting into the normal state. Transient and steady-state
characteristics were experimentally investigated. A mathematical model of the
device operation was proposed, and the calculated results were found to be in
good agreement with the experimental data. The application field and basic
requirements to such devices were discussed and it was shown that the proposed
device meets these requirements.Comment: 15 pages incl. 4 figures. Submitted to "Cryogenics
AC losses in type-II superconductors induced by nonuniform fluctuations of external magnetic field
Magnetic field fluctuations are inevitable in practical applications of
superconductors and it is often necessary to estimate the AC losses these
fluctuations induce. If the fluctuation wavelength is greater than the size of
a superconductor, known estimates for an alternating uniform external magnetic
field can be employed. Here we consider the opposite case and analyze, using a
model critical-state problem, penetration of spatially nonuniform fluctuations
into type-II superconductors. Numerical simulation is based on a variational
formulation of the Bean model. The analytical solutions, found in a weak
penetration limit, are used to evaluate AC losses for two types of
fluctuations: the running and standing waves. It is shown that for spatially
nonuniform fluctuations the losses are better characterized by the fluctuation
penetration depth than by the fluctuation amplitude. The results can be used to
estimate the AC losses in flywheels, electric motors, magnetic shields, etc.Comment: 18 pages, 5 fugure
Generalization of the Grad method in plasma physics
The Grad method is generalized based on the Bogolyubov idea of the functional
hypothesis for states at the end of relaxation processes in a system. The Grad
problem (i.e., description of the Maxwell relaxation) for a completely ionized
spatially uniform two-component electron-ion plasma is investigated using the
Landau kinetic equation. The component distribution functions and time
evolution equations for parameters describing the state of a system are
calculated, and corrections are obtained to the known results in a perturbation
theory in a small electron-to-ion mass ratio.Comment: 10 pages, 2 table
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