next up previous
Next: Soskin S.M. Up: Book of Abstracts Previous: Sirotkin O.L.

Skokov V.N. and Koverda V.P.

Thermal autowaves and dynamics of vapour bubbles during joule
self-heating of thin films of high-temperature superconductors
cooled by boiling nitrogen

V.N. Skokov and V.P. Koverda
Institute of Thermophysics, Urals Division of the Russian Academy of Sciences, Ekaterinburg, Russia

A rise and evolution of a thermal instability of superconductors with a transport current is a tipical example of nonequilibrium phase transitions in dissipative systems. In this case there may arise a state of thermal multistability, in which there are a few stable states with the same value of external governing parameter (transport current). In the report the results of the experimental study of nonequilibrium phase transtions under thermal destruction of the superconducting state of Y-Ba-Cu-O thin films are presented [1]. The voltage-current characteristics have been determined in the films during cooling in a nitrogen gas-flow cryostat and in liquid nitrogen directly. The conditions of an arising of stationary dissipative structures and autowaves propagations in a bistable superconductor have been realized experimentally. The velocities of waves switching thin films a superconducting state to normal are at different values of a transport current have been measured [2,3].

A formation and propagation of dissipative structures in a superconductor may be accompained by nonlinear thermal and hydrodynamic phenomena in a coolant. The effect on the thermal stability of films of phase transitions in liquid nitrogen, namely, transitions from one-phase convective heat exchange to a bubble boiling and from bubble boiling to a film boiling (boiling crisis) has been investigated. As shown experimentally, the existence of the superheated state of liquid nitrogen has considerable influence on the stability and dynamics of the thermal destruction of a superconducting state and results in the increase of possible types of switch waves.

In the experiments one could observed periodic and chaotic generations of vapour bubbles on a films surface.

It was shown that in the system there arose a non-equilibrium phase transition of a formation of a "dry spot" on a superconductive heater.

A critical behaviour of the system under a condition if a load line of an electric circuit was a tangent to a voltage - current characteristic has been investigated.

It was shown that under certain conditions there may arose a 1/f - noise.

A simultaneous proceeding and correlation of traditional phase transitions (superconductor - normal conductor and liquid - vapour) result in the variety of nonlinear phenomena in a complicated system: a current carrying nonlinear conductor - liquid coolant.

  1. V.N. Skokov and V.P. Koverda, Cryogenics, 33 (1993) 1072.
  2. V.N. Skokov and V.P. Koverda, Pisma v Zh. Tekh. Fiz., 19 (1993) 83.
  3. V.N. Skokov and V.P. Koverda, Phys. stat. sol.(a), 142 (1994) 193.

Climate dynamics as a nonlinear brownian motion
D.M. Sonechkin
Hydrometeorological Research Centre of Russia, Moscow, Russia

Although many natural processes can be depicted well by a certain governing equations based on thefundamentalphysical laws it is the usual casethat a natural process of interest is too complex and erratic. It does not admit such a depiction to be simple and effective. For example, the present-day system of the equations of the atmospheric hydro- and thermodynamics does not admit long-term weather forecasting, and moreover, there is a doubt that extended runs of these equations can be considered as reliable referees of the current climate change problem. The consequence is actually that empirically derived relative simple (low-dimensional) mappings need to be created by means of a processing of extended observational data time series in order to model such natural processes. For the purposes of the climate change detection and prediction one can be assumed that all climatic observational data time series reflect thecombinedeffects of internal statistically stationary processes, which are a inherent property of the real-world cliate dynamics, onthe one hand, and the effects of slowly varyed external forcings, which tend to introduce nonstationarity in the integral scale ofthe overall length of the time series under consideration, on the other hand. Ouraim here is to create a new processing of time series that would be capable of taking into account both these peculiarities of the climatic processes: their chaoticity in relative short time scales and their nonstationarity in the integral scale. The starting point of the processing is that any process watched on a finite time interval can be approximately splitted onto two components. The firstoneis a multiscale chaotic oscillation whosepower energy remains to be finite even if the interval of the observation increases to infinity. The second one is a trend whose power energy aspires to infinity under this circumstance. A powerfull tool of such an oriented processing is the so-called multiresolution wavelet transform of time series. The global and both Hemispheres surface air temperature time series were considered as most general descriptors of the current climate change. As well-known the series can be modelled by the heat balance equation that is a evolutionary form of the fractional Brownian motion. Scaling analysis of the motion revealed some prominent properties of the current climate change that admit to call the motion a Levy-type with divergent moments. At the same time, wavelet transform of the series revealed numerous linkages between different parts of the global climate system that are many-faced but self-similar in wide range of time scales. The main practical conclusion is that there exists one single common warming trend that was started from the early of the XXth century. Because the delay of the trend in the Southern Hemisphere as compared with the Northern one is equal the same value of delay of the greenhouse gases concentration growth one can say that the extracted warming trend is a first direct evidence of greenhouse gases phenomenon. One can suppose that the warming trend will be continued during the some next decades. But, because the contribution of the oscillation that imposed on the trend and positive now will become negative in several years, one can speculate that the global warming will be checked during the XXth century end as a result of adding the trend and the oscillation.


next up previous
Next: Soskin S.M. Up: Book of Abstracts Previous: Sirotkin O.L.

Book of abstracts
ICND-96