A.A. Pukhov

Normal zone acceleration: a new model to describe the quench process in superconductors with changing current

Abstract Quench propagation is studied in a superconductor with changing transport current l ( t ) placed in a varying magnetic field B ( t ). It is shown that the normal zone propagation velocity v increases and the quench energy e c decreases with an increase in the rate of change of transport current I and magnetic field B Such normal zone acceleration arises due to interaction between the propagating normal zone and thermomagnetic instability induced by varying I and B . This effect takes place both for increasing and decreasing currents and changes the values of v and e c by several orders of magnitude. The analytical and numerical dependences v( I , B ) and e c ( I , B ) are found. Analytical expressions for v and e c which are valid for a wide range of I , B values are obtained and compared with both experimental and numerical data. Good agreement is observed. The obtained results may be of importance for understanding the abnormally fast quench propagation in pulsed magnet systems (such as SMES), sectional magnets, thermally controlled switches and superconducting a.c. devices.

Normal zone propagation in the composite superconductor carrying varying transport current

Abstract The perturbations induced by the varying current I or magnetic field may result in the instability of the superconducting state at some I=Im dI dt . It is shown that cooperative effect of the instability and the normal zone propagation may be responsible for anomalies of quench processes. For example, the propagation velocity increases sharply and the quench energy tends to zero at I∼Im since at I > Im there is no stable homogeneous superconducting state. The obtained results may be essential for understanding of the quench in the pulse magnet systems (such as SMES) and sectional magnets where normal transition accomplished by the fast current exchange between the sections.

Current redistribution between strands and quench process in a superconducting cable

The influence of current redistribution between strands on the quench process in a superconducting cable is studied. A theoretical model is developed which accounts for non-uniformities in strands. In agreement with experimental data, the model predicts that three different current redistribution modes may be observed in the cable, depending on the initial current; namely the current sharing mode, and slow and fast quench modes. It is shown that fast current redistribution can initiate multiple normal zone nucleation in strands. This phenomenon is responsible for an abnormally large normal zone propagation velocity. The results obtained are of importance for understanding the quench process in multistrand superconducting cables.

Analytical method to calculate the quench energy of a superconductor carrying a transport current

Abstract The dynamics of the normal zone initiated by a local heat pulse in a superconductor carrying a transport current is studied theoretically. An approximate method is considered, from which analytical formulae are obtained for the minimum heat pulse energy E c required to destroy the superconductivity. The method described allows analytical results for E c available up to now to be generalized and also new ones to be obtained. Different models of Joule self-heating in a superconductor are considered. Analytical formulae for the dependencies of the quench energy E c on the current density j are obtained for one-, two- and three-dimensional heat propagation under adiabatic conditions. The influence of heat transfer to the coolant on the quench energy E c is discussed. The analytical results are confirmed by a direct numerical simulation of the normal zone dynamics. The results obtained may be of importance for studying the cryostability of both conventional and high-temperature superconductors.

Substrate effects on thermal instability development in high Tc superconducting thin films with transport current

Abstract The thermal instability development in a high Tc superconducting thin film carrying a transport current is studied both experimentally and theoretically. The normal phase propagation velocity ν is measured by means of both direct current and pulse current methods in a wide range of the transport currents I. It is shown that to explain the measured dependence ν(I) a non-linear heating over the thickness of a substrate, of which the reverse side is stabilized at a temperature T0, should be taken into account. The corresponding theory based on the solution of the non-stationary two-dimensional heat balance equation is developed and compared with experimental data. The results obtained may be of importance in the study of superconducting switching devices.

Main dimensionless parameters to describe the quench process in superconductors with changing current

Abstract The quench process in a superconductor with changing transport current /(t) or placed into a varying magnetic field B(t) is studied. It is shown that to describe the quench process in this case the Stekly parameter α is insufficient, and it is necessary to introduce additional parameters β, γ and δ. Parameter β determines a range of current rates where the quench can be described by means of normal zone propagation. A fast current variation results in normal zone nucleation within ‘weak’ spots of a superconductor. The following quench development depends on the value of parameter γ defining a characteristic region where the ‘weak’ spot can affect normal zone nucleation and propagation. Normal zone nucleation in one strand of a multi-strand superconducting cable gives rise to a fast current redistribution among strands. Parameter δ determines a range of currents where the local quench in one strand does not result in the quench of the whole cable. The considered hierarchy of dimensionless parameters may be of importance in the study of a.c. superconductors stability.

Local normal zone nucleation to 'global' transition: quench development in superconductors with changing current

The quench process in a superconductor carrying a varying transport current l(t) is studied. Thermomagnetic disturbances induced by the varying current cause normal zone nucleation at non-uniformities related to inhomogeneous physical properties along the superconductor. It is shown that, depending on the value of the current rate of change .l = dldt the quench can be either local in character or can occur over the whole sample length simultaneously due to the development of thermomagnetic instability. An equation is obtained for the threshold current rate of change .lc, above which the latter ‘global’ thermomagnetic instability occurs. The influence of non-uniformities of different types on the quench process is considered for cases of both increasing (.l > 0) and decreasing (.l < 0) currents. The analytical results obtained are confirmed by a numerical simulation of the quench process and they give a qualitative description of the experimental results. The processes considered may be of importance in understanding the effect of multiple zone nucleation in superconducting a.c. devices.

Quench energies of multistable composite superconductors

Abstract Thermal multistability of composite superconductor with transport current due to the boiling crisis is treated theoretically. In the case of multistability there are three thermally stable homogeneous states of superconductor with different temperatures T 1 T 3 T 5 . The minimum energy E c of local heat pulse, initiating normal zone propagation and the enthalpy of the minimum propagating zone E mpz are examined. It is shown that the multistability results in unusual dependence of E c and E mpz on current I . The quench energy E c is shown to be the multivalued function of I , whereas the dependence of E mpz on I has a singularity. The values of E c and E mpz are compared at various I and the analysis of most dangerous heat disturbances is carried out. The multistability effects may be essential for studying of thermal instabilities in high-T c superconductors.

The stability of wide high-temperature superconducting films with varying transport current

The current-carrying capacity of a wide high-temperature superconducting film under conditions of rapid injection of current is investigated theoretically. An analytic expression is derived for the rate of current injection into the film at which a marked degradation of the current-carrying capacity of the film occurs. The obtained results may be important from the standpoint of analyzing the stability of superconducting ac devices.

Normal Transition of Large-Area Superconducting Films under Non-Stationary Conditions

The thermal transition of superconducting films deposited on the insulating substrate is studied. The transition occurs either due to the instability in significant part of the film or due to the local normal phase nucleation at defects. The heat equations are solved only in the substrate while the heat release occurs in the film. The analytical expression for the maximum current-carrying capacity as the function of the current rate is obtained. The scaling law for the normal phase propagating velocity is found. The threshold power microwave breakdown is discussed.