A.L. Rakhmanov

Universal scaling law for quench development in HTSC devices

Abstract Full size HTSC magnets and other HTSC devices are becoming a reality. Current density in the HTSC tapes and wires is increasing, especially in the temperature range ∼20 K. On the other hand, smooth voltage–current characteristics, relatively high operating temperature and low resistive Ag-matrix used in HT superconductors make it difficult to distinguish clearly its normal and superconducting parts. This demands the new approaches for HTSC devices design to be developed. In many cases, quench in different superconducting devices develops as quasi-stationary overheating if the Joule heat exceeds the heat removal to ambient. For the relatively uniform samples, the quenching current and the quenching temperature at which quench occurs may be derived just from heat balance equations without any use of idea of superconductivity and normal zone propagation. We developed such an approach and showed that near the quench current the time dependencies of the temperature and the electric field obey the universal scaling laws different for the cases less and more than the quench current. The scaling parameters are quench temperature and current, parameters of voltage–current characteristics and cooling. The simple scaling law may be used as a first-step approach to HTSC devices design. In this paper, scaling theory and its comparison with experiments are presented. The methods to evaluate quench parameters of HTSC devices important for quench protection are suggested.

Thermal quench study in HTSC pancake coil

Abstract In spite of rather high general stability of high temperature superconducting (HTSC) Bi-based magnets, catastrophic thermal quench (TQ) may appear in them under certain circumstances. It happens because of non-linearity of voltage–current characteristics in HTSC superconductors. Starting with small samples in our previous works, we continue to study the TQ with large samples. We prepared a highly instrumented HTSC pancake coil. It is wound using the Bi-2223-based tape. We attached many potential taps to the tape and installed in the winding 10 cryogenic thermocouples (TC) and two heaters. Quench development in the coil was measured under different temperatures, different magnetic fields and different cooling conditions. In this paper, the experimental details and the results obtained are presented. The results are discussed from the point of view of scaling theory for quenching in HTSC devices.

Quench development analysis in HTSC coils by use of the universal scaling theory

The theory, describing thermal quench in HTSC devices, was developed. It permits evaluation of thermal quench conditions and development dependent on materials parameters and cooling. Thermal quench threshold current and time characteristics of the quench can be predicted. The theory has been extensively verified by experiments with different superconducting HTSC devices. Good correspondence between theory and experiments has been observed. We use this theory to analyze the thermal quench emergence conditions that are dependent on cooling, sizes of the device, material properties, etc. It was shown, that with increase of the size of the device threshold thermal quench current may become less than standard determined critical current. Time characteristics of the thermal quench increase with sizes. The analysis is supported by comparison with experiments with different HTSC coils.

Hybrid Energy Transfer Line With Liquid Hydrogen and Superconducting

The transfer of massive amounts of both electrical and chemical power over long distances will present a major challenge for the global energy enterprise in the future. Attraction of hydrogen is apparent as a chemical energy agent, possessing among the highest energy density content of various common fuels, whose combustive “waste” is simply water. It could be transferred via cryogenic tubes being liquid at temperatures ~18-26 K. The usage of “gratis” cold to cool a superconducting cable made of a proper superconductor permits to deliver extra electrical power with the same line. In this paper, we describe the experimental modeling of this concept via a combined MgB2-cryogenic dc superconducting cable refrigerated by “singlet” phase liquid hydrogen. We present the design, construction details, and test results of a 10-m prototype, focusing on choice of MgB2 cable and cryostat technologies. We also discuss the opportunities and possibilities for future practical deployment of such hybrid energy delivery systems.

\hbox{MgB}_{2}

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.

Cable—First Experimental Proof of Concept

It is demonstrated how to apply the theory of the thermal quench for the design of HTS magnets. Such issues as limiting quench current and quench development time, important for coil protection, are considered with their dependence on cooling and material parameters (“critical current,” index n, etc). The practical methods of determining of important quench parameters are considered. The quench development in HTS and LTS devices is compared. The analysis and evaluation methods are supported by comparison with experimental data from different HTS devices’ tests.

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

Stability and quench development in HTS magnets: Influence of cooling and material parameters

The numerical experiment to model heating development in long HTS object has been performed by use of the specially developed computer code. Cooling by liquid nitrogen was taken into account and parameters close to the real ones for HTS tapes were used in calculations. It was shown that depending on parameters combination, particularly on current density, stable and unstable regimes can happen. At unstable regime the fast temperature runaway is combined with the strong heat localization. The safety current density levels were determined for two model tapes. The results are important for HTS power cables that should withstand large overload currents during some time in case of faults in a grid.

Normal zone propagation in the composite superconductor carrying varying transport current

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.

Heating development analysis in long HTS objects with cooling

The normal transition of a superconducting NbTi-CuNi multifilamentary wire under fast transport current decrease (dI/dt<0) is studied both experimentally and theoretically. The normal zone propagation velocity v(t) is measured at.<<ETX>>