Electrical and thermal behavior of patterned superconducting disks

TL;DR: In this paper, a low and high T/sub c/ superconducting spirals on a normally conducting substrate have been designed and built, and the results of the tests of both high and low T/Sub c/ (Bi-Sr-Ca-Cu-O) and low-T/subc/ (Nb-Ti) thick-film spirals have been investigated experimentally.

Abstract: Apparatus for the investigation of low and high-T/sub c/ superconducting spirals has been designed and built. The device is capable of measuring the characteristic of superconducting spirals. The superconducting spirals are on a normally conducting substrate. The normally conducting substrate serves as a shunt between the superconducting turns, serving as a distributed quench protection resistor. Samples with both high and low electrical resistance substrates have been tested on this apparatus. Preliminary results of the tests of both high-T/sub c/ (Bi-Sr-Ca-Cu-O) and low-T/sub c/ (Nb-Ti) thick-film spirals have been investigated. Current distribution in films during quench has been studied experimentally. Generation of normal zone and hysteresis current-voltage characteristics have been discovered in high-T/sub c/ superconducting spirals on silver plate. It is shown that frequency of generation of normal zone depends at under certain conditions (transient current, magnetic field, temperature and resistivity of substrate). The results are being analyzed with models. >

Summary

I. INTRODUCTION

• The large upper magnetic field of high-T, superconductors gives a possibility in principle to build efficient magnets, operating at elevated temperatures (higher than 4K).
• A proposed method for building magnets is to manufacture the ceramic superconductor in spirals on substrates, with electrical contact on their ends (11.
• Quench protection is an issue with high-T, magnets.
• In superconducting Bitter magnets, the resistive shunts (actually the substrate) allows the current to bypass a normal zone in the superconductor, after which the heated zone cools down.
• In order to provide this underestanding, the distribution of current, temperature and voltages needs to be investigated.

111. SAMPLES

• Both samples with high and low-T' have been used.
• The low-T, tape was manufactured at the Plasma Fusion Cernter, while the high-Tc tape was manufactured at Los Alamos National Laboratory.

A . Nb-Ti TAPES

• The characteristics of the tape have been described in reference [2] .
• The tapes used in the experiment were produced by mechanically patterning a 2.5 x 2.5cm2 silver sheet with a spiral.
• After an additional 15 hours, the samples are cooled at 5C/min to room temperature.
• Bi loss during the melting process in Ar appears to have been significant only for samples processed above 82015.
• After the pulse is terminated, the voltage decreases with a time constant of -100s.

A . Nb-Ti

• Note the 'reveral of the direction of current flow between the nodes 4 and 5 , occuring shortly after the application of the heating pulse.
• In addition] the voltage in the innermost turns show a large oscillation (although sinusoidal in this case), with a perioud of about 1s.
• The quenching process in high-Tc superconducting spiral on silver plate is not simple.
• At even higher current, the behaviour is different.

V. DISCUSSION

• The calculated resistance across the turns for the Cu-Nb/Ti composite sample varies between 1.3 and 5 x Q.
• Therefore, it is not surprising that the critical current for the different turns.
• The dynarmcs of normal zone propagation can be investigated by using the heat and voltage balance equations for thermal and electrical processes occurring in the real spiral [4] .
• The problem of normal zone propagation in large composite superconductors have been studied many authors both analytically and experimentally [54].
• When the temperature crosses the critical temperature, the superconducting state is recovered, ,and current rediffuses back to superconductor.

VI. CONCLUSIONS

• Preliminary result of normal zone propagation in shunted superconducting spirals have been presented.
• It is shown that if a part of superconductor becomes normal, the current starts to redistribute between the superconductor and substrate.
• After the removal of the heating source, the spiral cools down and when the temperature crosses the critical temperature, the superconducting state is recovered, and the current reverses to its original flow pattern.
• Large heating inputs are required to quench the high-T, spirals.
• The propagation of the normal zone and, in particular, redistribution of the current, is being further investigated.