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Journal ArticleDOI

Electrical and thermal behavior of patterned superconducting disks

01 Jun 1995-IEEE Transactions on Applied Superconductivity (IEEE)-Vol. 5, Iss: 2, pp 321-324

AbstractApparatus 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 (2 min read)

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.

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Citations
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Journal ArticleDOI
Abstract: The use of high temperature superconductors (HTS) in long term tokamak fusion reactors is analyzed in this paper. The well-documented physical properties of high temperature superconductors are used in the evaluation. Short-sample wires and tapes of Bi2Sr2CanCun 1Oy materials, with n 1, 2 (referred to as BSCCO), and YBCO (YBa2CuO7 x) tapes approach their single-crystal performance limits. They provide a useful reference frame as well as a starting point of what may be possible in the future. In this paper, the design issues that arise when this technology is used in tokamak magnet applications will be investigated by extrapolating the properties of short sample YBCO thick films and BSCCO tapes to longer lengths. The properties of YBCO operating at elevated temperatures (\ 10 K) will also be summarized. The limitations imposed on the performance and lifetime of the HTS materials because of the fusion environment will be reviewed. The consequences of implementing an HTS configuration in a fusion environment will then be described. The use of HTS material may offer an opportunity for a fundamentally different approach to magnet design and construction leading to either lower cost or reduced maintenance. Reduced cost magnets requiring decreased shielding, with better access and easier maintenance may be feasible. © 2001 Elsevier Science B.V. All rights reserved.

64 citations


Journal ArticleDOI
Abstract: The environment close to the chamber of heavy ion inertial fusion energy reactors imposes severe constraints on magnets used for final focusing magnets. Space is at a premium, requiring close proximity of adjacent magnets, making magnet integration imperative. In addition, the high radiation flux imposes stringent shielding requirements. In this paper, the options for final focusing magnet topologies are described. Implications of using both high-temperature superconductors and conventional low-temperature superconductors are investigated. The use of high-temperature superconducting materials may offer an attractive, although speculative, opportunity for a fundamentally different approach to magnet construction, leading to either lower cost or reduced maintenance.

2 citations



References
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Journal ArticleDOI
Abstract: A detailed study of the crystallization process for compositions near Bi2Sr2Ca1Cu2Oy was undertaken using differential thermal analysis (DTA), transmission and scanning electron microscopy (TEM and SEM), and x-ray diffraction (XRD). Glasses prepared by a splat-quench technique were free of secondary phases in most cases. A two-step crystallization process in oxygen was observed in which partial crystallization of the glass occurs initially with the nucleation of “2201” and Cu2O, and is completed with the formation of SrO, CaO, and Bi2Sr3−xCaxOy. No specific thermal event could be associated with the formation of the “2212” phase. Rather, formation occurs via conversion of 2201 into 2212. This was a kinetically limited process at temperatures below 800 °C as other phases were found to evolve in addition to the 2212 phase during extended anneals. In contrast, a nearly full conversion to the 2212 phase occurred after only 1 min of annealing at 800 °C and above. However, changes in resistivity data, secondary phases, and the measured 2212 composition upon extended anneals at 865 °C showed that considerably longer heat treatments were necessary for the sample to reach its equilibrium state.

49 citations


Journal ArticleDOI
Abstract: With reference to the application of NbTi tape in a superconducting thermal switch, the critical current of a 20- mu m-thick NbTi tape was measured in several directions of the magnetic field. The critical current was found to behave strongly anisotropically, due to the deformation of the NbTi. The tape is extrasensitive to the component of the magnetic field perpendicular to the surface. Without an external field this component of the self-field reduces the critical current far below its intrinsic value. A one-dimensional model can describe the reduction of critical current due to the self-field in a thin tape. >

11 citations


Journal ArticleDOI
Abstract: The origination of propagating normal domains in large superconducting composites is studied numerically by means of an effective circuit model. The initial perturbation is considered to be a thermal pulse. The minimum energy required to form a propagating normal domain is calculated as a function of the dimensionless transport current, and three parameters characterizing the cooling conditions and the conductor. An analytical expression is proposed to determine this energy in the region of parameters of practical interest. >

1 citations