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Title: Superconductors in alternating fields
Author: Linford, Rodney Martin Forbes
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1968
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After a general introduction to the fundamentals of superconductivity, the concept of the critical state is introduced and its application to alternating field conditions discussed. A comprehensive review of the a.c. measurements in the literature and the relevance to the predictions of the theories is then given. Details of the comprehensive experimental facilities constructed are described, including the development of a sophisticated technique for the measurement of a.c. losses. The equipment is essentially a sensitive wattmeter, with a small analogue computer used to operate on the voltages, from pickup-coils wound directly on the samples. The experiments reported start with work undertaken to investigate the dissipation in applied fields, below the critical field. The part played by surface irregularities is explored and the general field dependence of the losses explained on a simple model. A large section is then devoted to the response of type II superconductors to large fields. Results are given on high-K materials and on both reversible and hysteretic niobium samples. The predictions of the critical state model are found to hold, if suitable modifications are made to account for the field region below Rcl and the surface currents. In particular, the applied field Hm can be replaced by (Hm - .6.H/2),where AH is the overall shielding effect of the surface currents. The value of these currents has been determined from the pickup-coil voltage waveforms and the interpretation of these waveshapes has also been advanced to determine the critical current density. A decrease in the surface currents, with roughening of the sample surface, is described, and their variation with bulk properties discussed. A correlation between the critical current density and the surface currents has been discovered, and examined on the basis of "surface barrier" pinning.
Supervisor: Not available Sponsor: Science Research Council (Great Britain)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; TK Electrical engineering. Electronics Nuclear engineering