Flux penetration and a.c. losses in superconducting niobium and tantalum films

Arton, Kenneth A. M (1970) Flux penetration and a.c. losses in superconducting niobium and tantalum films. PhD thesis, University of Glasgow.

Abstract

The penetration of magnetic flux into superconducting niobium and tantalum films, deposited by sputtering on the inside of cylindrical substrates, has been monitored under conditions of an a.c. magnetic field applied parallel to the film surface. The energy dissipation associated with irreversible flux motion in the film has been measured, and an attempt has been made to explain results on the basis of critical state models. Niobium and tantalum films were prepared by sputtering with a d.c. glow discharge in an argon atmosphere. A cylindrical geometry was adopted to deposit films on the inner surface of cylindrical substrates, and the gettering action of the sputtered niobium or tantalum was utilized to obtain a pure deposit at the substrate. Films up to 1 m thick were deposited on quartz cylinders, and films from 3 m to 25 m thick were sputtered onto copper or stainless steel. To obtain a measure of purity the critical temperatures of tantalum films have been obtained, TO values indicating a gaseous impurity content of typically 0.2 at.%. HO2 been measured approximately for some thick niobium films, the results indicating kappa values of about 5, corresponding to about 1 at.% of dissolved gaseous impurities. A 50 Hz a.c. field was applied to one side only of the film cylinders by means of a. solenoid positioned co-axially inside, the solenoid being shorter than the cylinder to reduce the effects of field fringing at the ends. Flux penetration into the film was monitored with a pickup coil in close proximity to the film surface in conjunction with a compensation system to remove unwanted pickup. Losses in the film were measured by means of an electronic wattmeter technique. Mean power loss and peak flux penetration for niobium films from 16 m to 25 m thick have been obtained at 4.2 K as a function of peak applied field, and comparative measurements have been made on a thick sputtered tantalum film and on a specimen machined from bulk niobium. The results have been compared with other work on a.c. losses in niobium. It was found that flux penetration and loss results cannot be readily explained by any critical state model. More detailed study of the manner of flux penetration revealed that the magnetic field penetrates preferentially at localized areas on the film surface, and that this behaviour dominates to such an extent that critical state concepts are not applicable. Examination of the films under optical microscope revealed the presence of relatively large scale defects (macrodefects) having a size of the order of several microns and being similar in structure to pinholes. It is concluded that such defects cause localized non-uniform flux penetration by acting as sites for flux line nucleation and penetration into the film. Causes of macrodefects and means of eliminating them are discussed, and the work has been related to a possible practical superconducting a.c. power cable system.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: P Hlawiczka
• Keywords: Electrical engineering, Electromagnetics
• Date of Award: 1970
• Depositing User: Enlighten Team
• Unique ID: glathesis:1970-74152
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 23 Sep 2019 15:33
• Last Modified: 23 Sep 2019 15:33
• URI: https://theses.gla.ac.uk/id/eprint/74152