Magneto-optical imaging of superconducting MgB2 joints

A technique for joining superconducting MgB2 pellets has been developed. For the first time, magneto-optical imaging has been used to visualize the superconducting behaviour of an MgB2 joint. No significant difference was found between the penetration of the magnetic flux in the joint and in the bulk at low magnetic fields and at temperatures less than 35 K. The critical current density of the joint in high magnetic field was determined by measuring the transversal and longitudinal magnetic moment of a bar cut from the sample. A description was developed to extract the critical current density from magnetization measurements. The magnetization measurements reveal a critical current anisotropy in the pressed MgB2 and a high critical current in the joint at low temperatures. A possibility to shape the superconductor by pressing, and the conditions preventing development and spread of fractures have been investigated. The joint technique, which utilizes high pressure, high temperature and high electrical current could be scaled up providing a basis for the wide use of superconductors at liquid hydrogen temperatures. (paper)