Non-Destructive Evaluation of Defects in Wires and Other Samples Using an 8-Channel High-Tc Scanning SQUID Microscope

Abstract : This project involves the development, construction, and operation of a sensitive magnetic microscope that is based on the dc Superconducting QUantum Interference Device (SQUID). During this project the authors constructed a system by modifying one of an existing 1-channel liquid-nitrogen cooled SQUID microscope to hold an array of 8 high-Tc YBCO SQUIDs. The SQUID chip was mounted on the end of a 77 K cold finger in the vacuum space of a dewar that has a thin (25 micrometer) window that separates the vacuum from room temperature air. The system will mainly be used to image yields, voids, and occlusions in Cu-clad NbTi superconducting magnet wires and in high-Tc superconducting wire samples, with the aim being to provide reliable detection of defects that cause reduced critical current. In the last six months of the project (September 2002 to the extended ending date of March 2003), there were three main tasks that needed to be completed. The first task was to complete the software for reading out and controlling multiple SQUIDS. The controlling software was rewritten, and now simultaneous data can be taken for two SQUIDS. The second task was to compare system performance to a flow-through SQUID system that was under development for testing long sections of wires. The last task for the system was to reinstall a chip with up to 8 working SQUIDs. The chip the author had used for the last year and a half started with seven working devices, but damage had accumulated (i.e., broken wire bonds, junction failure, and damage that was incurred when the thin window broke) and only two devices are presently working on the chip. However, a back-up chip is available with about 6 operating SQUIDs on it, and the author expects to swap out the existing chip in the next few months.