On the Characterization of coated conductors with the M-scan device

The characteristic feature of loss-free current transport in superconducting materials is -since its first discovery in 1911- stimulating ideas for fantastic technical applications. However, the requirements of cryogenic temperatures to reach the superconducting state, high critical fields and critical current densities as well as competitive prices impose limits on many superconducting compounds on their technical usage. Still, a handful of compounds have been discovered and enhanced such that they are suitable for implementation in applications as technical (super-)conductors.The era of high-temperature superconductivity (HTS) started with the discovery of superconductivity in copper oxides in 1986 in the Ba-La-Cu-O system with a critical temperature in the range of 30 K. A euphoric breakthrough was made the next year as superconductivity was discovered in the Y-Ba-Cu-O (YBCO) compound system at a critical temperature of 93 K which is above the boiling point of liquid nitrogen. Comprehensive efforts of research and development were necessary to overcome issues to design conductors out of this material which are reliable to be used in superconducting assemblies. At the current state of the art, YBCO coated conductor tapes with a length of several hundred meters are commercially available from a couple of manufacturers.YBCO coated conductors are also known as 2nd generation (2G) coated conductors, which -in comparison to the first generation of industrially manufactured Bi2Sr2Ca2Cu3O (BSCCO) tapes- are not based on a multifilament architecture or c-axis aligned grains but rather rely on a biaxially textured thin-film layer structure. On top of a polished (non-magnetic) metallic substrate, buffer layers enable biaxially alignment of the superconducting film when it epitaxially grows during its deposition. Charge doping is performed after coating the conductor with oxygen permeable silver. Stabilization of the conductor is done with copper plating. In the end, the tapes are electrically insulated and have a thickness of ~100-200 μm and typically a width of 4 mm.The outstanding properties of HTS coated conductors, especially at low temperatures (i.e. 4.2 K), are a driving factor to further push their development. For example, upper critical fields above 100 T allow to exceed the practical limit of 23.5 T of the Nb3Sn low temperature superconductor in magnet applications. A giant step in improving the flux density of superconducting laboratory magnets was performed in 2017/18, when a 32 T magnet with a HTS inlet coil became operational at the National High-Field Magnet Laboratory (NHFML) in Tallahassee. Beside magnet applications research is done to employ HTS coated conductors in fault-current limiters, superconducting generators or power transmission cables. Near-future prospects in large-scale facilities for example are HTS based magnet coils of nuclear fusion reactors such as SPARC from the Massachusetts Institute of Technology (MIT).Quality control of the tape along its entire length is inevitable prior to its implementation in a technical device. Local defects in the functional layer of the conductor might significantly degrade the critical current locally which limits the total dissipation-free current throughput.Routine characterization of long-length tapes is typically performed via contact-free Hall array scans at 77 K in self-field (SF). Direct, 4-probe characterization scan techniques typically struggle at establishing dynamic noise-free contacts between the tape and the measurement device. The natural formation of ice on the tape surface during the characterization process turns out to be an issue as well.In this work, a new continuous four-probe measurement method that scans superconducting tapes from one end to another in a 2.61 T flux zone and at 77 K was developed. In this device, called M-Scan, the tape continuously translates from a take-off reel through a bath of liquid nitrogen to a pick-up reel. Inside the bath, there is the four-probe measurement stage employing contact rollers and a permanent magnet with a channel through which the tape translates. This magnet generates the 4 cm long flux zone of 2.61 T. The voltage response of the tape, which is loaded with a pre-selected constant transport current, is continuously measured during its translation with rollers.This thesis is split into three major parts, namely: 1) Development and implementation of the M-Scan device with focus on the description of the permanent magnet, the developed low-noise rollers and gas-flow heated feedthroughs that prevent the formation of ice on the tape surface during the scan procedure.2) The development of the model to determine the critical current from the measured voltage response of the tape during its translation through the M-Scan device. The approach was experimentally verified with a sample in a 3 T pulsed electromagnet.3) The main result is the experimental confirmation of the feasibility of the developed M-Scan device. The measured Ic-curves obtained from the M-Scan were further analyzed and discussed with the developed model. Finally, the impact of defects on the critcal current was investigated and compared with results from Hall-array scans. (author)