UTFacultiesTNWEventsPhD Defence Tim Mulder

PhD Defence Tim Mulder

Advancing rebco-corc wire and cable-in-conduit conductor technology for superconducting magnets 

Tim Mulder is a PhD student in the Energy, Materials & Systems (EMS) group, his supervisor is prof.dr.ir. H.H.J. ten Kate from the Faculty of Science and Technology 

 ReBCO coated conductors are ceramic flat tape high temperature superconductors capable of carrying large currents in a broad temperature and magnetic field range. ReBCO tapes significantly exceed the performance reach of NbTi and Nb3Sn low temperature superconductors, which allows the creation of large-scale magnets capable of generating magnets field far beyond 20 T at 4.5 K and of magnets operating in the temperature range of 30 to 60 K, a range served uniquely by ReBCO material. Large-scale magnets require currents far beyond the capacity of a single ReBCO tape and therefore multiple ReBCO tapes must be combined into a multi-tape, high-current ReBCO cable. The three main cabling concepts today are the Roebel, Twisted Stack Tape and Conductor On Round Core (CORC) cable. The focus of the work presented in this thesis is on the development of CORC technology, which includes both the CORC conductor and its joint terminals. CORC cables are round and comprise many layers of ReBCO tapes wound around a central metal core. The cable is flexible and allows omni-directional bending due to its round shape. CORC can be grouped in three categories: CORC cable, which is a standalone general purpose HTS cable; thin CORC wire, that aims at a high-current density for application in accelerator magnets and insert coils; and CORC Cable-In-Conduit Conductor (CICC) designed for enabling stable, high-current magnets and their bus lines.

Joints for the CORC conductor are essential for its performance, as they have a dominant role in determining current distribution among ReBCO tapes and are a constant source of ohmic heating. Therefore, low-resistive joints that feature a uniform current distribution are key. The state-of-the-art joints for CORC that were available at the start of this work required improvements, as they were not able to provide a low-resistance connection with uniform current injection and extraction. A novel conductor joint design is developed that offers a significant improvement of current distribution among ReBCO tapes and a significant reduction in joint resistance compared to previous designs. This new design introduces staging, also known as tapering, to the layers of the CORC cable, to form a staircase-like geometry in the cable ends. In this design, current is able to flow directly to the more inner situated layers, without having to pass the resistive substrate and buffer layers of the tapes that were in the layers above. A joint preparation method was developed that offers practical realization of the new design and a computer model was made to predict the performance of the new and old joint terminals. Several demonstration joints were prepared to test and to fine-tune the preparation procedures and all CORC samples tested throughout this work included joint terminals of the new design.

CORC conductors and their production technique were new and in the early stage of development at the start of this work. A better understanding on the performance of the CORC conductor, current sharing between layers, current distribution between individual ReBCO coated conductors, CORC bending/handling and all details appertained to the production of the CORC conductor are required. Therefore, methods for predicting the performance of CORC conductors are developed using techniques to scale the performance of single ReBCO tapes at 77 K to the performance of a CORC wire or cable at 4.2 K in external magnetic field. A 12 m long CORC cable with 38 ReBCO tapes in 12 layers was procured by CERN. Part of this cable is characterized at 1.9 and 4.2 K in a magnetic field up to 9.6 T to test its performance, the scaling methods and its joint terminals that were prepared according to the new joint design. The remaining length of this cable is later used in the first CORC six-around-one Cable-In-Conduit Conductor.

Thinner substrate ReBCO tapes make it possible to wind thin CORC wires of 3 to 4 mm diameter. The reduction of the tape’s substrate thickness from 100 via 50 to 30 micron leads to a significantly reduced minimum bending radius of ReBCO tapes. The 30 micron substrate tapes can now also be manufactured with a width of only 2 mm. Thinner substrates and narrower tapes allow production of CORC wires with more flexibility and a high current density compared to the thicker, 5 to 8 mm in diameter, CORC cable. CORC wires with various tape layouts were tested in common effort of CERN, ACT and the University of Twente to develop this break-through technology. Four CORC wires, of 3.0 to 4.5 mm diameter, are tested in transverse magnetic field of up to 11 T as small solenoids at 4.2 K and in self field at 77K. The tests demonstrate the ease of use and high performance of the new CORC wires. In addition the tests provide feedback needed to optimize wire manufacturing and joint terminal production.

The ReBCO CORC six-around-one Cable-In-Conduit Conductor is a high-current multi-strand conductor aimed at application in large-scale magnets for detector- and fusion experiments, but also for the bus lines that feed high currents to such magnets. Three first-in-the-world CORC Cable-In-Conduit Conductors were prepared at CERN. The first CICC was 1.7 m long and had an aluminium alloy jacket. It has a design current of 48 kA at 10 T and 4.2 K and 13 kA at 77 K in self-field and was successfully tested at CERN. The conductor’s performance was according to prediction and showed that the preparation and prediction methods are valid. The second and third CICCs were especially designed for application in fusion and detector magnets. One conductor had a copper jacket that allowed conduction cooling, which is the more practical cooling solution for large magnets without large heat loads. The second CICC was designed for fusion magnets and comprised a stainless steel jacket and internal forced-flow cooling, a requirement for the large heat loads common to fusion environments. The six-around-one cable bundles in both jackets were both rated for 80 kA at 12 T.

The two conductors were tested at the SULTAN test facility of PSI, Villigen, Switzerland in a temperature range of 5 to 60 K in magnetic fields up to 11 T. The CORC CICC with the stainless steel jacket for fusion magnets performed as expected in the high temperature regime of 40 to 60 K. The CORC CICC with the copper jacket for detector magnets performed below expectations and demonstrated a critical current of approximately 30% to 40% of the expected value. The limited performance of the CICC for detector magnets prevented accurate measurements of the other CICC in the temperature range of 5 to 40 K as both conductors were tested in series. Both the forced-flow and conduction cooling proved to be effective cooling methods for the CORC Cable-In-Conduit Conductor. The strands from the CICC with the copper jacket were extracted and tested individually to localize and quantify degradation within each CORC strand. Degradation appeared to have occurred only in the high-field region, indicating that the large electro-magnetic loads locally pushed the strain in the ReBCO tapes over their intrinsic limit. No clear indication was observed that a similar type of degradation occurred in the CORC CICC for fusion magnets. Therefore the cause the degradation was traced back to a cabling parameter unique for the CORC strands in the CORC CICC for detector magnets.