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PhD Defence Peng Gao

transverse pressure effect on superconducting Nb3Sn rutherford and ReBCO roebel cables for accelerator magnets

Peng Gao is a PhD student in the research group Energy, Materials & Systems (EMS). His supervisor is H.H.J. ten Kate from the faculty of Science and Technology. 

This thesis seeks to expand the understanding of the transverse pressure effects on state-of-the-art Nb3Sn Rutherford and ReBCO Roebel cables for application in accelerator magnets. Various cable samples are exposed to transverse pressure and their critical current is measured at 4.2 K in a perpendicular applied magnetic field of 10 to 10.5 T. For impregnated ReBCO Roebel cables, the inter-strand resistance at 4.2 and 77 K, as well as the AC magnetization loss at 4.2 K for different applied magnetic field direction were measured as well.

In view of accelerator upgrades such as the High-Luminosity Large Hadron Collider (HL-LHC) or plans for next-generation machines such as the Future Circular Collider, the magnetic field generated by the dipole magnets needs to be enhanced from the present 8.33 to 11 and to 16 T, respectively. NbTi can no longer deliver these magnetic fields. Instead, the latest Nb3Sn wire technology, offering a non-copper critical current density in excess of 2500 A/mm2 at 4.2 K and 12 T, is used to assemble Restacked-Rod-Process (RRP) and Powder-In-Tube (PIT) based Rutherford cables. However, since the electronic properties of Nb3Sn are strain-sensitive and furthermore the material is mechanically brittle, it is a significant challenge to construct high-field dipole magnets with these advanced conductors. The correspondingly high Lorentz force results in a perpendicular stress level of 120 to 200 MPa that act on the wide face of the Rutherford cables. Such a stress causes a significant reversible reduction and eventually even a permanent degradation of the critical current. This thesis sets out to examine the critical current and the upper critical field of resin-impregnated state-of-the-art RRP and PIT cables in terms of transverse pressure at 4.2 K in a background magnetic field of 10 T. Also possible variation of the critical current and upper critical field due to thermal- and mechanical cycling is investigated. For reference, the magnetic field-dependent critical current of witness wires has been measured as well.

Two epoxy resins, CIBA GEIGY Araldite and CTD-101K, are successfully used for the vacuum impregnation of Nb3Sn Rutherford cables prepared at CERN. Key-stoned 40-strand PIT-114 cables with wire diameter of 0.7 mm designed for the HL-LHC 11 T dispersion suppression magnets, were impregnated with the CIBA GEIGY Araldite resin. Rectangular 18-strand Short Model Coil (SMC) cables comprising 1 mm diameter RRP-132/169 or PIT-192 strands were impregnated with CTD-101K. The latter epoxy resin is better suited for impregnating large-sized coils due to its substantially longer pot life. Glass sleeve for electrical turn-to-turn insulation causes a decrease of the thermal contraction difference between the resins and the reacted cables.

The measured non-copper critical current density at 4.2 K, 12 T for the RRP and PIT witness strands is 3100 and 2500 A/mm2, respectively. Critical current degradation due to cabling is found to be negligible. Under transverse pressure, the intrinsic and reversible critical current reduction reaches 10% at 130 and at 70 to 100 MPa for the RRP- and the PIT-type SMC cables, respectively. Irreversible degradation of the critical current for both cable types sets in at a transverse pressure larger than 190 MPa and 90 to 150 MPa, respectively. The RRP cable samples are thus concluded to be more robust than similar PIT cables and are suitable for the construction of high-field dipole magnets. However, the experiments also reveal significant critical current reduction due to stress concentrations when the pressure anvil and the impregnated cable surface are misaligned as little as 0.2°, illustrating that great care is needed to avoid stress concentrations during Nb3Sn coil assembly.

ReBCO Roebel cables represent one of the promising routes towards future accelerators that operate with bending magnetic fields beyond 16 T. The Lorentz force generated in such mag- nets leads to sizeable transverse pressure on the cables of the order of 100 to 150 MPa in the present 15 to 20 T magnet designs. Contrary to single ReBCO coated conductors, bare Roebel cables start to degrade already at 40 MPa due to their uneven surface and corresponding local stress concentrations. Proper vacuum impregnation with a suitable epoxy resin strongly improves the transverse pressure susceptibility of ReBCO Roebel cables in the first instance, independent of the tape- and cable layout.

Both CY5538 Araldite with silica filler and CTD-101K with glass-fiber sleeve are found to be suitable for vacuum impregnation of ReBCO Roebel cables. Six cable samples with an architecture that is directly relevant for CERN’s Feather demonstrator magnets were investigated exposed to a variable transverse pressure at 4.2 K in a perpendicular magnetic field of 10.5 T. Three cables comprise 10 SuperPower strands with a transposition length of 126 mm. Two were impregnated with CY5538 Araldite while the third one is a bare control sample. A further three cables comprise 15 strands of either SuperPower or Bruker tape and feature a relatively long transposition length of 226 mm. One of the SuperPower cables was impregnated with the filled Araldite, the others using CTD-101K.

The measurements show that the critical current of the SuperPower Roebel strands is limited by the local critical current density in the ’crossover’ sections, which is 42 kA/mm2 at 4.2 K and 10.8 T. Compared to these ’crossovers’, the critical current density reduction of the straight sections of the SuperPower ReBCO Roebel cables is insignificant with an appropriate strand punching and cable assembly process. The critical current density at 4.2 K, 11 T of the Bruker- tape cable is 30% higher than in SuperPower-tape with a similar architecture. Remarkably, no critical current reduction was observed up to transverse pressure levels exceeding 170 MPa in the 10-strand SuperPower cables. In the 15-strand cables, transverse pressure limits as high as ≥ 400 MPa for the SuperPower tape and 455 MPa for Bruker tape were observed for the first time. These values by far satisfy the design requirements of the presently envisaged 20 T class accelerator demonstrator magnets.

For accelerator magnets, also the dynamic magnetic field quality is a key parameter which is affected by the cable’s inter-strand resistance Ra. Therefore, direct transport measurements of inter-strand resistance in impregnated ReBCO Roebel cables were carried out at 4.2 and 77 K. These data also constitute essential input for cable simulation models. Various intra-strand contributions to the overall inter-strand resistance value are estimated with a straightforward electrical network model. The measured inter-strand resistances are used to estimate AC coupling losses in different magnetic field orientations. For validation, the AC loss of Roebel cables is also measured at 4.2 K. Three analytical models are used to calculate the hysteresis loss and the results are compared with the measured data.

The average inter-strand resistance of the cable samples impregnated with the unfilled epoxy resin CTD-101K ranges from 3 to 18 µΩ at 77 K and from 1 to 10 µΩ at 4.2 K, respectively. The corresponding contact surface resistivity is estimated at 1 to 20 nΩm2 at 77 K and 0.5 to 10 nΩm2 at 4.2 K, respectively. The copper to copper interface between the tapes dominates the overall inter-strand resistance between the ReBCO layers in the impregnated Roebel cables.

Both analytical estimates and measured data on the CTD-101K impregnated Roebel cable show that the AC coupling loss is lower than the hysteresis loss throughout the investigated measurement range. However, a similar cable impregnated with the alumina-filled resin CTD-101G showed considerable coupling loss when exposed to magnetic field parallel to the wide cable face of the cable. The AC loss in both cables was also measured in terms of the applied magnetic field angle, using an orthogonal set of pick-up coils to record the in- and out-of-plane components of the magnetization separately. Somewhat surprisingly, the loss at intermediate magnetic field angles can be predicted quite well by considering the perpendicular and parallel components of the applied magnetic field separately and by taking the sum of the corresponding losses.