The inter-tape resistance is a key parameter for REBCO coated conductors, cables, and coils for various applications. We explored the transverse resistance for sets of REBCO tape stacks, measured in a “through-the-stack” configuration. For the first measurements (Rig 1), the stacks were cooled to LN ₂ temperature and the tape stack resistance was measured as a function of applied pressure (i.e., the pressure was applied during the measurement) up to 417 kPa. We analyzed these direct measurements to examine the contributions of the tape surfaces vs the internal tape contributions, and the results were translated into a contact efficiency, η, defined as η = R _c *A _c (the contact resistance * contact area). It was found that the electrical contact efficiency, η, for an un-pressurized stack was ≅ 0.934 Ω·cm ² , but was reduced by a factor of 8 under 417 kPa of pressure to η ≅ 0.111 Ω·cm ² . A second test rig was used to explore tape stack resistances from 4.2 K - 77 K. This rig allowed pressures of up to 9.94 MPa to be applied. Here, the pressure was first applied, and then the sample was cooled. This difference between Rig 1 and Rig 2 allowed us to compare the protocols of fixed pressure (Rig 1) and fixed constraint (Rig 2). The contact efficiency reached 178 μΩ·cm ² at 4.2 K and 232 μΩ·cm ² at 77 K.

Rare earth barium copper oxide (REBCO) coated conductor tapes and cables are promising material candidates for constructing high field magnets. In both the magnet constructing stage and operation stage, transverse pressure is exerted on the tapes/cables. Therefore, understanding the mechanical behavior of tapes/cable under transverse pressure and figuring out certain properties, such as modulus and stress limits, will be helpful for magnet design and magnet simulation study. We accomplished these goals by performing compression tests on several commonly used tapes/cables in high field magnets. We found that the mechanical behavior of tape stacks can be characterized in three distinct regimes. In the first regime (60 MPa), the REBCO tape stack and its Ni-plated derivative had a modulus of 10 $ \pm $ 11% GPa and 4.6 $ \pm $ 9% GPa respectively. Roebel cable had similar mechanical behavior to the tape stack with three regimes, and the second and third regimes showed a modulus of 3.1 $ \pm $ 5% GPa and 9.1 $ \pm $ 11% GPa respectively. The conductor on round core (CORC) cable was more vulnerable to stress, and its fracture stress was around 27 MPa.

Rare-earth barium copper oxide (REBCO) coated conductors are of interest for fabricating high performance cables and magnets for magnetic field B > 22.5 T. One critical challenge is the control of the current sharing between conductor tapes to enable self-protection to take place when a localized disturbance (hot spot) appears in the cable or magnet coil. Current sharing can be enhanced by reducing the inter-strand contact resistance (ICR). In this work, we explore ICR reduction with several tape surface modification techniques. Under a pressure of 13.3 MPa the contact efficiency , η _int of a ten-layer REBCO tape stack was measured to be 106 $\mu$ Ω*cm ² at 4.2 K and 145 µΩ*cm ² at 77 K. Further increases in pressure had only a small effect on ICR. To further reduce ICR, we proposed two other techniques: (1) Heat treatment of the ten-layer YBCO coated conductor tape stack at 300 °C for 30 min and under various pressures. We concluded that 20 MPa was the minimum pressure required to initiate substantial conductor-to-conductor sintering and an optimum result was achieved by a processing pressure higher than 23.4 MPa which produced η _int of 10 µΩ*cm ² at 4.2 K and 19 µΩ*cm ² at 77 K when measured under 13.3 MPa. (2) The second approach was to Ni-plate the REBCO tapes. The acidic plating solution removes the native oxide on the Cu surface and the process replaces it with a thin layer of Ni. The result was an η _int of 2.7 µΩ*cm ² at 4.2 K and 4.6 µΩ*cm ² at 77 K. Both techniques led to η _int that were relatively insensitive to changes in temperature. We were able to achieve η _int < 10 µΩ*cm ² with sintering under pressure, and η _int < 3 µΩ*cm ² with Ni electroplating alone.

Current sharing is an important self-protection mechanism in rare earth barium copper oxide (REBCO) coated conductor (CC) tapes and cables that are intended for the construction of high field magnets. Because of the slower quench propagation rate of such cables, we expect that cooling is also required to facilitate the current sharing. We explored this idea by constructing FEM models that include liquid helium cooling and inter-strand contact properties. The results showed that given metallic inter-strand contact, a three-layer tape stack with a defective central tape could carry at least 3.05 I_c (where I_c is defined for an individual tape). When the inter-strand electrical contact efficiency, η ( η = inter-strand contact resistance ∗ contact area), was adjusted to be 5400 µΩ*cm ² and the inter-strand thermal insulance, ω ( ω = inter-strand thermal resistance ∗ contact area), was 5.54 K*m ² /W, the maximum current that could flow through the defective tape was 0.6 I_c with negligible current sharing to neighboring tapes. After reducing η to 10 µΩ*cm ² and ω to 0.01 K*m ² /W respectively, current sharing to neighboring tapes increased by 800 times, and the hot spot temperature was reduced. However, if the inter-strand thermal contact was insulating, regardless of the value of η, the same tape stack exhibited thermal runaway. Hence, current sharing in REBCO coated conductors is determined by both the inter-strand contact resistance (ICR) and the inter-strand thermal resistance (ITR).

Compared to Nb₃Sn- and NbTi-wound superconducting undulators (SCUs), MgB₂-wound SCUs are of interest for future electron synchrotron beam light sources owing to their higher temperature operating margin and associated stability. In this study, a three-period undulator consisting of twelve racetrack coils wound with 2nd generation (2 G) multifilamentary advanced-internal-magnesium-infiltration MgB₂ strands were fabricated and tested in liquid and gaseous helium (He) over a temperature range of 4.2 K–20 K. The coil winding cross sections (in each coil) were 5 mm wide and 4.8 mm thick. At 4.2 K, a critical current (I _c) of 325.7 Amps produced a maximum undulator bore field of 1.19 T. It should be noted that the short, 3-period nature of the coil led to an asymmetry in the field profile (the maximum positive field was 1.19 T, the maximum negative was −0.25 T), suggesting a peak field of 0.72 T in the absence of end effects. Finite element modeling (FEM) results of simulations for a one meter long undulator of otherwise identical design gave 0.85 T (larger because of higher currents enabled by the lower field). But in any case, the I _c value coil reached is 94% of that of the short sample (dictated by the 1.19 T positive field for the coil as tested). FEM was performed to study the magnetic field profile, which was validated experimentally. The magnetic field was measured using a Hall probe which was translated along the beam axis during measurement to explore the spatial field variation along the beam travel direction. The spatially alternating field was asymmetric, and the maximum field was more prominent in the positive direction than in the negative direction, the difference being due to broken symmetry, that is, short coil end effects. In this work, we show useful fields are possible for MgB₂ undulators; the use of such conductors can allow a larger thermal margin and enable conduction-cooled operation.

Carbon nanotubes (CNTs) are an ideal starting material for the development of strong, light‐weight conductors. In this work, we pursued the development of high specific electrical conductivity and high specific strength CNT/metal composites. We started with CNT yarns which had an initial electrical conductivity,σ_e, of 3.14 MS m^‐1and a density of 1.32 g cm^‐3. We brominated the yarns, demonstrating increases inσ_e, with our best samples reachingσ_e = 7.00 MS m^‐1. To increase the stability of the bromination, we electroplated a Cu layer onto the Br‐doped yarn which led to a stabilization of the conductivity improvement. This was quantified by monitoring periodically theσ_eof a Br‐doped and metallized CNT yarn over a period of 69 days, during which time the sample was otherwise exposed to ambient conditions. Analysis gives a value 6.15 MS m^‐1for the brominated yarn after metallization which is excellent for CNT yarns. Tensile tests on these Cu/CNT composites showed tensile strengths reaching 700 MPa, Young's modulus values of 22.8 GPa, and specific tensile strength values of 146 kN*m kg^‐1(this latter is 6X that of Cu). Our best CNT‐Cu composites show specific conductivity values comparable with that of Cu but with much higher specific tensile strengths.