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The design and fabrication of a 32 T, 32 mm cold bore superconducting magnet with high field REBCO inner coils is underway at the NHMFL. In support of the design, conductor characterization measurements have been made including critical current as a function of field, field orientation, temperature, and strain on conductors and joints. Various conductor and turn insulation systems were examined. The selected coil fabrication method for the 32 T magnet is pancake wind, dry wind coils with sol-gel insulation on a stainless steel co-wind. Quench protection of the REBCO coils by distributed heaters is under development. Small REBCO coils have been made and tested in a 20 T background field to demonstrate performance of the technology. The design of the 32 T magnet is described, including coil configuration and conductor lengths, fraction of critical current, selection of conductor copper content for protection, and stress in the windings.Index Terms-High field superconducting magnets, insulation, quench protection, REBCO.

In recent years, high temperature superconductors (HTS) have been developed intensively for different applications such as superconducting transformers, high field magnets, fault current limiters or current leads. HTS based conductors used for these applications are multilayered structures, which could be coated by a metal e.g. by Cu or stainless steel. Depending on the constituents, the thermal conductivity of the whole conductor could vary over several orders of magnitude. For the modelling of conductors with desired thermal properties the thermal conductivities of individual layers have to be known. For this goal, thermal transport properties of each constituent have to be measured separately. In this paper, we present results of thermal conductivity measurements of the commonly used substrates for the HTS, namely, the NiW tapes.

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No-insulation (NI) REBCO magnets have many advantages. They are self-protecting, therefore do not need quench detection and protection which can be very challenging in a high Tc superconducting magnet. Moreover, by removing insulation and allowing thinner copper stabilizer, NI REBCO magnets have significantly higher engineering current density and higher mechanical strength. On the other hand, NI REBCO magnets have drawbacks of long magnet charging time and high field-ramp-loss. In principle, these drawbacks can be mitigated by managing the turn-to-turn contact resistivity (Rc). Evidently the first step toward managing Rc is to establish a reliable method of accurate Rc measurement. In this paper, we present experimental Rc measurements of REBCO tapes as a function of mechanical load up to 144 MPa and load cycles up to 14 times. We found that Rc is in the range of 26-100 µΩ -cm 2 ; it decreases with increasing pressure, and gradually increases with number of load cycles. The results are discussed in the framework of Holm's electric contact theory.

Screening currents and their effect on the magnetic field and strain state have been shown to be a major problem in the design and operation of rare-earth-barium-copper-oxide magnets, distorting the field and rotating the conductor to potentially large strains. The latter is a possible catalyst for damage as both plastic deformation and degradation of the critical current leading to reduced fatigue life or even catostrophic failure. Due to the nonlinear dynamic behavior of the screening currents and the significant possible rotation, including this rotational effect in the electromagnetic state requires a new addition to the existing models. The effect of the changing rotation angle of the conductor on the electromagnetic and stress state is investigated by using a modified homogeneous T–A method. Numerical results are compared with experimental tests.

No-insulation (NI) superconducting REBCO magnets have advantages of self-quenchprotection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (R c ). In an effort to control R c , we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of R c of an as-received sample under cyclic contact pressure of 2.5-25 MPa up to 30 000 cycles. After an initial increase in R c for the first 10-20 cycles, R c decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low R c resulting from a previously high number of load cycles. We also studied R c of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase R c , we experimented with electro-and electroless plating of Ni, Cr, and Ni-P. We also measured R c with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases R c by about three orders of magnitude; while the Cu plated SS interlayer only increases R c by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the R c to be controlled over a wide range.

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