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.

In a no-insulation (NI) REBCO magnet, the turn-to-turn contact resistivity ( c ) determines its quench selfprotection capability, charging delay time and the energy loss during field ramps. Therefore it is critically important to be able to control a range of  c values suitable for various NI magnet coils. We used a commercial oxidizing agent Ebonol  C to treat the copper surface of REBCO tapes. The copper oxide layer was characterized by cross-sectional transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). The oxide layer formed in Ebonol  C at 98 °C for 1 min is Cu 2 O of about 0.5 m. The  c between two oxidized REBCO is in the order of 35 m-cm 2 at 4.2 K which decreases slowly with contact pressure cycles. The  c increases but only slightly at 77 K. We also investigated the effect of oxidation of stainless steel co-wind tape on  c . The native oxides on 316 stainless steel tape as well as those heated in air at 200 -600 °C were examined by TEM and XPS. The native oxides layer is about 3 nm thick. After heating at 300 °C for 8 min and 600 °C for 1 min, its thickness increases to about 10 and 30 nm respectively.For the stainless steel tapes with about 10 nm surface oxides, pressure cycling for 30,000 cycles decreases  c by almost 4 orders of magnitude. Whereas at 77 K, it only changes slightly. For a surface with 30 nm oxide, the  c decreases moderately with load cycles. The results suggest that for an oxidized stainless steel to achieve stable  c over large number of load cycles a relatively thick oxide film is needed.

With the successful test of the first two pre-series magnets the US HL-LHC Accelerator Upgrade Project has started production of the MQXFA magnets to be used in Q1/Q3 inner triplet elements of the HL-LHC. This good start comes after the test of two prototypes with limited performance, and it demonstrates the importance of learning from past issues.Therefore, in this paper we want to share the most important lessons learned so far, focusing on those which may be more interesting for similar projects. We will also present the status of MQXFA fabrication in the US.

The High-Luminosity LHC Accelerator Upgrade Project (AUP) in the U.S. will construct quadrupole magnets to be delivered to CERN. An initial 3 tons, over 600 km total length, of conductor was procured under the LHC Accelerator R&D Program (LARP) for this project. Programs for quality control (QC) at the supplier and quality verification (QV) at the laboratories were solidified into components of the overall quality plan for strand procurement under AUP. Measurements of the critical current (Ic) and residual resistance ratio (RRR), and related probes and techniques, are central to the quality plan. Described below is the verification testing that has taken place at the National High Magnetic Field Laboratory (NHMFL). Testing challenges are presented by the high sensitivity of these wires. In addition, new RRR test software was developed to accommodate challenges presented by meeting international standards with existing configurations of test strands.