In May 2022, the HL-LHC AUP project successfully completed an endurance test on an individual magnet (MQXFA05) in vertical position. This step, foreseen for the final cold mass assembly in horizontal position, was anticipated to this intermediate assembly stage to gather more information on the long term behavior of Nb3Sn magnets. The test involved completing a total of five thermal cycles and 50 high current quenches, eight of them being spontaneous quenches during the initial training and 42 provoked quenches at nominal current via the quench heaters. The first three thermal cycles were carried out in 2021, and the last two in April-May 2022. At the end of this campaign the magnet reached the required nominal current plus 300 A margin without quench. Moreover, nominal current was reached at 4.5 K, without quench, proving the large temperature margin already observed during the first powering (see Figure 1).
After the successful test of four magnets in the vertical test station (MQXFA03-06), two performance limitations were found during the vertical tests of magnets MQXFA07 and MQXFA08; both magnets did not meet the requirements with similar phenomenology to what was observed in the test of the second short model (MQXFS3). In both MQXFA07 and MQXFA08 the performance limitation is located in the lead end of a single coil and it shows a reverse behaviour pattern, i.e. nominal current was reached only at higher temperatures (4.5 K) and higher ramp rates. The disassembly of MQXFA07 has been completed in February 2022, and the limiting coil was sent to CERN for tomography and metallographic inspection. The disassembly of MQXFA08 has been completed in May 2022. Two possible mechanisms of performance limitation are being analyzed; the first hypothesis is an issue related to the magnet assembly, and in particular to the mechanical coupling between magnet collars and the alignment key in each coil pole. The second hypothesis is an issue related to the coil manufacturing, i.e. the presence of popped strands in the coil end revealed by the tomographies. Both coils had the manufacturing process paused during the COVID-19 lockdown of Spring 2020, and were completed after the pause imposed by the first pandemic wave in the USA. After the test of MQXFA08, the coil production was put on hold to review the QC documentation related to the coil fabrication and magnet assembly, with no determination of a “smoking gun”. A detailed root-cause analysis is still ongoing, but coil production was then restarted in March 2022. Tests of the next magnet, MQXFA10 is planned for after the termination of the endurance test of MQXFA05.
Coil production is ongoing at BNL and FNAL, with approximately 50% of the coils completed. Magnet assembly is ongoing at LBNL, and approximately 40% of the magnets have been completed. Following the endurance test of MQXFA05 (destined - together with MQXFA06 - to be assembled in the second cold mass later in 2022) magnet MQXFA10 is ready on the sideline for vertical test at BNL, with MQXFA11 following shortly thereafter.
At FNAL the construction of the first cryostated cold mass is ongoing, including MQXFA03 and MQXFA04 that were successfully tested in the BNL vertical setup by fall 2020. New alignment and welding tools were procured and setup in a new building made available to the HiLumi AUP project in FNAL. The experiences of CERN magnets MQXFBP1 and MQXFBP2 contributed to realize the criticality of maintaining a controlled and low additional pre-stress on the magnets due to the welding process of the stainless-steel shell. New cold mass assembly procedures were developed in 2021, allowing to proceed with the final assembly in early 2022. Low-Cobalt Stainless Steel shells for the He vessel, procured by AUP through CERN, have been formed and longitudinally welded around MQXFA03 and 04 in April 2022 (Figure 2). Coupon testing and visual inspection were satisfactory and preliminary QC measurements on the alignment of the two magnets performed with a stretched wire system showed compliance with the Q1/Q3 Acceptance Requirements at room temperature. Integration of various components (heat exchangers, beam tube, current buses...) followed, with the usual observation of integration issues expected during the first execution of a new assembly (see Figure 3). The cold mass/cryo-assembly team at FNAL with the full collaboration of CERN engineers and scientists were able to address all issues encountered. Recently, the team completed welding of the endplates (see Figure 3). In June the completed cold mass will be inserted in the vacuum vessel with CERN-provided tooling to obtain the first cryo-assembly. The horizontal test at FNAL is expected to take place in late-summer/fall 2022.
