Since Issue 3 of the HL-LHC CB Newsletter, three of the six series magnets were tested at cold [S1, S2, and S3], and another one [S4] is ready for testing on a test bench in SM18. On the side of production, the construction of the cold mass assembly of the first spare magnet has started. It is the fifth magnet of the series. As for the connection cryostats requested for the installation of collimators at IP2, they have been successfully installed and connected in the accelerator.

The first series magnet, S1-LMBHB002, was tested during summer 2019. It required only two quenches to reach the target current at 1.9 K, 11850 A (nominal design current for beam energy of 7 TeV) + 100 A. It also reached the target current at 4.5 K and did not require retraining after thermal cycle. Finally, it withstood 330 electromagnetic cycles between 5 and 11.85 kA at 1.9 K, with a ramp rate of 100 A/s.

The second series magnet, S2-LMBHA001, was tested at cold between November 2019 and June 2020 during four runs separated by full thermal cycles, including a forced interruption before the end of the second run because of the lock-down imposed by the COVID-19 crisis. The magnet had an insulation to ground weakness revealed at cold during the HV tests, at 2kV, and spikes on the coil voltage signals after quench. This required the exchange of the capillary tube and diagnostics cables between the first and the second runs. This action cured the insulation to ground fault, not the spikes. The magnet reached the target current after three quenches during the first run, and one quench during the second run. Although it went straight away to nominal current at the beginning of the third run, it started thereafter exhibiting instability with clear conductor degradation in one of the coils of Aperture 2. Further degradations were observed in another coil during the fourth run, this time in Aperture 1. There are strong indications that these degradations are linked to the repeated thermal cycles. However, electromagnetic cycles did not affect the conductor integrity.

In the meantime, the third series magnet, S3-LMBHA002, was also tested at cold. One of the four coils quickly showed limitations during the first run.

Overall, a lot has been learned about the behaviour of these first series Nb3Sn accelerator magnets but the magnets S2 and S3 cannot be used in the accelerator. Investigations and root cause analysis are ongoing to understand the reason of the spikes in S2, and of the degradations after repeated thermal cycles, also in S2.

Efforts now concentrate on the testing and finalization of S4 and S5 as the first pair for installation in the LHC during LS2.