The aim of the shift was to further progress in the CARM offset reduction procedure.
Both arms green lasers were locked at 7.10 UTC but the ALS_NEB_PD_REFL signal (reflection green laser signal) showed too large power fluctuations for the lock to be stable (see figure 1 and 2 for a comparison between the West arm and North arm signals fluctuations). These fluctuations were related to the SNEB oscillations. In particular, glitches on SNEB_LC_TY caused spikes in the reflection PD signals and, ultimately, unlocks.
The issue was solved by moving the SNEB_LC_Z from -310 to -280 um. While this removed the large spikes on TY and the North green laser unlocks, the fluctuations on ALS_NEB_PD_REFL remained highly correlated with SNEB_LC_TX, with a high coherence peak at ~0.5 Hz.
After this, we proceeded with locking CARM and DARm on the green signals 8.23 UTC, and then 2 kHz of CARM offset were subtracted.
The DRMI was then aligned and locked at 8.44 UTC and a successful handoff to the 3f signals was first obtained at 9.04 UTC.
We proceeded reducing the carm offset. Several attempts of reducing the carm offset were performed during the shift, all concluded with unlocks. The B1p and B4 DC triggers were disabled each time 500 Hz of offset was reached. The lowest carm offset reached was 150 Hz at 9.18.40 UTC. THis lock lasted ~30 seconds and then the DRMI unlocked simultaneously with a power spike on the B7 and B8 signals, indicating a sudden passage trough the IR arm resonance (see figure 4). No particular glitches were shown on the CARM and DARM signals in that moment. Following, the list of the other carm reduction attempts unlocks:
| UTC | OFFSET [Hz] | figure | probable cause |
| 9.19 | 150 | 4 | IR resonance spike |
| 10.37 | 500 | 6 | PRCL drift |
| 10.53 | 500 | 5 | MICH saturation |
| 12.03 | 1000 | ||
| 12.10 | 400 | 7 | MICH oscillation and saturation |
| 12.16 | 500 |
During most of these unlocks, the shutters on B2 PD1, B2 PD3, and B7 closed and required a rearm of the PDs.
During the shift the CARM and DARM loops unlocked 9.28 11.00, 12.20. Most of the times they relocked immediately without particular issues, except for two periods in which high frequency noise tampered with the lock stability (between 7.50 and 8.20 for WEB and between 12.40 and 12.55 on NEB). At 12.55 the NEB BPC tilt x setpoint was changed from 0.28 to 0.32 to recover the stability of the NARM ALS lock.
The activity continued in the following shift.
