Starting from Tuesday 22 afternoon, after the valve was open again, we started the recovery of the interferometer.
The first, very long step was to find again the West arm alignment and, then, signals on the B8 photodiodes. We could find rather easily the beam on the WE markers by changing orientation of the BS, so to have a rough alignment, then we used the locked North Arm to get a reasonable orientation of the WI by superposition on the B1p camera. However, after all this we could not find signals on B8 even scanning the WE orientation with the "snail".
We also used the reflection of the ALS beam back on the bench, and the spot on the WI mirror in order to improve such alignment.
This morning we changed strategy and we only aligned the West Arm, in order to see the flashes in reflection on the B4 PD up to a similar level to what only the North Arm aligned could provide.
While doing all of the above, we tried to follow with the SWEB alignment (and the vertical position of WE, this one with no effect), which could lead to marginal but visible improvements.
Later, we tried, after we were sure to have the cavity well aligned by hand, to make a "snail" on the SWEB bench itself; this did not work as expected, but a wide change in its orientation actually helped to find again good signals on the bench, which had a very different working point with respect to before. We've been probably chasing a ghost beam before.
We could then recover the lock of the West Arm, which went quite smooth:
- in order to do that, we already made a change in LSC_Acl yesterday, in order to rescale all the B8 signals given the specifications of the new WE mirror (3 ppm of transmission instead of 4 ppm). Mimicking the configuration of the NE (which changed in the same way at the end of 2023), we used a gain of 1.27. After the initial lock of the single West Arm (with a cold ITF and thermal actuators still to be tuned), this seems a bit overestimated, and 1.23 a better value for it. But we will need a more precise measurement in higher configurations;
- once the lock of the arm was engaged, the drift control of the SWEB bench moved considerably its alignment, so we waited a lot in LOCKED_ARMS_IR to have both the benches and the arms' dithering loop to converge; the new setpoints for SWEB have been saved, the configuration not reloaded;
- we recovered most of the Automation by reverting all the changes reported in entry 66517, with the exception of the ITF_STATUS and ITF_CONDITIONS nodes.
Then we moved on with the lock acquisition, getting up to the lock of the DRMI without any problems. We will have to check later if the working point of the West ALS is the best one, but we could lock easily and keep the lock. Once we locked the DRMI, we checked also the SDB1 alignment and we re-engaged the floating setpoints.
We could go then towards CARM_NULL_1F without the need of any adjustment. Once there, after an initial thermalization, we noticed the B1p_DC fringe signal being twice as usual; therefore we started the tuning of the WE Ring Heater, in order to at least reduce the fringe up to a reasonable level before opening of the SDB1 shutter.
We are currently in the RH transient, we'll monitor the situation for the next hours and decide when to do the next step of the recovery.
