Today after the input power was reduced to 27W (IMC transmitted power), we started with the realignment of the central interferometer (CITF). But first we locked both arms on the IR beam using the NARM and WARM nodes of metatron.
The alignment of the CITF consists basically in two steps. First to form an optical cavity between PR and NI and secondly between SR and WI.
For PR-NI cavity:
We first start misaligning NE, WE and WI. Since PR was misaligned at the beginning, with 26 urad in yaw and 22 urad in pitch. We had to align PR by taking B1p beam spot on B1p camera as reference, in order to study the power built up in the cavity formed by PR and NI. PR was moved until the secondary reflection superposed with the principal reflection. Once it was posible to see only one beam on B1p, we saved the values of this position of PR on metatron. Figure 1 shows this first step.
For SR-WI cavity:
Start by misaligning NI and aligning WI. SR has been on purpose misaligned by Paolo and Ettore in August 2020 (logbook entry #49249) and updated by DET subsystem in november 2020 (logbook entry #49985). With -672 urad in yaw and 28 urad in pitch. In order to study the power built up in the cavity formed by SR and WI, we moved SR checking the position of the multiple reflections shown on B1p camera. Once the multiple reflection superposed onto one beam on B1p camera, we saved the new values of SR position on metatron. They changed by 85 urad in pitch and -192 in yaw. Figure 2 summarises this second step.
Then the aligned again NI to have PR and SR cavities on resonance. We left this alignment alone some time, to check if there are some important drifts on the signal. But htere where no changes in ~30min.
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After confirmation that the mirrors were not drifting, we repeated this process again to confirm the steps of the process and for training.
Therefore, we first brought back the ITF to its initial state: We misaligned SR and PR in yaw. After that, we locked both arms using the metatron nodeos for NARM and WARM.
Again, we proceded with he misalignment of the NE, WE and WI. Also with the alignment of PR. We double checked that the cavity formed by PR and NI was well aligned with the values previously saved on the corresponding metatron nodes. This confirmation was done by misaligning PR by hand 10urad in pitch and coming back and then 10urad in yaw and coming back. From this misalignments, we could see the corresponding multiple reflection in vertical and horizontal on B1p camera correspondingly.
We continued with the misalignment and parking of PR mirror and alignment of the SR mirror together with the WI. At this point SR was 2 urad misaligned in yaw, therefore we corrected it. Then moving the SR in yaw by 10rad, we confirmed that it was well aligned. Then the new position was seved on the SR metatron node.
Finally we aligned again NI and PR. using the saved positions on metatron. This second alignment procedure can be checked in figure 3.
As it can be seen in the previous figures, B1p PD does not get more than 2mW in this entire process, which should assure that this can be repeated with no risk of damaging the DET PDs.
During preparation and conversation with DET subsystem about possible issues that might come up in the following shifts, we left the alignment like this for two hours.
In figure 4 can be seen that the mirrors were not drifting and therefore the power on B1p and B4 remained basically constant.
While we were trying to realign the green beam on the north arm. We noticed that in the cavities relocked on the IR, the transmitted power shared the same kind of oscillation and spikes in both directions (higher and lower power). Meaning that it is due possibly to slightly recycling effect. We thought about this, because the last parking position that we found for the PR was less far away that we used in the past. We then moved PR MAR TY by -10 urad and we saw immediately the reduction of this effect. However, overtime this effect comes back. So We moved it again -10 urad, and the effect remained there. Due to this misbehaivour, and for the sake of safety we left the arms unlocked. Figure 5 summarises this last part.
