Virgo Logbook

AdV-ISC (Commissioning up to first full interferometer lock)

ruggi, casanueva, spinicelli, chiummo - 17:48 Thursday 11 August 2022 (56723)

new PR parking position

As observed several time, the alignment of the cavities is not stable when the procedure starts after a long period of work with PR aligned (lock of CITF or CARM NULL). In fig 1, blue and red plots show two example: B7 and B8 demodulation w.r.t. ITM TX angular lines has a thermal transient. In one of the two cases, the effect is more evident because it comes from a longer period of work in CITF.

An exploration of different parking positions has been carried on. At first, the PR horizontal misalignment has been reduced from -60 to -25, while the cavity were locked. A backwards transient was triggered (see first 10000 s of fig 2), meaning that the difference between PR aligned and PR parked was reduced.

Later, a vertical misalignment has been added to the parking position, inducing a sudden additional offset to the cavity alignment error signals (fig 1, time 16000 s). The fact was interpreted as a further compensation of the difference with respect to the PR aligned positon.

At this point, PR has been aligned, CITF locked and left there for 45 min. Then PR has put back to the new parking positon and the cavity alignment has been engaged. No transient in the alignment error signals has been observed (see fig 1, yellow plot).

Looking now at the data, there is no reason to prefer this last parking position, with the additional misalignment in TX. So at the end the TX offset has been removed and the new parking position has been defined as deltaTY = -25 urad.

The next activity was the zeroing of the alignment error signal through the usual action of the beam, plus a refinement obtained engaging the loops on ITMs transversal position (fig 3, fig 4). The aim was to align the cavities as better as possible and see the difference in the quality of a subsequent CITF alignement. Unfortunately, this operation has been done in the 'diagonal' parking, compensating 1 mm additional vertical shift which probably should have not been compensated. In any case, troubles in the automation prevented the check of CITF alignment.

We will see the evolution of the error signals in this parking position and eventually we will make a further step for zeroing them.

Images attached to this report

Comments to this report:

ruggi - 12:16 Friday 12 August 2022 (56733)

New data can be added to the analysis (yellow plot in the attached figure). The alignment of the cavities done yesterday evening in the new parking position went on without big drifts, even if something was still visible, especially on north side.

Images attached to this comment

ruggi - 13:13 Friday 12 August 2022 (56734)

Two CITF locks are taken in account here:

1 - blue - gps=1344248818 - PR parking position and pre-alignment as bad as usual

2 - red - gps=1344248818 - first CITF lock after the attempt to optimize PR parking and pre-alignment.

Fig 1: error signals of cavity alignment in the conditions supposed to be referable to the respective CITF locks

Fig 2, Fig 3: figures of merit supposed to give information about the quality of CITF alignment.

The good pre-alignment appears to produce a CITF in a better shape. Quantitatively, the difference does not seem very important, but it is better to have an opinion from the experts.

To be taken into account that the second CITF lock occurred 6 hours after the first. The figures of merit have shown to be sensitive to other parameters, which stability in time is not perfectly known. Part of the changes we are showing could be due to drifts of those parameters.

Images attached to this comment

ruggi - 13:16 Monday 22 August 2022 (56795)

The new PR parking position has been used for 10 days. No disturbance from the fact that PR cavity is closer to the resonance has been observed: the power in the cavities during the pre-alignment phase looks as stable as usual.

The evolution of the out-of loop alignment error signals from dither lines is shown in fig 1 and fig 2: in the first part the parking position was the older one; then it was changed and the beam position was adjusted. The signals remained quite close to zero and the initial trend after the work with PR aligned looked more stable (this is better shown in figs 3-6 for each signal). To be more quantitative, the difference between the initial value and the value after 5000 s has been computed for NI TX and shown in fig 7. A reduction of the drift is evident, but the data say that a certain drift is still present.

Going back to fig 1, we have to remark that in the last data the north arm shows a progressive misalignment. Similar data for west arm are not available because the lock of the cavity was stuck for software reasons. We will evaluate in the next days the opportunity to recover a better alignment changing again the beam position.

Images attached to this comment

ruggi - 18:08 Monday 22 August 2022 (56804)

There is a mistake in the plots: the unit of the shifts is um, not mm.