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AdV-SGD (FDS commissioning)
bertolini, capocasa, de laurentis, guo, sequino, sorrentino, vardaro - 0:06 Saturday 15 May 2021 (51794) Print this report
Green beam aligned to FCIM

The collimation of the green beam was improved yesterday by acting on the EQB1 telescopes, and then aligned again on the center of the SQB2_M1 and SQB2_M2 mirrors on SQB2. This morning we found the green beam sensibly misaligned on SQB2, because the SQB1 suspension was left uncontrolled by yesterday. Before engaging the SQB1 local controls again, we balanced the bench in order to recover the reference set points on which the IR beam from EQB1 was originally aligned. We added  temporary masses at symetric positions to lower the bench down to the nominal height in air; we updated the TY set point, we disabled the TY low noise mode, and we engaged the SQB1 local controls, Fig 1.

We then moved the steering mirrors on EQB1 manually to get the green beam well centered again on SQB2 optics. We found the SQB2 too high by more than 3 mm w.r.t the nominal position, so we added symetric masses close to the northern and southern sides of the bench to lower it. Fig 2.

We worked to align the green beam towards FCIM. At the beginning, on visual inspection of scattered light in the SQB1-FC link, the beam was pointing a bit eastwards. We first tried to align the beam along the S-N direction by acting on the SQB2 suspension. We changed SQB2_LC_TY by more than 8 mrad, Fig 3, then we realized this was not enough and we decided to block the bench and move the SQB2_M1 steering mirror manually. As the blocking system is not available at the moment, we used some temporary spacers, resulting in a bench position with large residual tilts, Fig 4. By manually steering SQB2_M1 while looking at FCIM from the sout-western viewport, we eventually found the beam hitting the mirror cage, Fig 5. However, when we tried to center the beam on the filter cavity input mirror, we found that the beam got cut in the upper part of the pipe. We then started to move SQB2_M2, looking for a symetric configuration in which we could steer the beam on the upper and lower sides of the FCIM cage with SQB2_M1. We found that this is only possible when the beam on SQB2_M1 is much lower the the center of mirror, Fig6. This is suggestive of a vertical mismatch between SQB1 and FCIM, requiring further investigation.

We centered the beam on FCIM, and then we looked for the retro-reflected beam on SQB2. We moved FCIM by acting on the marionette. We applied horizontal and vertical tilts by a few mrad, Fig 7, but we didn't find the retro-reflected spot. So we gave up for the moment, and we decided to try next week with a more systematic approach by applying a spiral to the FCIM tilt angles.

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Comments to this report:
polini, vardaro - 13:26 Tuesday 18 May 2021 (51828) Print this report

In order to investigate better the beam height problem we measured the height between the bench and the bottom of the filter cavity, see fig. 1. It is 2.5 cm, as noticed also on the mirror.

Moreover, we checked also the height of the last diaphragm on SQB2 before EQB2, and it seems that the beam would arrive few centimeters high on the viewport.


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