After the work performed yesterday (42143) and the issue about the LB_L7 lens, whose picture is here attached, this morning we went to the laser lab to ultimate the alignment of the system.

 We started by aligning the acousto optics modulator on the LB along the beam path, because yesterday wa had to shift the beam of about 2 mm in order to realign it after the telescope whose lens was damaged and changed. Since it is the AOM used as an actuator for the power stabilization, we tuned the gain of the PSTAB loop.

 We realigned the PMC, whose transmitted power decreased during the night due to the thermal stabilization after the long intervention performed yesterday.

 When we tried to relock the injection system we encountered a problem with the alignment. We tuned the offsets on the AA error signals but this didn't solve the issue. We realized that the sudden oscillation and consequent IMC unlock was caused by alignment loop due to some glitches on the power of the SLED used for the local controls of the MC bench (look at the 2nd attached figure). Maybe these glitches are due to the aging of the SLED, since it has been operating for 4 years.

Anyway, an investigation about this issue will be done tomorrow by Ettore. In the meanwhile Paolo managed to close the automatic alignment loop and I measured the new AA sensing matrix. With this we were able to let the injection relocked, with the power stabilization loop engaged.

A comparative plot is given in figure 3. Since we lose around 8% of PMC transmitted power, we compensate this loss by adjusting the Input Power Control waveplate before the IMC to keep injecting 25 W into the ITF. As you can see from the plot we didn't go back exactly to the same power, but we increased it of 4% so that we have now 25.4 W instead of 24.4 W at the ITF port. From figure 3 it is also emerging a problem with the PSTAB loop. In fact, regarding the temporal signal many glitches are present (fig.4) which don't seem to come from the loop itself since they are at low frequency (around 20Hz). Despite these glitches the loop is closed, and work from tho other groups is going on. We'll investignate this problem as soon as possible.