Today we blocked the beam on the LB to allow HJ. Bulten to work on the control of EIB (details in another entry)
We took advantage of having INJ in down to remove the set up that we added few weeks ago with Walid to be able to inject laser phase noise. We were indeed suspecting that it could be the responsible for the loss of phase margin of the IMC loop that has been seen these last days.
When HJ finished, we try to relock the injection but it was not relocking. After multiple tests we eventually noticed that the BPC was locked on the reference values which were quite far from the ones it had before putting INJ in down.
The alignment of the whole system had indeed drifted away during the long lock of the night+day.to follow the ITF.

After having relocked we blocked the beam again to allow Alain to do the restart of rtpc19.

Then everyting relocked correctly.

We measured the OLTF of the IMC with the attenuation of 20 dB (10+10) we had an UGF of 100 kHz and 14 degrees of phase margin
We added few dB of attenuation 22 (20+2) and we had an UGF of 93 kHz with 16.5 degrees of phase margin (see figure).
It is better than what we had few days ago but still not at the level of what we add when we measured it on the 04/03.