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AdV-ISC (Alignment control scheme conceptual design)
vandael, Casanueva, Magazzu - 23:31 Tuesday 28 June 2022 (56297) Print this report
ISC Shift: Interferometer recovery

The goal of the shift was to recover the interferometer after changing the RH power.

Locking the CITF went very smoothly (we locked within 1 minute on the first attempt) and we didn't have to tune any phases except for B2_169MHz (minor change in demodulation phase) while doing the hand-off of CARM from the beating signals to the IR. After that we however got stuck trying to do the hand-off of DARM to the IR. We observed several things here:

  • We noticed that B1p_56MHz demodulation phase was drifting quite a bit. We left the interferometer locked for 10 minutes before doing the hand-off of DARM to observe the drift of the demodulation phase and attached are three different figures (last three) taken at different timestamps in the lock of the B1p_56MHz_I and B1p_56MHz_Q channel. We tune the phase by looking at the DARM line at 74 Hz and the figures show that the peaks of the I and Q channels change over time, which corresponds with the drift of the optimal demodulation phase in the LSC_B1p_DPHI channel that we saw. Nevertheless when we look at the entire spectrum to determine the optimal demodulation phase then the found demodulation phase seemed to be rather consistent. The question therefore is whether tuning the phase on the DARM line is a good figure of merit at this stage for finding the correct demodulation phase.
  • We noticed that going from 20 mW to 40 mW (On B7 & B8) by reducing the CARM_offset that the sign of the B1p_56MHz error signal would flip 180 degrees. 
  • At 40 mW the optimal demodulation phase seemed to be consistently good and not drift anymore. We therefore used the same demodulation phase for the last few hours of the shift and we didn't observe the drift of the demodulation phase anymore.

We tried to do the hand-off therefore at 40 mW which after some tuning of the gain seemed to work for just a few seconds. After optimizing the hand-off (tuning the gain and ramptime), we managed to do a hand-off lasting roughly 40 seconds after which we unlocked while playing with the gain of DARM (see first figure). We made one more attempt at the hand-off after this which lasted about 10 seconds after which we concluded the activity.

We leave the arms locked on the IR.

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casanueva, van dael - 10:22 Wednesday 29 June 2022 (56299) Print this report

Yesterday, at the end of the shift we made a last trial with a bit less gain, and the hand-off to the IR control filter just after. This procedure worked very well, and we got to a stable state that we unlocked by hand. Figure 1 shows the time plots of the hand-off: first we turn off the line of DARM, then we make the hand-off and just after we pass the filter to the IR one. FIgure 2 shows the fft of the final state and Figure 3 shows an fft done on the previous trial, that we just did the hand-off. Notice that a 74 Hz line with sidebands is present even when the DARM line is off.

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