Virgo Logbook

Reports 1-1 of 1

AdV-COM (AdV commissioning (1st part) )

bersanetti, casanueva, ruggi - 15:39 Friday 28 March 2025 (66459)

Lock in LOW_NOISE_3 with RFC out of resonance

Yesterday we wanted to take some data, in good weather condition, in LOW_NOISE_3 with the RFC out of resonance (i.e. with the CARM_SLOW loop open); here the sequence of actions:

15:39:34 UTC: we tried to open the loop in-lock, but the interferometer unlocked;
16:23:25 UTC: we reached again LOW_NOISE_3, without closing the loop in the first place; during the acquisition, another slower CARM loop was disengaged from the DSP, and one IP was moved enough to bring the RFC out of resonance;
from 16:23:25 UTC to 18:00:00 UTC we were stably in LOW_NOISE_3 with the RFC out of resonance;
18:00:00 UTC: we started to bring back by hand the RFC on resonance, rolling back the two changes we made in bullet #2;
18:14:29 UTC: we tried to close the loop but we unlocked (usually we close it with the HighPower actuation, which has much more range to cope with it);
the following relock was in standard conditions.

Comments to this report:

mwas - 20:24 Sunday 30 March 2025 (66469)

There was no noticeable difference in BNS range when the RFC was off resonance in LN3.

Figure 1. Comparing data with RFC off-resonance (blue) and on resonance (purple), there is no change in spectrum at any ports of the interferometer. The only difference is for the RFC itself, where there is no light in transmission, and the light reflection becomes an imperfect indepent measure of the laser intensity noise. The clearance between the 1501Hz PSTAB and the noise level looks the same for the RFC reflection and for the PSTAB out-of-loop channel (PDd).

Figure 2. Looking at the OMC length spectrum as expected the noise is much higher without the RFC loop (blue), especially at 30mHz. The only exception is a bump at ~3Hz which is present when the RFC loop is closed (purple), I guess this is the frequency at which the RFC control loop of CARM has the phase going through zero, creating gain peaking.

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