aLIGO LHO Logbook

H1 ISC (PSL)

jenne.driggers@LIGO.ORG - posted 10:58, Monday 07 August 2023 - last comment - 10:46, Wednesday 04 October 2023(72027)

Cycled ISS Second Loop - increased diffracted power

The ISS Second Loop engaged this lock with a low-ish diffracted power (about 1.5%). Oli had chatted with Jason about it, and Sheila noticed that perhaps it being low could be related to the number of glitches we've been seeing. A concern is that if the control loop needs to go "below" zero percent (which it can't do), this could cause a lockloss.

I "fixed" it by selecting IMC_LOCK to LOCKED (which opens the ISS second loop), and then selecting ISS_ON to re-close the second loop and put us back in our nominal Observing configuration. This set the diffracted power back much closer to 2.5%, which is where we want it to be.

Comments related to this report

jeffrey.kissel@LIGO.ORG - 12:33, Friday 08 September 2023 (72762)CAL, IOO

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This cycling of the ISS 2nd loop (a DC coupled loop) dropped the power into the PRM (H1:IMC-PWR_IN_OUT16) from 57.6899 W to 57.2255 over the course of ~1 minute 2023-Aug-07 17:49:28 UTC to 17:50:39 UTC. It caught my attention because I saw a discrete drop in arm cavity power of ~2.5W while trending around looking for thermalization periods. 

This serves as another lovely example where time dependent correction factors are doing their job well, and indeed quite accurately. If we repeat the math we used back in O3, (see LHO:56118 for math derivation), we can model the optical gain change in two ways:
    - the relative change estimated from the power on the beam splitter (assuming the power recycling gain is constant and cancels out)
    relative change = (np.sqrt(57.6858) - np.sqrt(57.2255)) / np.sqrt(57.6858) 
           = 0.0039977
           = 0.39977%
    
    - the relative change estimated by the TDCF system, via kappa_C
    relative change = (0.97803 - 0.974355)/0.97803
           = 0.0037576
           = 0.37576%
    
indeed the estimates agree quite well, especially given the noise / uncertainty in the TDCF (because we like to limit the height of the PCAL line that informs it). This gives me confidence that -- at least over the several minute time scales -- kappa_C is accurate to within 0.1 to 0.2%. This is consistent with how much we estimate the uncertainty is from converting the coherence between the PCAL excitation and DARM_ERR into uncertainty via Bendat & Piersol's unc = sqrt( (1-C) / (2NC) ).

It's nice to have these "sanity check" warm and fuzzies that the TDCFs are doing their job; but also nice to have detailed record of these weird random "what's that??" when trending around looking for things.

I also note that there's no change in cavity pole frequency, as expected.

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camilla.compton@LIGO.ORG - 10:46, Wednesday 04 October 2023 (73266)TCS

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When the circulating power dropped ~2.5kW, kappa_c trended down, plot attached. This implies that the lower circulating powers induced in previous RH tests 73093are not the reason kappa_c increases. Maybe see an slight increase in high frequency noise as the circulating power is turned up, plot attached.

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