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Entry  Tue Feb 4 02:13:02 2020, gautam, Update, LSC, Locking updates modifiedOutMat.pngOLTFs.pdf
    Reply  Wed Feb 5 16:35:12 2020, gautam, Update, LSC, Diagnosis plan ALSnoise.pdf
       Reply  Thu Feb 6 01:16:58 2020, gautam, Update, LSC, Diagnosis results IMCspec_ALS.pdfIMCspec_ALS_smallSpan.pdf
    Reply  Thu Feb 6 01:25:50 2020, gautam, Update, LSC, Locking updates semiStableArms.pngarmAngStability.png
       Reply  Fri Feb 7 02:24:24 2020, gautam, Update, LSC, Some short notes 
Message ID: 15191     Entry time: Thu Feb 6 01:16:58 2020     In reply to: 15188
Author: gautam 
Type: Update 
Category: LSC 
Subject: Diagnosis results 

Summary:

I did some more detailed tests to see if I could isolate where the excess ALS noise at low CARM offset is coming from, by measuring the spectrum of the IMC error point (in loop). The results, shown in Attachment #1 and #2, are inconclusive.

Details:

Since MC_F didn't show any signatures of elevated noise, I decided to hook up an SR785 to the A excitation bank TEST1 input of the IMC servo board to monitor the in-loop error signal. I initially took a few measurements spanning 800 Hz in frequency, and to my surprise, I found that there was elevated noise in the frequency band we see an increase in the ALS noise, even when the CARM feedback goes to the ETMs (so the IMC cavity is in principle isolated from the main interferometer). This is Attachment #1. So I re-took a couple of measurements (this time only for the case of CARM feedback to the ETMs), with a 200 Hz frequency span, and found no significant noise elevation. This is Attachment #2. I am led to conclude that the IMC error point level changes over time for reasons other than the CARM offset - it'd be nice to have a spectrogram of the IMC error point and compare excursions relative to the median level over a few 10s of minutes, but we don't have this data stream digitized by the CDS system - maybe I will hijack the MC_L channel temporarily to record this data stream. It seems a waste that we're not able to take full advantage of the measured <10pm RMS noise of the IR ALS system.

Attachment 1: IMCspec_ALS.pdf  251 kB  Uploaded Thu Feb 6 15:36:04 2020  | Hide | Hide all
IMCspec_ALS.pdf
Attachment 2: IMCspec_ALS_smallSpan.pdf  213 kB  Uploaded Thu Feb 6 15:36:11 2020  | Hide | Hide all
IMCspec_ALS_smallSpan.pdf
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