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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: 15192     Entry time: Thu Feb 6 01:25:50 2020     In reply to: 15185     Reply to this: 15195
Author: gautam 
Type: Update 
Category: LSC 
Subject: Locking updates 


I managed to partially stabilize the arm citculating powers - they stay in a region in which the REFL 11 signal is hopefully approximately linear and so I can now measure some loop TFs and tweak the transition appropriately. 


The main change I made tonight was to look at the REFL11 signal as I swept the ALS CARM offset through 0. I found that the maximum arm powers coincided with a non-zero REFL11 signal value (i.e. a small CARM offset was required at the input to the CARM_B filter bank). Not so long ago, I had measured the PM/AM ratio for 11 MHz to be ~10^5 - so it's not entirely clear to me where this offset is coming from. Then, I was able to turn on the integrator (z:p = 20:0) in the CARM_B filter bank while maintaining high POP_DC. At this point, I ramped up the IN2 gain on the IMC servo board (= AO path), and was able to further stabilize the power. 

Attachment #shows this sequence from earlier in the evening. Note that in this state, both ALS and IR control of CARM is in effect. The circulating power is fluctuating wildly - partly this is probably the noisy ALS control path, but there is also the issue of the (lack of) angular control - although looking at the transmon QPDs and the POP QPD signals, they seem pretty stable.

The next step will be to try and turn off the ALS control path. Eventually, I hope to transition DARM control to AS55 as well. But at this point, I can at least begin to make sense of some of the time series signals, and get some insight into how to improve the lock.


No systematic diagnosis scheme comes to mind.

Attachment 1: semiStableArms.png  324 kB  Uploaded Thu Feb 6 18:08:58 2020  | Hide | Hide all
Attachment 2: armAngStability.png  260 kB  Uploaded Thu Feb 6 18:30:45 2020  | Hide | Hide all
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