Jenne notes that this changing high frequency noise during the first hours of a lock since Wed 25th Oct (purple trace in 73798) may be caused by the new higher CARM gain 73738, changed on that day.
In 73798, I noted The 4.8kHz noise that suddenly changes ~ 1h40 into NLN, Jenne suggests this could be aliased down CARM gain peaking. Looking at the 64kHz channel, plot attached, noise disappears from 18.4 to 18.7kHz (v. large peak) and appears at 16.6kHz to 16.8kHz and at 21.1kHz.
We have had a weekend of shorter (~5hour) locks and two locklosses form LASER_NOISE_SUPPRESSION state#575 (73787, 73831), the state this CARM gain is changed. Maybe this gain change has made us less stable, we'll discuss today reverting it.
CARM sliders reverted back to 6dB in ISC_LOCK (svn) and loaded.
On Monday, Naoki and Sheila 73855 saw that even with the CARM gain back at 12dB, the high frequency squeezing was still bad and the optimal H1:SQZ-CLF_REFL_RF6_PHASE_PHASEDEG sqz angle has to be adjusted a lot.
Maybe the CARM gain increase was effecting stability, but we don't think it wasn't causing the high frequency noise that was present in FDS, FIS but not no SQZ, plot attached. With adjustment to SQZ angle the SQZ greatly improved. It wasn't clear to us why the SQZ angle changed.
Although the overall high frequency noise was still bad once the CARM gain was reduced from 12 back down to 6, the peak around 4600Hz did disappear once the CARM gain was reverted. See attached SQZ BLRM 6 purple trace with CARM at 12 and CARM at nominal 6.
See attached high frequency plot showing peaks at 16.4kHz and 18.7kHz (purple) and thermalized around 16.6kHz (red), peaks disappeared once CARM gain was reduced ( green to blue traces). Maybe this concludes that the peaks are CARM gain peaking as Jenne suggested.
