We have determined we want to operate with higher carm gain. Instead of slowly increasing the carm gain during thermalization, we decided to try increasing it from the start and avoid causing glitches. The SERVO gain for REFL A and B will both be set to 9 dB instead of 6 dB in LASER NOISE SUPPRESSION.
The CARM gain adjustment in the THERMALIZATION guardian has been disabled yesterday
I measured the CARM gain today 30 minutes and 1 hour into the lock stretch to see the effect of the 3 dB increase. See attached plot. I think this plot shows, as predicted, we start with the UGF around 25 kHz, which is a bit high. However, we thermalize back to about where we want the UGF to be. We have not completely suppressed the high frequency "tail" shape in DARM with this gain increase, but we are unlikely to win much more by increasing the gain further. Daniel and I ran a long coherence from the long lock this weekend, see plot, that shows that we are not limited very much at 1 kHz by frequency noise (this time stretch comes well after thermalization is complete). The high coherence around 100 Hz is probably coming from PRCL or SRCL. We think this is a decent place to run for O4. I declare our CARM adjustments complete! I will unplug the SR785 at the next possible opportunity.