Craig, D Brown, Georgia, Hang, Rana
Since last night, our adjustments to the SOFT loops has enabled stable operation at 25 W (> 1 hour) and several minutes of stability at 30 W.
- We dither the ETMs, and one ITM in both pitch and yaw to produce angle to length signals. We demod these to produce spot position error signals on the mirrors and lock down these spots by adding them to the SOFT loop error points.
- Lowered the CSOFT yaw loop gain by a factor of 2 to minimize 3 Hz suspension oscillation.
- mix REFL WFS into CSOFT to minimize 3 Hz gain
- Sidles-Sigg RP compensation set to 25 W
The attached image shows the 30W unlock. The step from 25 W to 30 W happens at -180 seconds on the plot. The growing oscillation at the end is at 0.42 Hz as we've seen before.
I set the RF phases for WFS 45 A&B. They were close already. Because of the large DARM offset, you can basically phase it by minimizing the ambient I phase signals (DARM length shwos up in the Q phase).
We are measuring the dHard, dSoft, and MICH sensing matrix in this ~2 AM lock. You can see the lines in the upper plot of the attachment. The lower one shows that the AS_A and AS_B signals are both good at low frequencies.
But the electronics/shot noise of B is less than A, so we would do better SNR-wise to use RF45_B, rather than A. This is not our main issue at the moment, but I note this in case people want to get lower DARM noise sometime before O3.
As an aside, I am reminded about the environmental coupling problem with our AS WFS: since we have a large DARM offset, there is a large, static carrier field on the AS WFS, making for a large signal. This makes us first order sensitive to motions of the beam and motions of the WFS head, whereas we would be immune to small WFS head vibrations if we had a true extinguished carrier field as we did in iLIGO and will in the future with BHD. (cf. Hefetz, et al "Principles of ...")
there's been no problem locking in the past 12 hours
: no ALS glitches, no Guardian crashes, no channel access timeouts, very low wind and the microseism in the 1-2 um/s range doesn't pose a problem any more