Jenne, Sheila

Today we wanted to get the SRC alingment under control, mainly motivated by the observation that both our noise couplings and noise change a lot over time (within and between locks)  (see 29984) 

SRC ASC

We have been able to close ASC loops for the SRC at full power, both pit and yaw.  We used to use AS_C to feedback to both SR2 and SRM, (SRC2), tonight we are sending this signal only to SRM, and we are using ASA45I to control SR3 angle. In the past we had a cage servo runnning for SR3 pit, but this is now disabled in full lock. I used a gain of +100 for pit and +30 for yaw, but both of these loops are verry slow as is and should probably have their gains increased a lot.  (input and output matrix elemetns are all positive as well).  I added some code to the guardian to use this in SRM)HIGH_POWER_ASC, but we should probably try it at 2 Watts, increase the gain and check the loop shaping, and try to power up with this configuration.  I'm levaing the IFO undisturbed, so hopefully we can see if our noise is any more stable tonight. 

REFL WFS detour

Since we were interested in using REFL 45 WFS as signals for SRC alignment, we wanted to prevent the 200 mHz PIT oscialltion which has dominated all REFL ASC for many months now.  It turns out that even without this oscialltion, we didn't see a good signal for SR3 in these WFS, but prevetning the oscialltion seems to have helped stabilize carrier power in the interferometer. 

Fixed oscilations in REFL DC centering loops

 I noticed earlier that the oscialtion around 200 mHz that is present in all of our REFL WFS signals seems to come from the DCP1 loop.  Jenne and I measured the loop, and found that indeed it had not much phase margin at all and a ugf around 200 mHz.  The only alog we found about these loops is 9966 (from 2014).  It seems that since then, someone added rather aggresive 0.9 Hz low passes that ate too much phase for all of the REFL centering loops. 

We replaced the filter design for these loops with the filters used for AS centering, which allow us to get bandwidths of 2.5 Hz with about 30 degrees of phase.  We also copied over the 20Hz low pass, but haven't engaged it.  

The attached screenshot shows the new loop shapes, the second shows the settings used when we measured these.   We do not understand why DC2 (which used REFL B DC for a combination of the 2 tip tilts) seems to fall as 1/f^4 after the resonsance. 

Increased gain of PRC1 P

With the DC centering loops no longer oscillating, the refl WFS signals were slightly better but still moved alot at microseism frequencies.  We found that we could improve the phase margin in the PRC1 P loop, and reduce these fluctuations, by increaseing the gain in this loop by a factor fo 4 (digital gain of 60).  This was what we had set the gain to in February, (see measurement attached to 26225), but it has slowly been decreased since then. The third attached screenshot shows the refl WFS pitch signals, the PRC1 gain was changed at -10 minutes.