I have updated all mode cleaner mirrors (MC1, MC2, MC3) and recycling cavity (SR2, SRM, PR2, PRM) HSTS damping loops to the Level 2 design that Jeff Kissel developed, and we worked on testing and implementing. You can read alog 65310 for a reminder of the implementation of these.
We have done this once before, but faced many stability issues related to the change. However, Jeff and I do not think the problems we faced were actually related to the damping loops. We think the various issues might have been related to the incorrect PRM M1/M3 crossover that Gabriele has noticed (and now fixed), as well as the ITM reaction chain motion. There is a whole saga related to this, but two things remained evident:
Therefore, Jeff and I think these are fine to implement again, especially given that Gabriele has solved the crossover issue on PRM (and is currently working on updating SRM).
All HSTS damping loops should be using FM1, 2, 4, 5, 9 and 10. All gains should be -1. I have SDFed these filters and gains accordingly. The "old" filters are located in FM6, 7, 8, and 9. (Note: FM9, the BR notches, is common to both configurations.)
We are relocking and there is no sign of the issues we experienced during the past saga of implementation.
I call this 90% successful, because once we got to NLN, Gabriele and I saw gain peaking in PRCL and SRCL around 3 Hz. Dropping all recycling cavity mirror damping gains by half reduced this gain peaking. I will SDF all these gains to now be -0.5. Jeff and I will investigate why the OLG measurements showed no more than a factor of 2 gain peaking, but we saw much more in lock. With the gain change, we are 100% successful!
In terms of the noise effects, we see some reduced noise in PRCL. PRC2 and SRC2 also show reduced noise. To make a more faithful comparison, I will wait until we are thermalized to compare with previous locks.