Gabriele, Evan

We spent some more time today on improving the interferometer recycling gain.

We started at 3 W with DARM controlled by AS45Q, and with the dETM, BS, cETM, and IM4 ASC loops engaged. We've switched the ordering back to an older configuration, so that cETM comes on before IM4 and PR2. This is intuitively appealing, since the motion of cETM appears in the IM4 and PR2 signals that we're using [REFL9IA − REFL9IB, and 0.5*(REFL9IA+REFL9IB)+0.83*(REFL45IA+REFL45IB), respectively], and therefore it is good to have it suppressed before these two loops engage.

Then by moving the ITMs (mostly ITMY; see attachment), Gabriele was able to bring the recyling gain from 32 W/W to 40 W/W. We had to move the SRM simultaneously to keep POP90 low.

At 40 W/W, the interferometer seemed pretty stable (and in fact lasted 90 minutes at this recycling gain), so we tried reengaging the rest of the ASC. We found that the PR2 loops had flipped sign, but otherwise engaged just fine. Then we tried engaging the ASC→SRM/SR2 loops after adjusting the QPD offsets. This seemed to work initially, but after about 60 seconds caused the interferometer to unlock.

After the lock loss, we relocked ALS (without touching the ITMs) and then updated the ITM camera spot positions. With a little more work, we can probably close the rest of the ASC tomorrow.