Sheila, Lisa, trying to summarize the work of many people today

* More  Intensity Stabilization Science   made possible to engage all of the ISS loops and lock several times at 50W;

* The lock at 50W had been broken earlier today due to the inability to damp the ETMY Mode 27 (18041 Hz). This triggered the decision to turn off the EMTY ring heater (see  Terra's elog ). After that we had a couple of stable locks at 50W, when Terra and Matt were able to keep all the Pis damped, and the sideband power and the recycling gain were stable. As Kiwamu already observed, it looks like the current combination of ITM ring heaters and CO2 powers is good (enough) to prevent the sideband power and the recycling gain from dropping (note: Corey and Kiwamu noticed a drop in AS90 in another 50W lock later in the evening (see   Corey's entry ) and they had to deal with some PI damping, but that lock was done with no CO2 X power, instead of 400 mW, as Kiwamu was exploring CO2 tuning);


* After more than 2 hours at 50W we started some low noise steps. Evan engaged the OMC whitening filters, but that didn't help at all unfortunately, as we are entirely dominated by intensity noise above 300 Hz. Looking at the noise budget from O1, a by-eye estimate tells us that the current level of intensity noise is at least ~300 times worse than during O1, even more at some frequencies. The noise is also worse than what it was during ER9 (when the HPO was operational). Sheila's entry  shows indeed that the intensity noise coupling to the interferometer is about 3 times worse than it was during O1; also, the ISS out of loop sensor is about 3 times higher than previously measured, showing coherence with PSL accelerometers. If you add that the current ISS loop has a factor of 2 less gain than during O1, while the intensity noise with the HPO is 30 times higher, well..the factor of 300 worse noise than during O1 makes sense.

So the bottom line is:

* we are close to a stable 50W configuration, it looks like PIs can be controlled, and the sideband power can be stable; need to work on robustness from lock to lock;

* the intensity noise is huge, and it dominates the noise above 300 Hz; trying to get more gain out of the current ISS loop didn't prove successful tonight, so we need a better strategy. Probably we should start by going in the PSL and realign the PMC and see if it improves the ISS out of loop PD spectrum.