We had difficulty locking most of the day today, but we were able to get to low noise with the 30W input power, which seems stable.

Difficulty locking

This morning we attempted to inject squeezing into the interferometer, which caused a lockloss.  We've had a tough time re-locking all day since then, it seems that the arms are moving a lot at 50mHz (up to 5um pp).  While it has been windy today it doesn't seem like the ground motion and tilt are large enough to explain the difficulty we've had.  We have had the seismic in the "WINDY", we have had max wind speeds around 30mph for most of the day but periods when it was gusting up to 40 mph. There have also been a few rather small earthquakes.  

ALS changes reverted

We spent some time having difficulty engaging ALS DIFF, we would loose lock reliably when it first ramped on the gain.  We have reverted a series of changes that were made recently, and things seem better now.  Yesterday we added a 120Hz notch to the 60Hz notch we use with ALS because we had bad ground loop problems at the PSL, today I removed this (46213).  A couple of months ago people made ALS DIFF and COMM lock simultaneously to save time, this doesn't save us much time and make it confusing to debug things, so we have gone back to locking DIFF then locking COMM.  Lastly, we went back to ramping the DIFF gain on first to 1/10th of its full bandwidth slowly, then ramping on to the full gain (this was removed over the break 46146).  We also reduced teh gain back to 400 (rather than 600, the 3dB increase Rana mentioned in his alog). 

Squeezer automation

We took some time to work on the automation of the squeezer while we were having trouble locking.  Nutsinee spent some time working on the squeezer guardians, and has a working guardian to close the LO loop which she tested on the homodyne.  We also spent some time testing some of the code that Haocun wrote for doing initial alignment of the squeezer, which is in the ALIGN_IFO guardian.  We don't know if we will really need to do this each time we do initial alignment, but since we have been doing it each time we inject squeezing it will be useful to have it automated.  We also added another branch to the ALIGN_IFO guardian that lets us engage the AS centering loops with a single bounce PSL beam, and offload the OM sliders, so that we know we are aligning the squeezer to the right alignment when we inject it.  

ISS problems

Keita and Jenne spent some time looking at problems with the ISS, I don't know all the details but when the second loop is either off or AC coupled the first loop diffracted power is fluctuating a lot.  Keita and I tried DC coupling the ISS with the interferometer at 30W input power, and DC coupling it caused the power to drop by about a Watt.  The first attachment shows that the problem is when the AC coupling output gets held it is not at the mean of the perviously noisy value, which we think is why the diffracted power changed.  We also saw that the diffracted power servo wasn't really doing anything, Keita increased the gain from 0.05 to 5.  This was tuned for 2W, so it makes sense that we need to adjust it for 30W input power.  This gain is now adjusted in the IMC_LOCK guardian before it turns on the ISS at 2W and before it gets DC coupled.  The ISS second loop is left commented out of the guardian, because DC coupling it could still cause a transient in the input power while the first loop is so noisy. 

30Watts input (~90kW circulating power) low noise 

Following the instructions given in 46190 we were able to get to 30W without an issues, and then go to low noise.  I moved ADJUST_POWER to after CHARD blends and it will not set the power to 25 W in one step, wait 5 minutes then set the power to 30W and wait 10 minutes.  I didn't try to shorten this time, and I haven't tested the code this way, but think it should work. I also lowered the amplitudes of the HARD yaw dithers another factor of 10 since those lines are very large in DARM once we are in low noise. 

We were locked in low noise (the only thing I skipped was reducing the 9 MHz modulation depth) from about 5:27 UTC to about 5:48 UTC.  We had a lot of non stationary noise at low frequencies, similar to last night.  I tried to drive ITMX CP to see if I could find a better alignment for it, but that unlocked the IFO.  DARM has coherence with MICH below 100 Hz (attached plot), we haven't re-tuned the feed-forwards yet for this these settings.