Kiwamu, Stefan
We were trying to nail down what is responsible for the Power Recycling Gain change during power-up, so we went through the whole sequence slowly, and gave the system time to settle in each state:
All times UTC of 20160629:
- 18:54:37 / Green dot: Power-up starts from 2Watt to 10Watt
- 19:00:32 / Red dot: Power-up restarts from 10Watt to 40Watt
- 19:06:10 / Black dot: : Power-up to 40W finishes, but additional SOFT offsets remain left off.
- 19:10:15 / Blue dot: Soft offsets in yaw start ramping on over 2 minutes
- 19:12:17: Soft offsets ramp in yaw is done
- 19:15:17: Soft offsets in pit start ramping on over 3 minutes
- 19:18:19: Soft offsets ramp in yaw is done
- 19:21:20: End of undisturbed period - guardian continued
In particular, we now there is a significan power recycling gain improvement with a soft yaw adjustment of the x-arm. Additionally, the beam splitter cannot affect the relative alignment of x-arm and power recycling cavity. SO attached are only yaw signals from x-arm and power recycling cavity.
Conclusions:
- During the very first step in power up (green dot) there is a momentary shift in PRM yaw and IM4 yaw, but it does not continue during the additional power increase.
- During the main power increase (between red dot and black dot). There is a consiostent signature in the ETMX control, oplev and IR camera signal, sggesting that we are slightly moving the x-arm with the ASC system. THe ITMX doesn't see much of this in control signal and oplev signal (its camera is broken and not plotted). However no significant motion is visible in any of the power recycling cavity mirrors (this includes PR2, which is not plotted).
- Once we start moving the soft loops with soft offsets, ETMX, ITMX, PR3 and BS clearly respond. Interestingly, the ETMX moves to the same direction as during the power increase.
Attached are 220days of recycling cavity alignment data (Nov 15 2014 to now)
The alignment offsets for IM1-3 are not a valid way to track the optic alignment, since IM1, IM2, and IM3 all have significant alignment shifts after a HAM2 ISI trip.
- IM2 shifts the most in pitch and it's shift is typically 40urad.
- IM3 typically shifts in pitch and it's shift is typically smaller than IM2's, around 20urad.
- IM1 typically shifts in yaw, and it's shift is typically smaller than IM3's, around 10urad.
I wrote a procedure to correct these after restoring the ISI and the optics but adjusting the alignment offsets to drive the optics back to their nominal alignments.
During O1 the procedure was to drive the optic back to it's most recent good value, however this can be hard to decipher, so in May I posted a Nominal Alignment for the IMs and have maintained it since.
I've been maintaining an alignment of the IMs based on our last good high power lock in April 2016, and this alignment choice is explained in alog 26916, and recently updated in alog 28016.
On a longer time scale, the alignment of the IMs is harder to track since I only started restoring their alignments after an ISI trip around September 2015, so before that time when IM2 pitch shifted 40urad we spent time realigning H1 to follow that change.
In my investigation, I see that the IM alignment shifts have caused down time in H1, and IM1-3 now have an alert in guardian to prevent the IM alignment shifts from causing down time in O2.
Here are some pertinent alogs from my investigation:
- alog 24696: alignment changes in yaw, for IM1, IM2, IM3, PRM and PR2, over 150 days (all of O1 plus a few days)
- alog 25075: IO IM2 alignment changes after a HAM2 ISI trip - January 20th power outage
- alog 25688: Alignment shifts of IO IMs (IM1-3) in H1 also occurring in L1:
- alog 26685: IM1, IM2, and IM3 now have a notification in DIAG_MAIN when alignment has shifted