Jenne, Hang, Sheila, Keita, Evan
Tonight it seems like we made some progress, although we haven't powered up yet.
- TMS X QPD A seemed to have some kind of saturation of segment 1, which was causing our soft loops not to work well (ie to sometimes drag the X arm alignment way off). reduced the analog whitening gain for all 4 TMS QPDs, which used to be 9 dB in full lock but is now 0 dB.
-
We were able to power up from 2 Watts to 8 Watts and see the oscialltion several times to check if configuration changes had any impact on our CSOFT oscillation. A list of things that had no impact:
- turning off ITM optical lever damping (4:34:05 UTC) (still off)
- turned on 0.44 Hz notches in SOFT loops (4:47)
- turned on a resG at 0.44 Hz in DARM (4:56) (reverted right away)
- set A2L gains using A2L script (5:07 UTC) (reverted right away)
- CHARD low gain (5:30 UTC)
- Lastly, we used the alignment dither system, with all the A2L gains set to zero, and tuned the soft offsets to center the beams on the ETM actuation nodes. (the soft offsets don't have much of an impact on the ITM spot positions.) For future reference, the demod phases we used for pit were -25 degrees, for yaw we used -40 for osc1, -30 for the rest.
- With these offsets we were able to power up to 12 Watts before seeing the oscialltion, where previously we had been seeing the oscillation at 8 Watts. This was before we understood the problem with the QPD saturations, so the soft loops didn't really keep us centered on ETMX as we powered up. We stopped for a while at 10 Watts, ran the A2L script to set gains, and did a little bit of work to update the X arm green QPD offsets.
- We have a new guardian state ENGAGE_SRC_ASC, which will just be convient because we have been going back and forth between turning these loops off after DRMI and leaving them on. It is not tested yet.
The first screenshot attached shows the symptom of the xarm soft loop not working, as we power up the optical levers move, and the loops work to bring the QPDs back to their starting points but bring the optical levers in the wrong direction, causing the green to become misalined. The second screenshot shows the individual segments of the X arm QPDs durring this time. The third screenshot shows a power up (to 20 Watts) towards the end of O1, with no evidence of saturation, despite almost twice the power on each segment. The last screenshot shows the noise of each segment, the bottom panel is before reducing the whitening gain, the top panel is after (the A2L dither lines were on, that is the forest of lines around 20 Hz). Although it seems like we have more sensible signals coming from the QPDs with the lower whitening gain, something still doesn't quite add up when you compare the second and third screenshots.
I re-did the spot centering on the ETMs using the ADS procedure described above, but after we reduced the analog gain of all the transmon QPDs. I was able to sit stably at 12W, and lost lock at 15W from our dear friend the ~0.4Hz oscillation. Note that here, as earlier, we moved around and found the offsets we wanted while in the X and Y arm bases, rather than common and differential. After letting the alignments offload to the top stages of the quads, the loops were turned off again, and reverted to common and differential bases and the individual QPD pitch and yaw offsets were set, before re-engaging the loops and trying to power up.
This time around, CHARD was high gain with boosts, no resg in the DARM length loop, no notches in the SOFT loops (all these are nominal settings, undoing some of our tests tonight), but also no oplevs for the ITM pitch damping (not nominal, although we want it to be nominal in the future). Also, the A2L gains in the L2 stages of the test masses are still set to zero for now.
I tried powering up one more time, with the ITM oplevs re-engaged, and again I saw no difference. I was still stable at 12W, and unstable at 15W. I'm leaving the ITM oplevs off.
There's clearly something wrong with TMS X QPD A, segment 1, and maybe segment 2 as well. The second image shows them flat-lining at just under 3000 counts, which is way below the hardware saturation levels when things are working right. I think you're going to have to look into the transimpedance amp and whitening/vga chassis for this unit.
Looks like the PI interface chassis that parallels the signal path was causing problems. We have removed the chassis and plugged the QPDs directly into the ISC chassis again. We will look into the PI interface to see what might be happening.