Patrick, Sheila, Carl and Ross damping PIs
After Keita's ISS fixes we had a few hours of bad weather and earthquakes that contributed to dificulty locking. Once things calmed down we locked and sat at several stages to see if the lock was stable, DC readout, 25 Watts, ISS 2nd loop engaged, and now we have made it to some kind of "nominal low noise" state. Patrick hit the intent bit.
There are several things wrong with this spectrum, some of which could be solved by going back to 40 Watts in the morning:
- We skipped the coil drivers state, since we don't expect it to help our noise and we lost lock in this state earlier. We don't see any reason not to try doing this tomorow though.
- There is a lot of intensity noise in DARM. (1st attached screenshot shows 2nd loop PDs before ISS was on 6:40 UTC July 8th, and after the 2nd loop is engaged at 7:30 UTC. The 2nd attachement shows that there is still a lot of coherence of intensity noise in DARM.)
- The CO2 guardians were in preheating mode for the first bit of time we had the intent bit set (this is a result of not thinking through all the consequences of changing the nominal power down to 25 Watts). We set the CO2 guardians to NOMINAL_LOCKED_CO2 level, which means that they are now at the settings that are appropriate for 40 Watts I think, which is probably closer to the right settings for 25 Watts than preheating at least.
- Our low noise hard ASC loops were designed for 40 Watts, so the decision to go to 25 Watts means that we would need to do some measurements and loop design work to have a low noise state for 25 Watts.
- We probably have different spot positions on our optics now than we would at 40Watts, so we would need to run a2l for 25 Watts if we want low noise.
- DIAG main is complaining that the POP beam diverter is not closed, this is because we are still locking on the POPX WFS, this will be open throughout ER9.
- The OMC alingment is clearly off, on the camera it is on the left half of the screen.
- The DCPD whitening is off, which makes our high frequency noise worse.
The last attached screenshot is just to show how many locking attempts everyone made in the last 14 hours. The range displayed by the DMT viewer in the control room seems to have a problem, it should be stable at some low range but on the DMT viewer it looks like we lost lock in the last half hour.
Sheila's comment
There were several things which had to be done today because of the change to run ER9 at 25 Watts.
- Getting rid of soft loop offsets that were tuned to improve recycling gain at 40 Watts
- setting ISS 2nd loop offsets
- commenting out low noise ASC states
- changing TCS CO2 guardians by hand
We should remember to undo these things as soon as we go back to 40 Watts. According to Ross and Carl, there isn't a reason to think our PI situation has gotten worse than it was for the last 2 weeks where we had stable locks more than 4 hours long, but the PI damping would need to be babysat at 40 Watts. Maybe an engineering run is a good time for detector engineers and operators to learn how to do this.
There are several things that should be done if we want to continue the ER at 25 Watts with a reasonable sensitivity:
- run A2L
- make 25 Watt low noise hard loops
- get CO2 powers set to something OK in the guardian
none of these are all that hard to do, but to me it seems like it would be more productive to try continuing the ER at 40 Watts, to see what we can learn about the IFO in a state that is more similar to the configuration we would like to run at for O2.
DARM-ISS 2nd loop coherence doesn't change by increasing the ISS 2nd loop gain by 6dB. Jitter or whatever, ISS is imprinting noise onto intensity.
Keita, this is something I think we saw in the past too:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=20394
CO2 related comment:
- CO2 Nominal state of the guardian gets turned on during Coil Driver state. If you have skipped Coil Driver (which you did according to the alog) then CO2 power guardian stays at pre-heating. This can be changed easily.
- CO2 power at its Nominal state is calculated based in PSL input. There might be a message complaining that the CO2 setting is not nominal (since we used to operate at 20W) but the output CO2 power should be correct at 40W.