Here is a list of the things we need to change to return as closely as possible to a desirable 60W configuration.
- Ring heaters: 1W ring heater power on both ETMX and ETMY. No change to the current ITM ring heater settings because we are running at the same settings as 60W
- CO2 settings: 1.7W annular on both ITMX and ITMY (we are currently running with these settings). Depending on the amount of time we have to do TCS additional commissioning, it might be worth some time to see if we can improve frequency noise by changing these settings differentially (increase CO2X power). See this alog comment for more details: alog 68842
- OM2 TSAMS: no change
- ASC, control design: I don't believe we need to make any changes to the loops. There has been significant update to the recycling cavity loop design at 76W, but these are power independent. The HARD loops are running with loop compensators designed for about 400 kW of power. I think dropping to 380 kW should not negatively impact the loops, as the hard loops have been stable through thermalization which can change the arm power by 30 kW. The soft loop compensator design is the same. We may even be able to reduce some soft loops gains, as most of the soft loop gains were increased to compensate for the 430 kW operation.
- ASC, HAM1 FF: pending another 5-10 minute on/off test to collect data, we can either use the current filters, or revert to ones that I tuned on April 6 while at 60W. See alog. The filters are named by the date they were installed (so just turn on the ones labeled 0406). We did not need HAM1 FF in the yaw loops at 60W, so we can turn off all those filters first and then check the necessity again.
- LSC, control design: we should consider reverting the PRCL control loop design. Gabriele designed a new loop that could manage the gain loss at 430 kW in addition to adjusting the loop gains. We will likely still need to adjust the PRCL loop gain throughout the lock at 60W. Once we go down in power we should dedicate some commissioning time to determine the gain loss trend and commission accordingly. We should also check that the UGF of MICH and SRCL are where we expect them to be. The PRCL1 filters are FM2, 3, 4, 5, and 6 with a gain of 11.6 and none of the PRCL2 filters were in use.
- LSC, feedforward: we will need to tune the LSC feedforward. We had been running with an outdated LSC feedforward with the plan to remeasure and update it once we settled on a final configuration. All recent updates occurred at 76W. The filter for SRCL was FM2, labelled EDH used with a gain of 1. The filters for MICH were FMs6-9, however FM9 has been overwritten by a new filter. I will use the archive to recover it. The MICH filters used a gain of 0.97.
- CARM gain: the CARM gain was increase by 3 dB to counteract the loss of gain that occurred during thermalization. We should remeasure CARM and determine if we still need that gain increase. alog 69152
- ISS second loop gain: we reduced the ISS second loop gain to -5 dB. We should remeasure the loop and determine a better gain setting for 60W. alog 68789
- SRCL offset: we had been running with an offset of -175. This was also with the previous LSC-POP_RF45 whitening at 21 dB. We could revert the whitening change as well if we think it's better for noise considerations
- DARM offset: 20 mA, not sure if we want to revert this value
If you are looking for a timestamp to determine when the power change occurred, the last full lock at 60W was on April 6 from about 17:00 UTC to April 7 3:30 UTC.
Under LSC controls, I claimed that we should revert the PRCL loop design, however Gabriele reminded me that the new PRCL design has better suppression, see alog 68817. We should keep this new design, but we should still determine how/if we need to change the gain to ensure the loop UGF is around 30 Hz.
Under LSC feedforward, I forgot to mention that we did not run with PRCL feedforward at 60W, so we can turn that back off at 60W.
I have also recovered the old MICH FF filter that was in FM9, called "May_d". At 60W, we will need to engage FM6-9. labeled May a-d.
J. Kissel, J. Driggers, N. Aritomi, S. Dwyer Just FYI I brought up the open question in Elenna's aLOG about DARM offset: 20 mA, not sure if we want to revert this value The quick consensus (without agreeing to write it in stone) is that we "plan" to *not* revert the DARM offset, leaving us with 40 mA of current on the DCPDs, as has been the case since May 05 2023 (see LHO:69358).J. Kissel, J. Driggers, S. Dwyer Regarding the following setting suggestions in this bullet point, SRCL offset: we had been running with an offset of -175. This was also with the previous LSC-POP_RF45 whitening at 21 dB. We could revert the whitening change as well if we think it's better for noise considerations The plan is to *definitely* go to the -175 ct SRCL offset, however -- upon discussion this morning -- we've decided *not* to revert the reduction in POP A RF45 whitening gain from +21 dB to +15 dB. Said with all positives to avoid confusion, we'll continue to reduce the gain to +15 dB rather than revert to +21 dB. We think - the extra ADC range head room is nice, - the sacrifice in SRCL / PRCL sensing noise is minimal, and/or has minimal impact*** - for now, today, when we power down, we want to change as little as is need to achieve stability, rather than revert absolutely everything. ***One may find the assessment of the noise impact in LHO:69350.