Sheila, Alexa, Evan

Summary

We have recovered the usual low-noise locking state (except for the outstanding issue of the HAM6 centering scheme) and have turned up the power to 15 W.

Details

• Alexa and Sheila traced the ALS DIFF problems to the wrong coil state in ETMX L1; it was in low range mode, and hence saturating when DIFF engaged.
• IMC-F was saturating every 10 minutes or so. First we increased the UGF for the slow common feedback from 20 mHz to 40 mHz. That helped a little, but we had much more success by adjusting the settings for red tidal feedback. It now engages when each arm's TMS WFS B goes above 50 ct (with a 1 s holdoff), and disengages below 20 ct. This way, the tidal engages during the CARM reduction sequence, right before control of DARM is handed over from ALS DIFF to AS45Q. Excursions of IMC-F are now about 100 kHz pkpk (albeit with low wind).
• ITMX CO₂ laser heating was off. We turned it on to the usual 0.2 W.
• The OLTF gain for the dETM WFS seems to have increased; we had to turn down the filter module gains to 10 for pitch and 5 for yaw (whereas they used to be 14 and 21).
• The DARM optical gain appears to have changed. Our usual DARM ERR offset of 2e-5 ct, which previously gave something like 23 mA total from the two OMC DCPDs at 10 W PSL power, now gives more like 35 mA at 10 W PSL power. When we remeasured the DARM OLTF, we found that we needed a DARM filter module gain of 2000 ct/ct to achieve the same UGF as last week (compared to 1300 ct/ct that was used previously).
• Consequently, the DARM calibration was wrong. Using Kiwamu's script, I reckoned that the optical gain had to be increased from 9.09e-7 m/ct to 1.40e-6 m/ct (see attached plot and xml file). So I changed this value in CAL-CS_DARM_ERR FM5. That made the spectrum more resonable, but it seems that the high-frequency noise floor is still at or above the GWINC 10 W curve, even after bumping up the CARM gain. [Note that the old optical gain produced a phantasmagorical 50 Mpc lock stretch in the summary pages.]
• For 15 W, we closed the POP shutter on ISCT1, since POPAIR_A already has 25 mW dc at 10 W of PSL power.
• For 15 W, the DARM ERR offset was reduced to 1e-5 ct, giving 14.6 mA total dc on the OMC DCPDs. If (IF) the current calibration is correct, the DARM offset is about 12 pm. We could always bump this up to the 15 pm we were using previously.

As before, HV in HAM6 has been turned off for the night.