J. Kissel While Sheila and Jenne were commissioning the settings for the CARM loop (LHO:70662) as we revert to 60W PSL input power (see LHO:70648), we noticed that the DARM ASD on the front wall (8 sec FFT, 75% overlap, 3 averages) showed a lot of excess noise and comb-like behavior, especially while Sheila was measuring the open loop gain of the CARM loop by injection SR785 voltage into the analog CARM servo, injecting a 100 Hz to 100 kHz swept sine excitation around the UGF of the CARM loop. We figure this ~10 minute stretch of data might be interesting to compare against the list of combs that Ansel sees (e.g. LHO:70219), in case there are clues that some of the combs seen are due to this loop. The approximate times of injection/gain steps/commissioning are between 16:40 UTC and 16:50 UTC, with combs definitely seen at 16:45 UTC (on June 21 2023). One can confirm the times by trending the CARM gains listed in Jenne's aLOG (again, LHO:70662).
Unfortunately, it doesn't look like these combs are the same ones we've been tracking. I looked at 200s FFTs averaged over the 10-minute period and made a few plots. There's a clear ~12.5 Hz comb and a stronger overlapping ~50 Hz comb, but these are new and not persistent as far as I'm aware.
Figure 1 shows this time period overlaid with known combs-- it's a messy plot, intended to show all locations where comb peaks would be. The only one that sticks up above the background is a 99.99864 Hz comb, which overlaps roughly with every other harmonic of the 50 Hz. But zooming in (figure 2) shows that the alignment is not great, and that the feature in this data set is actually fairly broad. Contrast with figure 3, showing the same frequency range in an Fscan weekly data set, where this comb and another ~99.99 Hz comb are both visible, narrow, and offset from zero (and we don't see peaks between them).