Craig Georgia Danny Dan Sheila

Following on from RF9 coupling tests earlier this week (alog 47008), tonight we ran some more excitations in the RF9 amplitude modulation.

We found that over the course of the lock the coupling of our excitation to DARM changed. See first attachment top left plot. The dotted references are traces from earlier this week, the pink was measured 10 mins after reaching NLN, and the blue trace an hour and a half into the lock. This supports Dan's observation of the coupling of his 70 Hz RF9 AM line (alog 47081). I only measured the broadband coupling at these two times, it might have been useful to measure ~1000 seconds into the lock, where Dan observed maximal coupling.

We then changed the DARM offset and ran the same RF9 coupling measurements, as well as the OMC DCPD cross-correlation, for a few values of the DARM offset. We found the RF9 coupling was reduced with higher DARM offset, maybe this isn't surprising given that the ratio of 9MHz to carrier at the AS port is changing as we change the DARM offset. The second attached figure shows several DARM offsets, dotted traces are from earlier in the lock, and have nominal (14.8) DARM offset. At 19.3 (33.65 mA DC current on the DCPD sum) we started to saturate the DCPD's with the second order violin modes and so backed off. The change in DARM offset did not show any change in the DCPD cross-correlated spectrum. We did not alter the feedforward as we were changing the DARM offset, and Craig noticed there was coherence between SRCL and DARM between 10 and 20 Hz when we had the large DARM offset.