daniel.hoak@LIGO.ORG - posted 02:22, Thursday 16 April 2015 - last comment - 12:19, Thursday 16 April 2015(17904)
notes from tonight
Alexa, Dan, Elli, Evan, Koji, Sheila
- With the youngest parametric instability on the continent tamed by the ring heater, we locked a few times tonight, each time for an hour or more. At the start of each lock the OMC DCPDs were either saturating or approaching their nonlinear region, due to an excited violin mode harmonic around 1009.62Hz. We believe this is an ITMY mode but haven't been able to damp it yet.
- A quick transfer function of the OM1 suspension showed that the mode splitting we saw in the yaw d.o.f. last week is no longer there, see the first image attached. This is good, but it means that this suspension is very close to some kind of mechanical contact that changes the plant. I have reverted the yaw damping gain to the value from before, and committed new L and Y TFs to the SUS svn.
- The green light transmitted to the corner through the X arm has decreased by ~15% in the past day, see the second plot. Sheila renormalized the X_TR signal so that the guardian will be satisfied with the X arm alignment. Changes in the alignment of PR3 didn't recover the power (so we're not misaligned on the PD), and the power level at the endstation hasn't changed. Baffled.
- We studied the OMC SUS excitation data from last night to measure the scattering noise from the OMC, Sheila will post more information later. For now, compare the first PDF attached, which shows a spectrogram during the large longitudinal excitation of the OMC SUS from last night, with the second PDF, which is the same plot from tonight with no excitation. The quiescient motion of the OMC does not appear to generate scattering noise. There is clear nonstationary noise around 100Hz and also around 40Hz, but this does not have the arch-shaped features from scattering, and isn't correlated to the OMC suspension motion. There's still a turbo pump running outside of HAM6.
Images attached to this report
Non-image files attached to this report
Comments related to this report
We also locked with both end station beam diverters closed last night, we had no problems locking this way, but also saw no improvement in DARM. Now we are leaving the beam diverters at both end stations and OMC refl closed all the time. AS air and POP we have been closing sometimes in full lock, but we don't see a notiicable change in DARM. The AS air beam diverter has krytox, so we should feel free to close it whenever we want a low noise lock, but the POP beam diverter has not had krytox so we may want to avoid closing it without a specific motivation. We have not yet tried closing the refl beam diverter.