Philippe, Sharan, Kara, Anamaria, Robert

The HVAC reduces our range by 3 or 4 MPc (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=48649 ). Shutting down the various stations showed that the corner station is the worst culprit but there also appears to be some coupling at EX. Figure 1 shows that the HVAC is affecting DARM in the 30-90 Hz region. The lower plot in Figure 1 shows that, at the corner station, the HVAC increases ground and septum motion above a few Hz. This is in the frequency band that we can shake with the big shaker, so we would hope to be able to reproduce the HVAC effect, though, of course, local shaking would differ some from the more global shaking produced by the HVAC.

A preview of the LVEA vibration coupling summary from the recent PEM injection program suggests that the strongest coupling site in the 30-90 Hz band is in the HAM5/6 area (Figure 2a). Impulse injections narrow the coupling site down to the septum (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=48886 ).  Figure 2b suggests that the noise from the motion of the septum when the HVAC is on would be a factor of about 3 below the DARM floor. This would be enough to produce the several percent change in DARM when the HVAC was turned off and the septum motion was reduced by a factor of 1.5 to 2.