Sharan Banagiri, Kara Merfeld, Anamaria Effler, Robert Schofield

A large line appeared just below 80 Hz on Tuesday (visible in Mar. 13 summary), that went away when the squeezer beam diverter was closed. We noticed a similar, though not coherent, line in the ISCT6 accelerometer, and tracked it down to the squeezer laser controller fan. Figure 1 shows that the 80 Hz line and a line at the fifth harmonic, ~398 Hz, appeared and disappeared several times as we moved the laser controller (on top of ISCT6) on and off of ¾ inch rubber legs. The seismic isolation provided by the rubber legs reduced the amplitude of the 80 Hz peak in the table accelerometer by more than an order of magnitude.  We have left it on these legs.

We are not sure why the peak seemed to suddenly appear, I checked back in time a little more than a month and the fan peak at 80 Hz has not increased in the ISCT6 accelerometer signal, so I think the difference is coupling, such as an alignment change, rather than an increase in the source that might indicate imminent fan failure.

Figure 2 shows results of injection with a speaker placed right on the table, we moved the speaker to HAM6 and SQZT 6 and the coupling was lower, so, for this speaker location, we think the coupling at ISCT6 is dominating. The accuracy of the prediction can be assessed by comparing the prediction from the injection on either side of the 80 and 400 Hz peaks to the no-injection level of the 80 and 400 Hz peaks in DARM.  So, we are fairly confident that the coupling at ISCT6 is within a factor of a couple of DARM in bands around 400 Hz and possibly other bands.

If the appearance of the peaks is an indication of increased coupling, we may be able to reduce it, there are a couple of other investigations we could do.