Anamaria, Adrian, Robert

In Sept. of 2020 I checked all four cryobaffles and recommended that all but the ITMX cryobaffle be damped (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=56857).  We damped 2 of the 3 recommended ones and we even damped the one I didn’t recommend, but we were unable to damp the ETMX  cryobaffle because there was no incursion at that station.  Damping worked well on the cryobaffles that were damped (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=65621).

The undamped ETMX cryobaffle appears to be the source of the 4.1 Hz noise comb in DARM. The figure shows a spectrogram comparison of a 3-5 Hz swept sine injection using the architectural shaker attached to the manifold reduction flange at EX in 2020 and now. The 4.1 Hz frequency and the Q of the decaying scatter shelf  are very similar in 2020 and now, but the SNR of the scattered light noise is much higher now, at least ten times worse.  The figure also shows, on the second page, DTT spectra as the 4.1 Hz harmonics become a shelf as the shaking amplitude increases.  While the mechanical shaker produced the 4.1 Hz noise in DARM, the HEPI shaking that I did at EX a few weeks ago did not, further supporting the contention that it is the cryobaffle and not the ACB (which may also have a 4 Hz resonance), or anything else isolated by HEPI.

One possibility is that the increase in arm power resulted in more scattered light at EX, but the first appearance that Gabriele found of the 4 Hz comb, Jan. 31 of this year (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=69406), doesn’t seem to coincide with a power increase.  Scattered light might also have increased from alignment changes.

I think that I could damp the EX cryobaffle through a viewport in a short vent like I did with the Swiss Cheese baffle (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=36147 ).