Summary: Acoustic coupling at HAM6 (dark port) is likely to be a borderline noise source for aLIGO, near the expected noise floor in the several hundred Hz region, as it was at the end of eLIGO. As risk mitigation, I began investigating the resonance features that couple acoustically driven external vibrations to the table surface with transfer functions approaching unity in the several hundred Hz region. The resonance features consist of broad resonances that are likely ISI blade spring and flexure and/or table resonances, with higher-Q resonances riding on them that are likely from components: the GS13 pods, the mass pegs, the individual panels, and the optic structures on the table. The damping planned for other reasons is unlikely to significantly change the Qs of the peaks. We may want to begin developing damping schemes in case this coupling is a problem at HAM6 or elsewhere.
Vibrational coupling, driven by external acoustic noise, was a problem in eLIGO, requiring us to suspend HAM6 steering optics and install blade springs on tip tilts (reduction in 400-550 & 750-900 Hz peaks with suspension). Even with this work, DARM was still somewhat contaminated by acoustically-driven vibrational coupling at the end of S6 (here). Figure 1 shows that there was coherence between the HAM6 geophones and DARM in the 400-550 and 750-900 Hz band late in S6 after our HAM6 interventions. I had hoped that the passive damping planned for the HAMs (for other reasons) would also help reduce the higher frequency motion. However, Guillermo Valdes (UTB), took a recent look at LLO HAM3, which had the damping installed, that made me want to conduct a damping experiment at HAM4. I found little or no decrease in the Qs of the high-frequency peaks even after I more than doubled the planned tabletop damping (7 dampers instead of 3). Thus I wanted to gain a better understanding of the source of this vibrational coupling in case it is a problem in aLIGO.
The ISI transfer functions from ground motion to table motion suggest an isolation of less than 10 at certain bands in the several hundred Hz region (for example see 500 Hz here). The ISI transfer functions typically show broad horizontal resonances in the 400-550 and 750-900 Hz regions, even before the tables are populated (Figure 2). Modeling by High Precision Devices (G-0701156-00-R) suggested lowest table resonances between 300 and 400 Hz. There are resonances in this region, but the higher frequency resonances are typically more pronounced on the geophone signals. A competing, and, to me, more likely possibility, is that the broad resonances are resonances of the blade springs and flexures, possibly matching table resonances (see resonances of the BSC ISI blade springs here). Geophone spectra show these broad resonances, as well as more narrow resonances riding on top of them (Figure 3). Thus the tallest peaks, the ones that are most likely to contaminate DARM, could be reduced by damping either the broad resonances or the peaks that ride on them. As an example of how the features appeared in DARM during early eLIGO, Figure 4 shows DARM from early S6 with the broad ISI resonances and narrow peaks riding on top. While reducing the broad ISI peaks would have been the best option, reducing the narrow peaks looks like it might have reduced the maximum peak height by a factor of 2 or 3.
In an effort to identify the sources of some of the narrow peaks that ride on the broad resonances, I did a series of tap tests on HAM4. Figure 5 shows that taps on the ISI side pegs for balance masses, the centers of the “X” shaped cutouts in ISI side panels, optic supports on the tables and GS13 pods all excited individual peaks or families of peaks that rode on top of the broader resonances. Figure 6 is a photograph showing these structures.
Figure 7 shows that each GS13 pod has its own characteristic family of several high-Q peaks. The pods consist of GS13s inside vacuum enclosures. The vacuum enclosures can be heard ringing long after they are tapped. These pod peaks are evident in geophone signals even when there is no tapping. The resonance peaks for a particular pod are largest in signals for the geophone in the pod, but are also evident in signals from other geophones. Fortunately, the frequencies of the geophone pods are, typically, slightly below the broad 400-550 Hz resonance, so it is likely that only a few of the higher frequency pod peaks will be among the highest amplitude peaks that could show up in DARM. Nevertheless, we may want to consider a passive damping scheme for the geophone pods, or a scheme to move the resonances a little lower in frequency.
The individual panels that make up the body of the ISI also have resonances that may coincide in frequency with the broad 400-550 and 750-900 ISI resonances. I took two of the several panel types and suspended them by “strings”. One had resonances at about 400, 700 and 800 Hz, the other at 490 and 670 Hz. Some of the resonances may be associated with the “X” shapes left by the cutouts (see Figure 6). Fabrice and I imagined a couple of passive damping schemes for these, if needed, but access to all of them would be tough and we might just want to instead put damping that is tuned to the 400-550 and 750-900 Hz resonances onto the tabletop structures that touch the beam.
Finally we might want to be thinking about a simple scheme for passive damping of individual optic structures or the ISI mass pegs.
There may also be ways to damp the broad resonances, such as tuned damping on the blade springs, as the SEI group developed for the bucket peaks in eLIGO, or even active schemes. I would like to repeat my HAM4 work at HAM6 just before it is closed up, so that we have a record of the frequencies of the pods and optics on the surface of HAM6 in case features show up in DARM. It might also make sense to investigate whether blade spring and flexure resonances could produce such high transfer functions. In conclusion, I think we should begin to consider mitigation routes in advance in case this coupling is a problem at HAM6 or elsewhere.
