Ansel reported that a peak in DARM that interfered with the sensitivity of the Crab pulsar followed a similar time frequency path as a peak in the beam splitter microphone signal. I found that this was also the case on a shorter time scale and took advantage of the long down times last weekend to use a movable microphone to find the source of the peak. Microphone signals don’t usually show coherence with DARM even when they are causing noise, probably because the coherence length of the sound is smaller than the spacing between the coupling sites and the microphones, hence the importance of precise time-frequency paths.
Figure 1 shows DARM and the problematic peak in microphone signals. The second page of Figure 1 shows the portable microphone signal at a location by the staging building and a location near the TCS chillers. I used accelerometers to confirm the microphone identification of the TCS chillers, and to distinguish between the two chillers (Figure 2).
I was surprised that the acoustic signal was so strong that I could see it at the staging building - when I found the signal outside, I assumed it was coming from some external HVAC component and spent quite a bit of time searching outside. I think that this may be because the suspended mezzanine (see photos on second page of Figure 2) acts as a sort of soundboard, helping couple the chiller vibrations to the air.
Any direct vibrational coupling can be solved by vibrationally isolating the chillers. This may even help with acoustic coupling if the soundboard theory is correct. We might try this first. However, the safest solution is to either try to change the load to move the peaks to a different frequency, or put the chillers on vibration isolation in the hallway of the cinder-block HVAC housing so that the stiff room blocks the low-frequency sound.
Reducing the coupling is another mitigation route. Vibrational coupling has apparently increased, so I think we should check jitter coupling at the DCPDs in case recent damage has made them more sensitive to beam spot position.
For next generation detectors, it might be a good idea to make the mechanical room of cinder blocks or equivalent to reduce acoustic coupling of the low frequency sources.
This afternoon TJ and I placed pieces of damping and elastic foam under the wheels of both CO2X and CO2Y TCS chillers. We placed thicker foam under CO2Y but this did make the chiller wobbly so we placed thinner foam under CO2X.
Unfortunately, I'm not seeing any improvement of the Crab contamination in the strain spectra this week, following the foam insertion. Attached are ASD zoom-ins (daily and cumulative) from Nov 24, 25, 26 and 27.
This morning at 17:00UTC we turned the CO2X and CO2Y TCS chiller off and then on again, hoping this might change the frequency they are injecting into DARM. We do not expect it to effect it much we had the chillers off for a ling period 25th October 80882 when we flushed the chiller line and the issue was seen before this date.
Opened FRS 32812.
There were no expilcit changes to the TCS chillers bettween O4a and O4b although we swapped a chiller for a spare chiller in October 2023 73704.
Between 19:11 and 19:21 UTC, Robert and I swapped the foam from under CO2Y chiller (it was flattened and not providing any damping now) to new, thicker foam and 4 layers of rubber. Photo's attached.
Thanks for the interventions, but I'm still not seeing improvement in the Crab region. Attached are daily snapshots from UTC Monday to Friday (Dec 2-6).
I changed the flow of the TCSY chiller from 4.0gpm to 3.7gpm.
These Thermoflex1400 chillers have their flow rate adjusted by opening or closing a 3 way valve at the back of the chiller. for both X and Y chillers, these have been in the full open position, with the lever pointed straight up. The Y chiller has been running with 4.0gpm, so our only change was a lower flow rate. The X chiller has been at 3.7gpm already, and the manual states that these chillers shouldn't be ran below 3.8gpm. Though this was a small note in the manual and could be easily missed. Since the flow couldn't be increased via the 3 way valve on back, I didn't want to lower it further and left it as is.
Two questions came from this:
- Why are we running so close to the 3.8gpm minimum?
- Why is the flow rate for the X chiller so low?
The flow rate has been consistent for the last year+, so I don't suspect that the pumps are getting worn out. As far back as I can trend they have been around 4.0 and 3.7, with some brief periods above or below.
Thanks for the latest intervention. It does appear to have shifted the frequency up just enough to clear the Crab band. Can it be nudged any farther, to reduce spectral leakage into the Crab? Attached are sample spectra from before the intervention (Dec 7 and 10) and afterward (Dec 11 and 12). Spectra from Dec 8-9 are too noisy to be helpful here.
TJ touched the CO2 flow on Dec 12th around 19:45UTC 81791 so the flowrate further reduced to 3.55 GPM. Plot attached.
The flow of the TCSY chiller was further reduced to 3.3gpm. This should push the chiller peak lower in frequency and further away from the crab nebula.
The further reduced flow rate seems to have given the Crab band more isolation from nearby peaks, although I'm not sure I understand the improvement in detail. Attached is a spectrum from yesterday's data in the usual form. Since the zoomed-in plots suggest (unexpectedly) that lowering flow rate moves an offending peak up in frequency, I tried broadening the band and looking at data from December 7 (before 1st flow reduction), December 16 (before most recent flow reduction) and December 18 (after most recent flow reduction). If I look at one of the accelerometer channels Robert highlighted, I do see a large peak indeed move to lower frequencies, as expected. Attachments: 1) Usual daily h(t) spectral zoom near Crab band - December 18 2) Zoom-out for December 7, 16 and 18 overlain 3) Zoom-out for December 7, 16 and 18 overlain but with vertical offsets 4) Accelerometer spectrum for December 7 (sample starting at 18:00 UTC) 5) Accelerometer spectrum for December 16 6) Accelerometer spectrum for December 18
