On Friday 06/07/2024 Dave Barker sent an email to the vacuum group noting 3 spikes on the pressure of the main vacuum envelope, I took a closer look at the 3 different events and noticed that the events correlated to the IFO losing lock. I contacted Dave, and together we contacted the operator, Corey, who made others aware of our findings.
The pressure "spikes" were noted by different components integral to the vacuum envelope. Gauges noted the sudden rise on pressure, and almost at the same time ion pumps reacted to the rise on pressure. The outgassing was noted on all stations, very noticeable a the mid stations, and with less effect at both end stations, and for both with a delay.
The largest spike for all 3 events is noted at HAM6 gauge, we do not have a gauge at HAM5 or HAM4. The one near HAM6 is the one on the relay tube that joins HAM5/7 (PT152), with the restriction of the relay tube, then the next gauge is at BSC2 (PT120), however the spike is not as "high" as the one noted on HAM6 gauge.
A list of aLOGs made by others related to the pressure anomalies and their findings:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=78308
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=78320
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=78323
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=78343
Note: the oscillation visible on the plot of the outer stations (Mids and Ends) is the diurnal cycle, nominal behavior.
Of live working gauges, PT110 appears closest to the source based on time signature. This is on the HAM5-7 relay tube and only indirectly samples HAM6*. It registered a peak of 2e-6 8e-6 Torr with decay time of order 30 17s. Taking a HAM as sample volume (optimistic) this indicates at least 0.02 0.08 torr-liters of "something" must have been released at once. The strong visible signal at mid- and end-stations suggests it was not entirely water vapor, as this should have been trapped in CP's.
For reference, a mirror in a 2e-6 Torr environment intercepts about 1 molecular monolayer per second. Depending on sticking fraction, each of these gas pulses could deposit of order 100 monolayers of contaminant on everything.
The observation that the IFO still works is comforting; maybe we should feel lucky. However it seems critical to understand how (for example) the lock loss energy transient could possibly hit something thermally unstable, and to at least guess what material that might be. Recall we have previously noted evidence of melted glass on an OMC shroud.
Based on the above order-of-magnitude limits, similar gas pulses far too small to see on pressure gauges could be damaging the optics.
It would be instructive to compare before/after measures of arm, MC, OMC, etc. losses, to at least bound any acquired absorption
*corrected, thanks Gerardo
Corner RGA scans were collected today during maintenance, using RGA on Output tube. RGA volume has been open to main volume since last pumpdown ~March 2024, but electronics head/filament was turned off due to the small fan on the electronics head not spinning during observing. Unable to connect to HAM6 RGA, through either RGA computer in control room, or locally at unit with laptop. Only Output tube RGA available at this time.
Small aux cart and turbo was connected to RGA volume on output tube, then RGA Volume isolated from main volume and the filament turned on. The filament had warmed for ~2 hours prior to RGA scans being collected.
RGA Model: Pfeiffer PrismaPlus
AMU Range: 0-100
Chamber Pressure: 1.24E-8 torr on PT131(BSC 3), and 9.54E-8 torr on PT110 (HAM6), NOTE: Cold Cathode gauge interlocks tripped during filming activings in LVEA today, BSC2 pressure not recorded
Pumping Conditions: 4x 2500 l/s Ion Pumps and 2x 10^5 l/s cryopumps, HAM6 IP and HAM7/Relay tube
SEM voltage: 1200V
Dwell time: 500ms
Pts/AMU: 10
RGAVolume scans collected with main volume valve closed, only pumping with 80 l/s turbo aux cart
Corner scans collected with main volume valve open, and aux cart valve closed
Comparison to March 2024 scan provided as well.
RGA is still powered on and connected to main volume with a continuous 0-100AMU scan sweep at 5ms dwell time
Richard posting from Robert S.
I had a work permit to remove viewports so I opened the two viewports on the -Y side of HAM6. I used one of the bright LED arrays at one viewport and looked through the other viewport so everything was well lit. I looked for any evidence of burned spots, most specifically on the fast shutter or in the area where the fast shutter directs the beam to the cylindrical dump. I did not see a damaged spot but there are a lot of blocking components, so not surprising. I also looked at OM1 which is right in front of the viewports. I looked for burned spots on the cables etc but didnt see any. I tried to see if there were any spots on the OMC shroud, or around OM2 and OM3, the portions that I could see. I didnt see anything, but I think its pretty unlikely that I could have seen something.
Repeated 0-100AMU scans of the corner today, after filament had full 24 hours to warm up. Same scan parameters as above June 11th scans. Corner pressure 9.36e-9 Torr, PT 120.
Dwell time 500 ms
Attached is comparison to yesterday's scan, and compared to the March 4th 2024 scan after corner pumpdown.
There is a significant decrease in AMU 41, 43 and 64 compared to yesterday's scan.
Raw RGA text files stored on DCC at T2400198