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Reports until 12:48, Friday 20 January 2012
robert.schofield@LIGO.ORG - posted 12:48, Friday 20 January 2012 (2071)
Arm cavity baffle: installation photos, glint search and contamination control

Summary: Photos show arm cavity baffle installation and the view of the baffle from the ITM and ETM. Part of a reflective bellows can be seen from the test mass through the H1 hole. I include a photo of the interior baffle surface that could be used as a dust witness plate near the test mass. Particle counts measured during installation did not exceed 100/ft^3 (>=0.5um).


Installation photos:

Figure 1 is an annotated selection of installation photos that Lisa A. took.

Glint search:

Figures 2 and 3 show photos taken to look for potential retro-reflections  - in particular, to find surfaces that reflect light scattered from the beam spot on one of the test masses back to the beam spot. This retro-reflection, and reflection to the beam spot on the opposite test mass, are thought to be the dominant paths for scattered light noise. For the side of the baffle viewed by the ETM beam spot, Figure 2, the camera and flash were set up in the beam path many meters from the baffle. The brightest retro-reflections from within the clear aperture were from the diodes and their mounting screws and from the edges of the bends in the baffle.

For the ITM side of the baffle (wide-angle scattering side), Figure 3, I placed the camera at the location of the beam spot on the test mass, except that I had to use the side that didn’t have an optic. The baffle is close to symmetrical so I believe that the actual beam spot would have a similar view of retro-reflections from the inside of the baffle. The brightest retro-reflectors (at least at visible wavelengths) are the backs of the photodiodes, screw heads, and the bellows on the spool between the manifold and BSC8, which can be seen through the hole on the opposite side of the baffle. The outside of this bellows would be a good place to mount a shaker to test for scattering problems. It would be possible to block the view of this bellows with a divider between the two sides of the baffle.

Contamination control:

The large equipment was first staged in the Y-manifold while the dam was located in the spool piece, separating cleaned BSC8 from the un-cleaned manifold. I had recently found that particle levels were ten times higher in the manifold than in BSC8 for similar activity levels, here, and wanted to use the dam to keep manifold activities from contaminating BSC8 and the ITM. Immediately after transport, I measured particle levels in the manifold next to the dam to be about 1000/ft^3 (0.5 um or larger). After the particle level reached about 150/ft^3 (~20 minutes of sitting), I removed the dam and left carefully. As I was leaving, I measured the particle level at about 200/ft^3.  

For our work in BSC8, we had to walk a short distance into the manifold to retrieve the staged equipment, so I wanted to have clean air flowing out of  BSC8 into the manifold (clean to dirty) rather than the other way around. I made sure that the clean room at BSC8 would overpower the clean room at the spool piece, where the Y manifold was open, driving air into BSC8 from its clean room and down the manifold to the opening at the spool. To do this, I turned off the purge air, had the C3 removed from over the BSC8 ISI, removed the C3 from the manifold opening and pulled aside some of the clean room curtains there to reduce pressure in the clean room at the spool. Previous experiments had shown that this produced a good air flow from BSC8 into the manifold and out at the spool.  Most of the air was coming in through the BSC8 dome opening so I checked the particulate level, finding under 50/ft^3 inside BSC8, even when a person was on the upper level working on the ISI.

Immediately after installation we carried the railing down the manifold, and I measured particle levels of about 50/ft^3 in BSC8 as Art walked down the manifold.

Figure 4 is a photo of the inside bottom surface of the baffle, showing a few dust particles. This surface is quite close to the test mass and may be a good witness plate to monitor accumulation of dust. I think we should photograph it again when we button up.

Robert S., Lisa A., Scott S., Art R., Manuel R., Thomas V., Jodi F., Chris K.

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