Here are some pictures of the beam dumps we installed last night to dump ghost beams originating from the beam splitters on the REFL path. This is the status as of Tuesday night, Keita and Ryan went in this morning to finish up this work.

Some of the earlier pictures are out of date now since we took the beam dump we installed near REFLAIR and used it to dump the original ghost beam upstream, removing the need for the REFLAIR beam dump.

• First Keita installed a beam dump by the REFLAIR beam diverter to catch a ghost beam that originated at the first beam splitter on the REFL path (first 4 photos).
  • NOTE this beam dump was later removed!!
• Then we looked at a ghost beam that was grazing the side of the high power beam dump, and then hitting the back of ASC-REFL_B
  • 5th photo shows this viewed through the IR camera. The beam dump is very bright and we were confused, thinking the beam was missing the Silcon Carbide piece, but it is just scattering and a very high power beam (6th photo).
  • We installed a black glass beam dump to catch the beam before it hit the side of the high power beam dump. It does not interrupt the main beams going to the high power beam dump (photos 7, 8, 9)
• There was another ghost beam passing just to the -X of M2 (the final 50/50 splitter installed last week). We had to scootch the beam dump behing M2 to make way for a new beam dump to catch the ghost (photos 10, 11, 12)
  • 13th photo is a top down view near the high power beam dump showing the 3 black glass beam dumps
• Finally we sought to dump a ghost beam that was hitting the barrel of M14.
  • Dumping this beam removed the need for the first beam dump described above, near REFLAIR, so we took this back out, removed one of the glass panels (since there is not a lot of space between HW1 and M14) and inserted it to catch the ghost beam
  • The reflection off this black glass was quite significant (visible on the card) and we needed to dump this reflection, but had run out of beam dumps and called it a night.

Continued the work on Wednesday morning with Ryan. See 1st attachment, that's  cartoon showing which additional beam dumps were installed. Circled in green are for ghost beams in REFL path.

It took some time but we figured out that, not surprisingly, the strongest ghost beam was the AR reflection of the 1st 90:10 (M14) reflected by the 90:10 itself, going into the REFL path direction. On the 1st 50:50 (M17), that ghost beam was half reflected and hit the back of WFSB and half transmtted and hit the inner barrel of M17 and came out as a funny beam, going to the next 50:50 and reflected. I don't have any "before" picture but Georgia might.

We inserted a beam dump with only one plate ("A"), then installed a V-shaped one ("B") to receive the reflection by A.  2nd picture shows the ghost beam and the main beam on a viewer card in front of the "A" beam dump. The ghost beam is not clipped by the beam dump edge but is very close, but I wanted to make sure that the main beam is not clipped. The 3rd picture shows the ghost beam caugut by the beam dump. Here the translucent edge of the card is aligned with the edge of the glass plate, so there is some room before it's clipped. 4th picture shows the V-shaped beam dump at "B" position.

There were also two ghost beams we couldn't identify exactly, but we installed two beam dumps ("C" and "D"). See 5th and 6th picture. It might have been possible to just use one for both beams but the beams were separated enough that we couldn't comfortably do it at "D" location, and "C" location was not really ideal for precise positioning of the beam dumps due to the fact that it's close to the center of the table and we had to reach deep into the chamber.

BTW, in the 5th/6th pic you can also see that the 90% beam on the high power beam dump is super bright and scatter very strongly. Yesterday Georgia and I were worried if it was hitting the metal part, but after moving the beam down by a few mm on the beam dump (there wasn't any change in the brightness) we concluded that the beam was/is hitting the SiC plate and it's the nature of the plate.

The rest of the pictures show the newly installed beam dumps (as well as the old ones).

All in all, in this incursion we installed 8 new beam dumps. Only one of them (the one to receive the transmission of M2 corner mirror which is now 50:50) was planned to be installed.  One was installed as we were able to see the ghost beam of M2. Four were installed to catch ghost beams that comes from upstread of M2. Two were installed because we saw two ghost beams from the main septum viewport for the PSL beam, which landed on and were reflected by the newly installed nozzle baffle (alog 63587), and we wanted to properly damp these reflections on the table.