N. SatÅ, Akutsu; following 21654
Summary
As shown in Fig. 1. Continued beam dump installation. Most of the, except for one, ghost beams were dumped. Instead, new scattered light field started to be seen.
About beam dump #11
We have a concern on how to install/tune beam dump #11 in Fig. 1. This one should catch a pair of reflection beams from a viewport window through which IFO_REFL beam exits. It would be hard to access here and do fine tuning after installing the viewport window, but this twin beams would not appear without this window installed, and so hard to do fine tuning; a tentative deadlock. Fortunately there is another ICF203 viewport in the +X direction, which is now sealed with a blank flange, through which we may be able to aceess this beam dump, hopefully.
Beam dump #12 installed
This beam dump should catch a pair of reflection beams from a viewport window installed on Day 1 (see 21564). This viewport windows is, as explained in 21564, to extract IFI-pickoff beam, which is unfortunately about 2 cm higher than the center of nominal beam height in this chamber (see 21476). Today the required parts were delivered, so we assembled them to make a taller beam dump, and installed it so that it could catch the twin beams (Fig. 2).
Note that we forgot this beam is in S-pol, and assumed P-pol, so we only prepared the P-pol beam dump, but maybe this would work enough...
Beam dump #13 installed
This beam dump was not firstly planned. But as shown in Fig. 1, the ghost beam from CWP1 (maybe) flyes to the beam dump in the side of STM1, and the beam dump reflected this beam to, accidentally, the mirror (see again Fig. 1) for reflecting IFI-pickoff beam!! Moreover, unfortunately, on Day 1 (see 21564), the ghost beam #13 was hit at the edge of the IFI pickoff mirror!!! This beam path is drawn in Fig. 1 with a yellow dashed lines.
Today we slightly rotated the beam dump at the side of STM1 with hoping if this ghost beam #13 can reach more center of the IFI-pickoff mirror... and fortunately, we did it!! (Fig. 3). Now the ghost beam #13 comes at well within the IFI-pickoff mirror (note that the actual IFI pickoff beam is more edge of this mirror...) and so the reflected beam was caught with the newly installed beam dump #13 (Fig. 4).
Beam dump #14 installed
This beam dump was also not planned, but we found it would be needed. A pair of ghost beams, maybe from inside of the Faraday rotator were found on Day 1 (maybe not reported...). They were separated from the main beam line at least on STM2, and then they reached bottom of STM1 mirror region; so annoying. Unlike beam dump #13, these ghost beams need a lower beam dump. The ordered part in the last week (we did so immediately after we found these on Day 1) were delivered, so we assembled them, and installed it. As this beam dump top is close to the main beam line, SiC plates (one side polished to Ra0.01; used also on Day 3 in IMM chamber; see 21638). Of course the polished sufraces are set to face inside each other in the V shape (Figs. 5 and 6).
By the way,I did not check the polarization of these ghost beams (we can do such identification with some expensive polarizer from Thorlabs, manually), and the plates are standing as if the ghost beams would be P-pol. Hope the V-shape structure would dump the beams sufficiently, for whatever poralization would be...
Additional scattered light field!!
I found new stray light field, having shapes of multiple arcs, maybe coming from the Faraday rotator, are illuminating upper side of STM2, and partly reflected to STM1, and also illuminating STM1's upper side and made also arc-lke stray light field... today I have no idea sure how to overcome this situation.
