Hirata, Akutsu on 26 Apr 2024; following 29334; also refer to some historical record in 2022's finalization Day 1, 2, 3, 4AM, 4PM, 5, 6, and 7.
Summary
Checked if ghost beams were within the relevant beam dumps in IFI chamber. Found some spread stray field but left regretfully today.
Setup
- Request PROVIDING_STABLE_LIGHT to IO Guardian, then request HOLD_ALIGNMENT to IMC Guardian.
- IMMT1 and 2 -> ALIGNED
- PRM -> LOCK_ACQUISITION so that backward light beam from PRM can be seen at the same time.
- Don't hesitate to monitor IMMT1T, POP_FORWARD, and REFL.
Details
Firstly see Fig. 1, which is the same one as Fig.1 of 21655, Day 4 in 2022.
Around STM1
Around STM1, There are four beam dumps;
- a KG5 glass behind STM1
- a KG5 glass at the +Y side of STM1
- a beam dump (nameless) located above STM1 as in 22084; made of SiC.
- a beam dump #14 below STM1; made of SiC.
Among these, the first two was not be accessible today. The 3rd one can be seen in Fig. 2, catching two ghost beams and an arch-like scattered light. This photo was taken by Miyakawa-san's IR camera without using a tripod; I holded it with my hands. IR wavelength seems assigned to be shown in violet, interestingly. Unfortunately, there seems stray light field out of the existing dumps; actually, with this IR camera, such faint scattered light fields that have not been seen by sensor cards nor IR viewers, can become clearly seen; so useful. Prepare dozens of this kind of cameras to set everywhere.
Fig. 3 shows the same scene as of Fig. 2 but taken from different viewpoint. In addition to the already-mentioned stray light, CWP1 seems shining in IR. By the way, in Figs. 2 and 3, the two beam spots on STM1 are due to forward and backward beams. Seemingly they are located not too much close to the mirror holder's edge; good. Fig. 4 shows #14 beam dump still caught a kind of ghost beam.
Around STM2
Fig. 5 was taken from behind of STM2. There is a KG5 beam dump behind STM2, and maybe caught transmitting (ghost) beam through STM2. I could not take nice photos to show that the KG5 dump catcing the ghost beams. What would be good would be that the KG5 is not so shining with IR.
Carefully looking at this photo, you can see two spots on STM2, and they should be forward and backward beams. Figs. 6 and 7 was taken from the front of STM2, which is located deep inside the chamber, and the forward and backward beam spots can be clearly seen within the holder edge. Again, there are severe scattered light fields maybe due to IFI (not the chamber's name, but the Faraday isolator itself) can be clearly seen. There might be possiblity that some of the scattered light might be due to a window on GV between MCF and IFI chambers; in Fig. 7, three beam spots can be seen on this window; from the left, these are backward beam to REFL, forward beam from IMC to STM1, and backward beam from PRM. Hopefully some of the scattered field might be mitigated after GV will be opened.
Around IMMT2
There is a KG5 beam dump behind IMMT2. Fig. 8 shows the two ghost beams are within the dump.
Beam dump #13
Fig. 9 is of beam dump #13. As shown in Fig. 1, this ghost beam won't exist if the mirror attached on IMMT2 structure would not exist. This mirror is to take out one of GWP1's ghost beams to out of the IFI chamber for monitoring the input beam alignment before IFI, but never be used at all so far. I personally guess, although making these beam paths took much efforts of us, but this kind of complexity might increase the difficulty of stray light control, so this mirror should be replaced with a beam dump, or making a shroud around IFI.
Beam dump #12
Fig. 10 is of beam dump #12. As shown in Fig. 1, these ghost beams are genereted when the "IFI pick off" beam passing through a viewport window. Again, if we discard the (so far) meaningless plan to use this pick off beam, this beam dump is not needed.
Beam dump #5
Fig. 11 is of beam dump #5, which catches a transmission (ghost) beam of a pick off mirror to reflect the backward beam to REFL table.
Beam dump #11
Fig. 12 is of beam dump #11, which catches two AR reflected (ghost) beams generated at a viewport window through which the backward beam to REFL table passes.
Beam dump #8
Fig. 13 is of beam dump #8; maybe a ghost beam generated at CWP2 by backward beam.
Around input Faraday isolator (IFI)
Figs. 14, 15, and 16 are around the isolator structure. Many ghost beams and/or scattered diffused field are found.
IFI Chamber itself
Fig. 17 shows the inner surface of the IFI chamber seems shining due to stray light field. Regretfully I could not identify from where this came today. Before O5, it may be nice to do
- Detach the unused pick off mirror, and relavant beam dumps to reduce complexity of this chamber.
- Put IFI Shroud
- (Hopefully STM1, 2, and IFI should be suspended...)
