N. Satō, R. Takahashi, Miyoki, Akutsu; following 22851

Summary

(Re-)installed steering mirrors, beam dumps, and shield plates in the OMM and OMC chambers; still on the way. We did the following mostly with IR-Y locked.

Preparation: confirming output beam alignment

I checked main IR beam spots on OMMT1, OMMT2, and OFI. In fact, it was difficult to check if they were strictly centered or not, but feelingly and seemingly they would be so with my eye balls. Together with this, I checked the beam spot on OMMT2T-QPD1 again. As no RTmodel and MEDM for OMMT2T yet, I could only read raw outputs from the relevant ADC (Fig. 1); around -80 ~ -100 counts when being illuminated with IR. Whichever turning on/off ceiling light of the tunnel, the OMMT2T-QPD1 read seemed unchanged, meaning that the ambient-light shielding would be sufficient. Note that the OMMT2T-QPD1 gain would be set to the higher now.

I found the OMC QPD2 sum was ~8000 but it was ~10000 on the last Friday, while the counts are almost unchaged for OMC QPD1. Strange. To check things, I and (remote) Aso-san confirmed that the OMC-ASC loop (actuators: OMMT2 and OSTM) could be closed without problems; during the loop closed, the OMC QPD2 sum increased to ~9000. Now, considering the IMC output was about 98% (according to Yokozawa-san) of that of the last Friday, the difference of sum from the last week in OMC QPD 1 and 2 could be consistent. Anyway, it is interesting the OMC QPD2's behavior....

In the course, I was slight worried offset in the OMC QPD1 and 2 when the OMC ASC loop opened, so I asked Aso-san to do some offload. Aso-san offloaded them only with OMMT2 picomotors, and that seemed sufficient.

Reinstalling shield plates for output suspension structures

I found some black shield plates for the output suspension structures have been still unmounted, so asked Satō-san and Takahashi-san to reinstall them. We did this together with monitoring the OMC QPD1 and 2, as too much torqueing may cause plastic change of the whole system and the QPD signals might go far away as happend in the input optics sometimes, which should be avoided.

Anyway, the shiled plates were reinstalled onto the OMMT1, OMMT2, and OSTM suspension structures somehow, and we once confirmed the OMC ASC loop could be closed.

Finalizing OMM chamber (mostly done)

I found two ghost beam spots coming back from OFI in the OMM chamber (Fig. 2). Both ghost beams might be seemingly shut with the shield plates installed at the front side of the OMMT2 structure, but we installed the beam dump for the brighter one (left to the mirror). With a polarizer, I confirmed this ghost beam was in S-pol, and so we installed the beam dump for S-pol against this (Fig. 3).

Finalizing OMC chamber (still on the way)

Please refer to Fig. 4 by Satō-san. For the names of optics in/around OMC, see JGW-T2113071. The current situation is summarized as

• Beam dump 1: not yet installed; the relevant ghost beam has an issue; discussed later.
• Beam dump 2: installed since O3GK, and today I confirmed it is located properly
• Beam dump 3 and 11: today I slightly relocated this so that this could catch ghost beam 3 and 11 simultaneously; the usuable space was not sufficient for installing additional beam dump for 11.
• Beam dump 4: not yet installed; we confirmed there are relevent ghost beams not only from main beam 3 (OMC_REFL) but also main beam 2 (AS RF).
• Beam dump 5: not yet installed.
• Beam dump 6: not applicable, as we could not see any ghost beam here. Maybe the relevant mirror (OBS3) would have nice quality.
• Beam dump 7 and 8: not applicable, as we could not see any ghost beam here. Maybe the relevant mirror (OMC1) would have nice quality.
• Beam dump 9: reinstalled today. I confirmed there was a ghost beam due to that OMC_REFL parthally thrrough the mirror 1.
• Beam dump 10: already installed.
• Beam dump 12: not applicable, as we could not see any ghost beam here. Maybe the relevant beam would be sufficiently weak(?)
• Beam dump 13: newly installed today for confirmed ghost beam.
• Beam dump 14: not applicable, as we could not see any ghost beam here. Maybe the relevant beam would be sufficiently weak(?)
• Steering mirror 1: replaced with the newer mirror (30-50 deg one of JGW-D2113030), which was prepared before by Satō-san (22672). This is to steer OMC_REFL.
• Steering mirror 2: replaced with the newer mirror (30-50 deg one of JGW-D2113030), and the downstream optical path was re-aligned mostly hopefully. This is to steer AS_RF.
• Steering mirror 3: reinstalled with the newer mirror (30-50 deg one of JGW-D2113030), and the downstream optical path was aligned to the outside periscope. Note that this is to steer OMC_REFL.
• Steering mirror 4: already installed the other day (22900) with the newer mirror (30-50 deg one of JGW-D2113030). This is to steer OMC_TRANS.

Among these, or among all the in-vac optics installation in my KAGRA life, I have to give the award of the worst hell to the compound of steering mirror 1 and beam dump 9; it was almost impossible for me to access here and also do precise adjustment of these. I never ever want to repeat.

All the installation work were done together with monitoring OMC QPD 1 and 2. Note: never weigh on the optical bench in the OMC chamber, as weighing it might plastically vary the balance of the optical bench to the level that the IR main beam would go off OMC QPD1 and/or 2.

For the ghost beam to be caught with beam dump 1, born at OMC QPD2 surface reflectoin, this ghost beam is now illuminating back of OMC QPD1. The alignment of OMC QPD2 has some play for tuning, and unfortunately the tuning was wrong (or just had forgot of this ghost beam treatment?). Anyway, this ghost is visible with the sensor card, so this must be properly caught. To do so, there would be at least two options:

1.  with a specially designed dump to be inserted before the ghost beam would reach OMC QPD1 back
2. adjust OMC QPD2 alignment somehow.

However, we have no way to do whichever of these, considering the remaining time for O4a starting.

Notes

• In Fig. 2, the streak is just due to that OMC_REFL beam had not been properly treated at this time; after the proper treatment, this streak disappeared, so forget of this.
• There is still an iris diagphram in the OMM chamber. This should be removed before closing this chamber.
• For getting OMMT2T, the rear side shield for the OMMT2 suspension structure was not reinstalled, of course.
• In the OMC chamber, re-checking if we would need beam dump12 should be done later.
• We could sometimes work inside the OMC chamber together with turning on the OMC ASC loop. I was so quietly working in the chamber in this case. Particulally this kind of trick was required when looking for ghost beams for beam dump 6 and 14, as only (even tentatively) OMC locked (as stable as possible...) state could provide "viewable" beams (with the sensor cards F-IRC4) off the OMC; well, but, as written in the above, I could not see any ghost beams.
• At OMC chmanber, the viewport at +X+Y was opened today to make more accessibility inside the chamber.