KAGRA Logbook

Ushiba, Akutsu; following 30143.

Summary

We checked ghost beams and scattered light in the OMC chamber, and relocated one of the beam dump. Except for some concerns, as for what we can do immediately, all seems fine for closing the OMC chamber, in the viewpoint of optics. In short, ready for full chamber-closing of the KAGRA interferometer.

Other reference for the works

klog27252 says there are some ghost beam spots that can be seen with 10W IMC output.
klog22942 reported reasoning and explanation of some beam dumps
klog22934 Fig. 4 of this would be the original drawing for ghost beams and beam dumps in the OMC chamber, and then some modifications have been stuck on it since then (for example, Fig. 1 of 27252).

AS_RF (or RF_AS?) beam

As mentioned in 30143 as well, there has been a mystery remained from the last week; AS_RF beam was about 3 cm off the relevant steering mirror. Our discussion In the morning meeting brought us an idea that this might be explained if the OSTM's wedge was accidentaly flipped and installed. I checked it with the simulator (as kind of my hobby), and confirmed this should happen. Fig. 1 is before flipped, and Fig. 2 is after flipped. On the other hand, it was difficult to in-situ-ly identify the wedge direction. So we took a photo of OSTM; no severe scattering of IR light could be seen (Fig. 3), so it would be fine. By the way, if OSTM OSEMs are turned on, their sensor light illuminates the mirror (Fig. 4).

Check with 10W IMC ouput beam

I firstly put an obstacle strucure used by Aso-san et al. the other day to cut the OMC bow-tie path to avoid accidental OMC resonance, which will severly damage OMC's DCPDs. Then Ushiba-kun increased the IMC output power up to (maybe) 10W or so. It seemed 10W IMC output could not be stably achived together with X arm locked, so X arm locking was turned off; the detail of this method may be reported by Ushiba-kun later. With this setup, we checked still faint ghost beam spots at beam dump #7 and 8 (about the names, refer to Fig. 1 of 27252). Maybe I forgot to check beam dump #12. Ghost beams at beam dump#15 could be seen even yesterday (30143), but with this situation, they could be seen clearer. The ghost beam at beam dump#3 was properly caught.

Check with OMC occasional flash

Reducing back the IMC output to 1 W or so, we detached the obstacle inserted into the OMC resonant path, and started to check ghost beams that could be generated only when OMC being in resonance. In fact today we need to expect some occasional flashing, because OMC was still in air, so it was hard to keep the resonance for long time. Ushiba-kun did some efforts to make the occasionality higher, so he may be report the detail in a separate post, hopefully. For beam dump#2, it might be difficult to access, but the geometrical relationship might not change, so this should catch the relevant ghost beam. Beam dump#5 is the one we need OMC resonance. It was very hard to identify this ghost beam, but thanks to Ushiba-kun, we somehow identify this beam and I re-located (after some iteration) this beam dump properly.

By the way, beam dump#1 and #4 was already checked yesterday.

Homework and concerns

Please cross check if three each beam after the relevant each steering mirror would not be interfere against some structure or not.
As shown in Fig. 5, this has been long known that OFI has a severe scattering light spot; we would need to consider re-design or at least modification of OFI itself.
A unknown beam spot? was found at the upper part of the steering mirror for OMC refl (mirror#1 in Fig. 1 of klog27252). See Fig .6. The beam spot gone when taking photos in the different directions (Fig. 7). We have no idea today... Need to be clarified later. As already mentioned several times, this IR camera also has a repsonse to OSEM light, even from the faraway suspensions, so well-considered tools would be needed to do more speefy checks.
The top inside, wall inside, and also some upper structures of OMC suspension seemed shining in IR with the IR camera. I have no idea what they are; need further investigation with turning off OSEMs (Figs. 8 and 9).