KAGRA Logbook

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IOO (IFI)

yoichi.aso - 20:57 Thursday 07 October 2021 (18503)

Ghost beam tracing around IFI

Yasui, Aso

After wasting some time for the clumsy picomotor cables. we finally checked the ghost beams around the IFI.

We found a lot of ghost beams around the main beam on STM2.

By blocking the main beam at various places, we found that most of the ghost beams are coming from components in the IFI.

For example, the surface reflection from the output CWP was creating an arc like ghost beam seen near STM2.

Reflections from the HWP, TGG, DKDP are also creating ghost beams.

We may have to install a lot of beam dumps to kill them all. We may want to ask LIGO how they are dealing with these beams.

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Comments to this report:

yoichi.aso - 12:01 Saturday 09 October 2021 (18517)

I adjusted the position of the SiC aperture on the output side of the IFI so that the beam is centered on it.

I also centered the beam on the IMMT1 trans QPD1.

However, currently the beam is blocked by a temporary beam dump in front of the STM1, so no beam reaches the QPD.

This is because we have not recovered the aluminum foil duct between the IFI and IMMT chambers, and these chambers are closed with white flange covers.

Tomaru-san will measure the height of these chambers next week. We plan to recover the duct after that measurement.

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