K. Tanaka, Nakano, Akutsu: continued from; 15970.
With P-pol input, searched for the ghost beams. I confirmed that every ghost beam found with S-pol could be observed with P-pol in MCF. No strange ghost beams.
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OMG
With further careful check I found the MCo ghost beam ("a" in15970) evaded (was not caught by) the dedicated beam dump! This means the ghost beam illuminated the MCF-IFI bellows during O3GK. This might support the PEM injection result, which said the MCF-IFI bellows was sensitive to DARM when PEM injection . With P, ghost "f" could be also observed. - Around MCi, With P-pol input, I can even find ghost beams reflected by the viewport window for IMC_REFL ("K" and "L" in 15970. See Fig. 1.)
- Ghost beams around MCe are hard to see even though we aligned IMC flashing with the P-pol beam. Furher investigations will be done.
Detail
- Around MCo; yesterday we thought the ghost beem "a" was in the beam dump, but that was not true. Today I confirmed that the ghost beam went through the side of the beam dump and illuminated the edge of the "2nd" viewport window on the gate valve between MCF and IFI (the 1st is for the main beam); see Fig. 2 and 3. Although the gate valve was opened during O3GK, the ghost beam would illuminate the bellows or the inner wall of the vacuum chamber that has high reflectivity like a mirror. I have no idea about the concrete coupling path to DARM, but this might contribute to the noise level of DARM during O3GK, as PEM injection experiments suggested.
- Also around MCo, at the inner frame structure, with P-pol input, I found the ghost beam that would correspond to "f"; see Fig 4. Now I start wondering whether the default black shield would be sufficient for O4 sensitivity. But consiering the separation from the main beam, there seems no other choices....????
