K. Tanaka, Akutsu, Nakano(remotely)
To overcome those situation 9846, 9817 and 9681, we replaced the oplev light source for ITMX, but the sum was not recovered. There would be some mixture of causes behind this degrading issue...
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We replaced the original oplev light source installed for ITMX (the serial number was 225022). The new one's was 228144. But the sum was only slightly revived (284cnts -> 546cnts)
See Fig. 1 - Then I tried to switch the gain setting on the tilt-QPD circuit to the higher. This is shown in Fig. 1 as a sudden jump up of the sum in-between 546-cnt-state to 740-cnt-state. By the way, why was the gain switch set to the lower here? For lower sensing noise? But for me it seems the setting concept is not the same for the other test masses... Need to check.
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Anyway, we again replaced the new one to the new-new one (228141). The sum was only slightly revived (284cnts -> 546 cnts -> 740cnts) as shown in Fig. 1. Considering that the sum was around 1000 two days ago (Fig 2), the number is too small, and THIS degrading would happen not only due to the light source aging. Note that 1000 was still too small; probably the light source reached its lifetime anyway, but I do not have sufficient evidences. As mentioned above, we need careful when we say something about the degradation of oplev light sources for cryostat. It would not only due to simple frosting...
So what is the reason? In Fig 1 and 2, I also plot the other signals such as temperature of 8K shiled's wall, temperature of the sapphire mirror, and vacuum gauge for IX. But it is difficult to say any definite things so far. Did some particles coming out from the mirror adhere to the oplev viewport windows???? Last year, for example, just after finishing the WAB cooling test at IYC, and the cooler was turned off, then the similar phonomenon was observed; see 4882 and Fig 3, which was a trend of the oplev sum for WAB, and Hasegawa-kun sent me on the day. As you see the behavior cannot be understood easily...
By the way, see Fig 4. During my work, I found the inside of the IXC chamber was quite shinig in green. When AOS folks had been installing WABs, we sometimes needed to work along with green-beam illumination. But such scattering light had not been observed. At that time the sapphire mirrors were not so shiny.
