KAGRA Logbook

Yesterday, in the morning meeting, Inoue-san introduced his observation of scattered light from ETMY. Also, there was a klog entry in the past by Kimura-san and Yokozawa-san(http://klog.icrr.u-tokyo.ac.jp/osl/?r=7695). So, just before we started working on PR3's First Contact removal, with help of Yokozawa-san, I went to the Y-end to take a look at ETMY with a light source that we recently started using for inspection and cleaning of core optics. The purpose of the visit was of course to judge whether the reported point-looking scattering source was dust on the mirror surface or not. Although space inside the inner shield is very busy(!), I was able to take a look at the HR surface with the light source. I observed dust on the surface, and it is likely that the point-looking scattering sources reported in the above are in fact particulate contaminants (and some fiber-looking stuff) on the surface, not defects in the coating nor the sapphire crystal. I could not take a picture that could show the observed dust, but attached are pictures of the breadboard and barrel of the mirror (I am sorry for its being out-of-focus, but you can identify contaminants). Since I hear that CRY group extensively clean things inside the chamber, I suspect there must be continuous contamination sources somewhere, possibly inside the chamber or duct connected to the chamber.

As I reported in the past (http://klog.icrr.u-tokyo.ac.jp/osl/?r=7621), there is a phenomenon, triggered by contaminants, where coating will be damaged at a threshold in laser power density. According to LIGO-G1400209, the average threshold is about 92[W/mm^2]. If IOO inject 20[W] at the very upstream, roughly considering buid-up factor of PRC(10) and arm cavity (1000), laser power inside the cavity will be 10[W]x10x0.5x1000=50[kW]. The beam spot area on the sapphire mirror is about pi*(35mm)^2=3850[mm^2]. So, the power density will be ~13[W/mm^2], and I am not sure at all if we have enough margin. I have a bad feeling about this..