R. Takahashi, Aso, Akutsu on Oct 18, 2023 JST; related to 27245. In some sense, following 25944 (2023 July opening OMC to install a beam dump: JGW-T2314840-v2).

Synopsis

In the OMC chamber, we added three beam dumps that were removed last year.

Details

In the last year before KAGRA's O4a run, some beam dumps were detached (see 22942), because I could not see ghost beams to be dumped by them at that time. Note that putting beam dumps randomly without identifying the ghost beam paths would make another scattered light due to unwanted collision of the ghost beam against the randomly-put beam dumps, which should be avoided. Today we took advantage of using 10 W laser input (27246) at IMC, which would make power excitation of 10W * 0.1 * 0.5*0.5 = (PRM transmission, BS x2) = 250 mW at around OMC for a single-bounce-at-whichever-ITM. Note that we have only 1-1.5W input at IMC in usual, and I could not do these checks last time opening the OMC chamber (see 25944).

The I was able to see some ghost beams with a sensor card, although they were quite faint. They correspond to the ghost beams to be caught by the beam dumps #7, 8, and 12, respectively in Fig. 1, so we re-installed these three dumps. So far Fig. 1 does not necessarily reflect the real locations of them; need to be modified according to some photos later. Anyway, conceptually, Fig. 1 is the as-of-today situation for the beam dumps.

Especially for the beam dump#12, firstly it was clipped by the mirror holder itself, as the rotation of this mirror holder was 90-deg wrongly installed for some reason. So,with iris diaphragms to save the virtual optical path, we once detached the mirror holder from the pillar, then rotated it in 90 deg in roll and fixed it again, then took alignment. Now we believe the reflected beam from this steering mirror would illuminate the AS RFPD and AS WFS PDs. I also confirmed that the ghost beams generated at a viewport were caught by the beam dump#15 with this "new" optical path.