Synopsis
Installed an in-vac beam dump to catch a ghost beam from one of the DC QPDs on the OMC breadboard in the OMC chamber; this is homework from the last time (22942). Also reinforced steering mirror pedestal fixing with more clamps.
Additional complex beam dump
As reported in 22934 and 22942 last November, in the OMC chamber, "beam dump 1" could not have been placed; this beam dump should have caught the ghost beam from one of the QPDs on the OMC breadboard. The ghost beam was clearly visible with a sensor card, and so we understood/agreed that this would mean a severe scattered light was left in the OMC chamber before closing the chamber in the course of the O4a commissioing. Indeed during O4a commissioning, some PEM investigation revealed there might be scattered-light noise contribution right below the current sensitivity curve, which should be managed before starting O4b.
To catch the ghost beam concerned, we need a special-shaped beam dump, which has already been designed by SatÅ-san (see JGW-T2314840). As the workplace is so complicated, it was anticipated that we may need to try several combinations of the relevant parts to insert the KG5 glass on the way of the ghost beam.
I firstly detached the old beam dump 1, which was for some reason left close to the position where the new beam dump will be. With the old beam dump, according to the memos in 22934 and 22942, no ghost beams are caught. Then, after trying several combination of the parts, I was able to find a resolution to set the new beam dump (Figs. 1 and 2) there. One of the complex parts was to take the relative position of the supporting bar arm of the new beam dump and the cable wires from the back of the QPD (Fig. 3). To install the beam dump, firstly I assembled all of them outside the chamber, and then SOMEHOW bright it to this position; in the end those cables should not touch the beam dump. I firstly thought that I may be able to put the glass unit at the tip of the arm after installing the main body of the beam dump, but due to the limited space/access, it was not possible.
Anyway, the beam dump position was adjusted to catch the ghost beam and fixed finally (Fig. 4). The ghost beam would be in p-pol., although I could not confirm with a polarizer, as the working space was so narrow and so dangerous. Anyway basing on this assumption, I adjusted the whole alignment of the beam dump structure so that the angle of incident of the ghost beam to the KG5 glass would be close to its Brewster angle, around 56 deg; see the measured data shown in page 3 of JGW-T1503265.
Further clamping the steering mirror pedestals
In Fig. 1 of 22942, there are four unsuspended steering mirrors in the OMC chamber, named 1, 2, 3, and 4. The pedestals of them were all fixed with a single clamp. Regarding the number of clamps, we had a discussion and agreed that each one should have three clamps. So taking advantage of this chance, I increased the number of the clamps for 2, 3, and 4, while it was hard to access 1.
