N. Satō, Uehara, Akutsu; following 21564. Ref: 19585.

Summary

Today's work is summarized in Fig. 1. Looked for aimed/not-aimed ghost beams in the IMM chamber; installed some beam dumps for observable ghost beams in this chamber.

Details

First step: checking beams  on IMMT1T-POM

Today we focused on the IMM chamber, leaving the IFI chamber. The first and the most important step is to confirm that the current beam lines around IMMT1T are what we aimed at. Referring 19585, we saw that  the IMMT1T_FORWARD was illuminating at about 15 mm away from the left edge of the IMMT1T-POM (Fig, 2), and I thought this would be within tolerable region; compare the situation in January (19585). With calling PRM as ALIGNED state, the IMMT1T_BACKWARD also came to the surface of the IMMT1T.

About the ghost beam (GB) generated at IMMT1 from the forward beam must illuminate the beam dump behind the IMMT1. And this seemed slightly off... maybe this would be realted to that the beam around IFI and IMM seemd shifted in the plus Y direction(?) as reported in 21564, and so, also as mentioned already, the IMMT1T_FORWARD seemed shifted in the plus Y direction compared with that of January, and so the "ghost beam generated at IMMT1 from the forward beam" would be the similar case... should we need to re-adjust these slightly??

Anyway I thouht these beam spots locations on IMMT1T-POM would be tolerable, we continued the beam dump installation.

Beam dumps #3 and #4

The most difficult beam dumps are #3 and #4 in Fig. 1; they are to catch ghost beams originated from the interferometer backward beam. By the way, it was hard to access them without cut-in the backward beam path, in the end, we opened a flange in the +Y+X direction on the IMM chamber to increase the accessibility for them.

Before installing the beam dumps, we checked if there would be actually observable ghost beam spots, and there were, maybe (Fig. 3). After taking some long time to check how they were, and I found that, for some reason, their separation was narrower than that of shown in Fig. 1, so I determined to set only single  beam dump to catch both of them. Due to 45-deg like incident angle, but we can see these dumps from a K400 flange in the minus Y direction, the mapped image seem my eyeballs was so degenerated and it was very hard to judge whether these ghost beams were properly caught with this beam dump or not... so it took long time (Fig. 4).

Beam dumps #1 and #2

In the same manner developped for the beam dumps #3 and #4, we searched for the ghost beams from IMMT1T_FORWARD, but to our efforts, we could not observe such things. So today we omit to put beam dumps #1 and #2.

Beam dump #5; reflected from the viewport window for the ISS beam

Two ghost beams created at the viewport window for extracting the light beam for ISS (intensity stabilization system) was observable with a sensor card. So we put a beam dump to catch these two ghost beams at once before they started separating (Fig. 5).

Notes

Currently, we detached a tentative beam dump located in January (19585) to dump the ISS beam, so this beam comes out of the relevant viewport window. Without proper treatment such as setting a beam dump in air, do not increase input power from the PSL room!! otherwith it may fire. This is the similar case as the IFI pickoff beam at the IFI chamber (21564).