Following 21655.

Summary

Installed an additional beam dump in front of STM1 to catch some stray light field from the inside of IFI (Figs. 1-4).

Details

As reported the other day (see the last section of 21655), there is a set of stray ligh field contating an arc and two beam spots on STM2 and STM1, and it seemed they came from the inside of IFI. Unfortunately a separation gap from the main beam to these stray light field is not sufficient on STM2, and probably sufficient on STM1 so that we could put a beam dump to catch only such unwanted stray light.

Since this August, N. Satō-san has designed the additional beam dump, which holds an SiC plate above the top of STM1 to hide the mirror surface of the STM1 from the stray light incidence. As the pillar for this beam dump should be placed very close to that of STM1, I detached one of the clamps of the STM1 pillar together with monitoring the transmission beam spot positions (partly help with Ushiba remotely), and re-attached it, and then located the beam dump structure close to STM1.

The SiC plate (JGW-D1504158) has one side polished while the other side is bare. In this application today, it is difficult for me to choose whether I should use the polished surface or un-polished surface, but after trying both cases, I chose the latter; the bare suface is facing the stray light. With the polished surface, unfortunately, it seemed the incident stray light field was not well absorved and reflected in the other direction. Anyway, I am not sure which option was nice.

The current situation around STM1 is shown in Figs. 5 and 6. The main beam from IMC is separated from the upper and lower beam dumps at STM1 (Fig. 5), and the upper beam dump catches the stray light field. This situation can be also seen with an IR viewer (Fig. 6).