KAGRA Logbook

MIF (General)

kenta.tanaka - 17:55 Wednesday 15 January 2025 (32348)

Beam path investigation on the AS table

Yokozawa, Ushiba, Tanaka

related to klog31986 (REFL), klog31992 (MCE TRANS), klog32330 (IMC REFL)

The optical layout on the AS table is here. Optics name convention as below is followed as the ones in this layout. Working photo are saved in the KAGRA dropbox.

## Abstract

We checked the beam path (OMC {TRANS, REFL}, AS) on the AS table when X-arm was locked. The beam position of almost all of the optics seem to be fine in the OMC TRANS and AS path, except for the HWP3 in the AS path. We shifted the HWP3 and tweaked the mirrors in front of PDs and QPDs for the centering. We found that there are some ghosts around the beam dump and HWP1. We should dump them after the new dumpers are delivered.

On the other hand, the beam position on the periscope upper mirror in OMC REFL path seems to be shifted from the center and clipped. So we need to lower the position of this mirror. However, we have no time today. We will do it later.

## What we did

We checked the beam path (OMC {TRANS, REFL}, AS) on the AS table when X-arm was locked.

### OMC TRANS path

First, we checked the beam in the OMC TRANS path. We found that the beam power in this path was too low to see the beam postion on the optics with the IR camera. So we used a sensor card to check the beam position. The beam positions seem to be fine.

### AS path

Next, we checked the beam in the AS path. The beam position on HWP3 was shifted in the -Y direction. So we shifted HWP3 to the -Y direction. The beam postions on the other optics until the QPD path seems to be almost center. The beam positions on the mirrors in the QPD path seemed to be off center slightly. Therefore, we tweaked STM4 and ST12 for QPD2 centering and BS3 and ST3 for QPD1 centering.

After that, we tweaked the steering mirrors to center RFPDs and the AS camera.

Also, we found that there are some ghost beams around the beam dumper (fig.1) and HWP1 (fig.2). We should dump them after the new dumpers are delivered.

### OMC REFL path

Finally, we started to check the OMC REFL path. We turned off the ceiling light above the OMC booth and we took the photo of the periscope upper mirror in the path (Fig. 3). The beam position seems to be shifted from the center and clipped. So we need to lower the position of this mirror. However, we have no time today. We will do it later.

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## Note

We left the SD card of the camera in front of the k1ctr9 PC. Do not forget to bring the card before using the IR camera.

Images attached to this report

Comments to this report:

kenta.tanaka - 19:24 Thursday 16 January 2025 (32361)

Michimura, Ushiba, Tanaka

We lowered the upper mirror of the periscope to solve the clipping the beam at the mirror. Fig. 1 shows the result. The beam itself became able to be seen so the beam seems not to be clipped at the mirror.

Then, we found that the beam height in beam path seems to be lower than the nominal height in this state. So we lefted up the lower mirror position of the periscope to adjust the beam height..

After that, we realigned the OMC REFL beam path.

Also, we tried to dump one of the ghosts from LNS3 which hit on the 2-inch mirror holder after HWP3 (fig.2) but we could not find the optimal position for a beam dumper to dump the beam. We gave up to dump this today.

Images attached to this comment

kenta.tanaka - 20:37 Monday 20 January 2025 (32398)

[Ushiba, Uchiyama, Komori, Tanaka]

Today's working photo are saved in the KAGRA dropbox. We updated the current layout on AS table (JGWdoc)

## Abstract

We bustered some ghost beams around the OMC REFL beam path. As for the ghost on the HWP in the AS path, therer seems to be no space to put the beam dumper. But, the ghost beam hit on the rotater and the beam power is too low. We left it for now.

## What we did

The 1st ghost was on the mirror holder as reported in previous klog32361. This ghost seemed to originate from LNS3 and this ghost beam path was very close to the main beam path. So we tweaked LNS3 to separate each path. After the tweaking, the ghost beam passes thourgh the beside of the ST6. Then we dumped it (fig.1).
The 2nd ghost was on the lens holder of LNS5 from BS5 (fig.2). We installed an iris and block the ghost beam with the iris (fig.3).
The 3rd ghost was on OMC REFL QPD1 (fig.4). (There are 2 beams on QPD1 in fig.4, the one of higher beam position was the 2nd ghost. This is dumped by the iris in front of LNS3. The another of the same height as the QPD element was the the 3rd ghost beam.) We moved the beam dumper near QPD1 and dumped it with the dumper (fig.5).
The 4th ghost was on the ST14 holder. We moved ST14 and ST15 to the -Y direction. Then, the beam passes through the beside of ST15. We installed the dumper to dump it (fig.6).
Last ghost beams passed thourgh above ST6 from the ND filter in front of CCD. We know the ND filter has been broken previously and the CCD is not used. So we replaced from the ND filter to a beam dumper.
After that, we confirmed each beam position was on each center of each optics.
As for the ghost on the HWP in the AS path, therer seems to be no space to put the beam dumper. But, the ghost beam hit on the rotater and the beam power is too low. We left it for now.

Images attached to this comment