[Hirose, Saito]

We constructed the optical layout for the PLL.

• For the PLL, we built an optical system to inject both the sub-laser IR beam and the IR beam coming from the interferometer into the RFPD simultaneously (Photo 1). The current optical configuration is shown in Fig. 1. Next, we adjusted the HWP to maximize the power of the sub-laser beam entering the interferometer. Under this condition, the power of the sub-laser beam incident on the RFPD was measured with a power meter to be 1.78 mW without the ND filter, and 8.9 μW with the ND filter. The power of the beam coming from the interferometer was 3.8 μW. In total, 12.7 μW of light is incident on the RFPD, which is well below the maximum allowable input power of 1 mW, so this is not an issue. The next step is to check the signal from the RFPD.

[mTakahashi, hSawada, Nakagaki, Yasui]

1. Vacuum evacuation by DSP
We tightened top- and +X-side- flanges of SRM, and started pumping down by the Dry Pump.

2. Leak check of Q-mass
Also, we had a leak test around Q-mass that installed yesterday.
Since the leak level was less than 1×10^-13Pam^3/s, we opened the valve and connected to the OMMT chamber.

3. Vacuum evacuation by TMP
After rough pumping by the dry pump, I started evacuating by the TMP.       

Date: 2026/04/24

Member: Dan Chen, Misato Onishi, Seiya Matsuo

We performed our usual WSK calibration at UToyama.

The results look no problem.

Results

Comparison with Previous Results

Comparing with previous results, no significant issues were found.
Attached graph is the result summary including the latest measured data.

[Kimura, Tomaru, M. Takahashi, H. Sawada and R. Takahashi]
 "Inspection results for the sealing surface of the +X side flange"
 A vacuum leak in the range of 1×10⁻⁹ Pa·m³/s was detected, so we inspected the sealing surface of the side flange on the +X side.
As a result, we found a slight scratch on the elastomer surface.
Based on the location of the scratch, we believe it is the cause of the vacuum leak.
After installing a new elastomer on the sealing surface of the +X side flange, the flange was closed.
The flange fastening bolts are currently in a temporarily secured state.

"Inspection results for the sealing surface of the SRM top flange"
 After removing the SRM top flange, the condition of the flange sealing surface was inspected.
No dirt or scratches were found on the surface of the elastomer.
Therefore, it was decided to reuse the SRM top flange as a seal.

Date of work:
23, Apr, 2026
 
Workers:
Kimura and Tomaru
 
Abstract:
 We pressurized SRM atmospheric pressure for the investigation of outgassing source in SRM (k-log 36811).
The following is a record of the opening and closing of valves and the pressure of the pressurization process.
The same process is used for k-log 26030. It is noted here for reference.
 
~9:50 GVoomt Close
          Set safety valve to SRM vacuum pump unit
          Turn on main SW of TPM (Not acceration, only raise the rotor blades to protect the TMP)
9:55    Pressurization was  started by G-2class grade air (klog-25912).   Cylinder is filled with 7 m³ of compressed dry air.
          Pressure inside SRM was    1.7 x 10^1 Pa    Cylinder pressure 15.2 MPaG
10:00          "                         2.1 x 10^3 Pa
10:22          "                         3.7 x 10^3 Pa
10:33          "                         4.2 x 10^3 Pa    Cylinder pressure 14.2 MPaG
10:40          "                         7.8 x 10^3 Pa
10:55          "                         1.6 x 10^4 Pa    Cylinder pressure 11.9 MPaG
11:21          "                         3.3 x 10^4 Pa                "               8.9 MPaG
11:30          "                         3.9 x 10^4 Pa                 "              7.9 MPaG    Increased air flow  (Secondary pressure 1.1  kg/cm^2G) 
11:34          "                         4.1 x 10^4 Pa                 "              7.5 MPaG
11:54          "                         5.3 x 10^4 Pa
12:12          "                         6.4 x 10^4 Pa
12:12          "                         6.4 x 10^4 Pa
12:19          "                         6.9 x 10^4 Pa
12:27          "                         7.4 x 10^4 Pa    Cylinder pressure   2.0 MPaG
12:34          "                         7.8 x 10^4 Pa                 "              1.1 MPaG
Change to a new cylinder filled with 7 m³ of compressed dry air
12:41          "                         7.9 x 10^4 Pa    Cylinder pressure   15.2 MPaG
12:45          "                         8.1 x 10^4 Pa                 "              15.1 MPaG
12:53          "                         8.4 x 10^4 Pa
12:58          "                         8.9 x 10^4 Pa
13:02          "                         9.0 x 10^4 Pa                "               13.2 MPaG
13:04          "                         9.1 x 10^4 Pa
13:07          "                         9.4 x 10^4 Pa               "                12.6 MPaG
13:10          "                         9.6 x 10^4 Pa
13:11          "                         9.7 x 10^4 Pa
  The pressurization process is terminated because the pressure in the SRM has reached almost atmospheric pressure at 13:11. 
Based on the volume of dry air, the internal volume of the SRM was estimated to be approximately 9 m³.

I checked the IM transfer functions after closing the chamber. The behavior did not change after opening the chamber to remove the geophone pods.

I checked the IM transfer functions after the pressure reached 8E4 Pa. The behavior returned to the situation before starting the evacuation.

[Washimi, Tomaru, Kimura, M.Takahashi, Sawada, R.Takahashi]

We opened the X+ side hatch and the top belljar. We could not find any dirty items inside the chamber. Pictures are here.

We removed the geophone pods. The weight of each pod was 12.6kg. After closing the chamber, we measured the IP transfer functions. The resonant frequencies became 0.16Hz for L and T, and 0.4Hz for Y.

KAGRA Pcal-Y updates (2026/04/23)

Workers: Dan Chen, Misato Onishi

We performed monthly Pcal-Y calibration on 2026/04/23.

After the calibration, we updated EPICS parameters related to the Pcal-Y system. No issues were found.

Thank to klog36803, the picos are available. So I adjusted the Pcal-Y beam position.

The TM positions on the Tcam photo as the ref for Pcal were also updated:

The factor converting mm to dot is 11.3[pixel/mm]

Through this work:
the Pcal-Y path 1 was moved down by ~4 mm on ETM = by ~8 mm on RxPD, and
the Pcal-Y path 2 was moved down by ~2 mm on ETM = by ~4 mm on RxPD, and
the RxPD output recovered by ~1.5%.

[Kimura, M.Takahashi, H. Sawada, R. Takahashi and Tomaru]
 On April 22, we continued conducting leak tests on the SRM vacuum vessel.
Since vacuum leaks were suspected due to cracks in the weld lines,
we performed leak tests by blowing helium gas onto the weld lines and using the hood method; however,we were unable to locate the leak. (See attached photo.)
 The build-up test was conducted with the SRM and OMMT sharing a common vacuum, and the pressure was recorded using the OMMT’s pressure gauge.
The plot of the build-up test is attached.
The total duration of the build-up test was 935 minutes.
The estimated leak rate from the build-up test was 9.7 × 10⁻⁴ Pa·m³/s.
The slope of the pressure rise was approximately 1.

I changed the setpoint for the F1 GAS from -3170 to -3700, and the F2 GAS from 1280 to 1240, so that the MN V Oplev can be zero.

I changed the setpoint for the F2 GAS from -1109 to -1198 so that the MN V Oplev can be zero.

I checked the IM transfer functions after the TMP stopped. The situation was not changed (The DC gain of transfer functions is smaller than the reference, except for L).

[Tanaka, Hirose, Saito]

To align the optical path of the sub-laser beam with the IR beam on the POS table, we reinstalled two irises and performed alignment. In addition, we prepared mirrors, a BS, lenses, and ND filters to combine the sub-laser beam and the IR beam on the POS table and inject them simultaneously into the RFPD.
 

• Since the alignment in klog:36773 was not successful, we changed the positions of the irises and the mirrors used for alignment (Photo 1). To more precisely match the optical path of the sub-laser beam with the IR beam on the POS table, we moved the position of the second iris (in klog:36773) to 2125 mm from the sub-laser. We adjusted the setup so that the IR beam on the POS table hit the center of the mirror located after the second iris, and then fine-tuned the iris so that the beam passed through its center. Using the second and third mirrors after the second iris, we aligned the sub-laser beam so that it passed through both irises.
   
• We prepared mirrors, a BS, lenses, and ND filters to combine the sub-laser beam and the IR beam on the POS table and inject them into the RFPD. The lenses are used to make the beam diameter smaller than the RFPD sensor diameter and to match the beam sizes of the sub-laser and the IR beam on the POS table at the RFPD location. The ND filters are used to reduce the sub-laser power entering the RFPD, since the maximum allowable input power to the RFPD is 1 mW.

I changed the setpoint for the BF GAS from 3860 to 3490 so that the MN V Oplev can be zero. The BF GAS was offloaded with the FR.