[Tanaka, Hirose, Saito]

We matched the polarization of the sub-laser beam to that of the beam coming from the interferometer. Next, we increased the power of the beam coming from the interferometer by opening the iris. Then, we adjusted the alignment so that the sub-laser beam overlapped with the beam coming from the interferometer at the mirror just before the RFPD. As a result, we were able to observe the beat signal between the beam coming from the interferometer and the sub-laser beam.
 

• First, we checked the polarization of the sub-laser by inserting a PBS after the FI and found that it was mostly P-polarized. Since the beam coming from the interferometer is S-polarized, we installed a HWP after the FI and used the PBS to adjust the polarization to S-polarization. Next, the iris used for alignment had been partially closed; by opening it, the power of the beam coming from the interferometer incident on the RFPD increased from about 2 μW to about 15 μW. Then, with both the beam coming from the interferometer and the sub-laser beam incident on the RFPD, we found that their spot positions differed at the mirror just before the RFPD. We adjusted the alignment so that the sub-laser beam overlapped with the beam coming from the interferometer. At this time, observing the DC output of the RFPD with the oscilloscope on Moku:Lab, we found an offset of about 12 mV; the beam coming from the interferometer contributed about 29 mV and the sub-laser beam about 83 mV. Next, we attempted to observe the beat signal using the spectrum analyzer on Moku:Lab. When we set the sub-laser temperature to 31.57 ℃ to match the main laser frequency, a beat signal was observed (Photo 1). The amplitude of the beat signal fluctuated significantly, and its frequency changed by about 3.5 MHz over the course of approximately one minute.

[Kimura and Yasui]

 Attached is the residual gas distribution during the SRM build-up test using the Qmass installed in the OMMT.

Since the pressure inside the Qmass analysis tube exceeded 1×10² Pa, the Qmass measurement was interrupted at 12:45 p.m. due to the interlock mechanism.
 

[Kimura, Yasui]
1. Build-up test
We had the build-up test by closing the GV between the duct and the pumping unit.
The slope of the pressure rise rate was approximately 0.6, which is the same rate as yesterday.
During the build-up test, I checked the rate of the pressure decrease with the TMP alone.
According to our logbook, when we tried to swich the vacuum pump from TMP to IP on 7 January 2026, we closed the GV between duct and pumping unit. At that time, the pressure value immediately droped from 2E-5 to 6E-6 Pa. Sine it took 1hour 47minites to reach the pressureof 6E-6Pa from 2E-5Pa this time, there seems to be something affecting the TMP performance.

2. Pressurization
We pressurized SRM again.
13:27 duct-side GV open
13:33 start pressurization 2.0Pa (cylinder pressure: 11.8 MPa)
13:40 3.0E3 Pa
13:57 1.0E4Pa
14:05 1.4E4pa
14:23 2.3E4Pa
14:37 3.0E4Pa
15:00 4.1E4Pa
15:08 4.7E4Pa
15:20 5.5E4Pa
15:30 5.9E4Pa(cylinder pressure: 1MPa->replace with a new cylinder(15.8MPa)
15:40 6.1E4Pa
16:00 7.3E4Pa
16:10 7.9E4Pa
16:20 8.7E4Pa(cylinder pressure:10.5MPa)
We will continue to pressurize it to atmospheric pressure on 30 April morning.
 

[Tanaka, Hirose, Saito]

We performed alignment to inject both the beam coming from the interferometer and the sub-laser beam into the RFPD. While the DC components of each beam were confirmed, the AC component of the interference signal could not be observed.
 

• We aligned the system so that both the beam coming from the interferometer  and the sub-laser beam were incident on the RFPD. The incident optical powers measured with a power meter were approximately 2.04 μW for the beam coming from the interferometer and 7.95 μW for the sub-laser beam. Observing the DC output of the RFPD with the oscilloscope on Moku:Lab, we found an offset of about 10 mV; the beam coming from the interferometer contributed about 18.09 mV and the sub-laser beam about 47.63 mV.　Next, we attempted to observe the AC component using the spectrum analyzer on Moku:Lab. To match the main laser frequency, we set the sub-laser temperature to around 31℃ and tuned it in that range to check for the AC component, but none was observed. Possible reasons include insufficient optical power incident on the RFPD or a low degree of interference between the beam coming from the interferometer and the sub-laser beam. We plan to change the ND filter and further investigate the interference condition.

[kimura, hSawada, Yasui]

We had the build-up test again. The slope of the pressure rise rate was approximately 0.5 for the first 30minutes and then rose to about1.2. 

During the buildup test, we checked the Q-mass data and found that the N₂/O₂ ratio was not 4, indicating that it differs from the composition of the air.

We also had the helium leak test, and a small leak ( ~1.0E-10 Pam^3/s) was detected on the +X-side-flange.

Finally, we replaced the dry pump with a repaired one to improve its performance.

The log-log plot of the SRM pressure after TMP startup indicates that the pressure is still decreasing.

[Ikeda, Sawada, Kimura, Takahashi]

We checked the leak of the geophone pods. First, the background level of the test chamber, which was taken from the CLIO site, was measured by the He leak detector. The level was smaller than the detectable limit, 1.0e-13 Pa ‣m^3 /s. After that, we measured the leak level of the H1, H2, and H3 geophoen pods one by one. In any case, the leak level was smaller than the limit.

I compared the vacuum evacuation speed this time with that at 28 February 2025.
Figure 1 shows the time trend of the vacuum pressure in 2025.
It takes 1 day and 7.5 hours to reach the pressure of 1.0e-4 Pa from 1.0e-2 Pa.

Figure 2 shows the time trend of the vacuum pressure this time.
After 1 day and 7.5 hours passed from the time that the vacuum pressure was 1.0e-2 Pa, the vacuum pressure was 1.9e-4 Pa.
Even after 2 day and 15 hours, the vacuum pressure was 1.2e-4 Pa.

So, the vacuum codition seems still worse thatn before.

I checked the signals of the IM OSEMs during the evacuation. The signals in count were increasing slightly during the pump-down with the DRY pump, due to the buoyancy. Just after switching the pump from the DRY to the TMP, the signals decreased rapidly in 10 min. The pressure change was from 10 Pa to 0.02 Pa. In this pressure region, the thermal conductivity of the gas changes a lot. I think this signal drop is due to the temperature change of the LED in the OSEMs.

I checked the IM transfer functions in a vacuum, changing the OSEM positions. The position in um was changed by the DC actuation of +10000 count to some COILOUTFs, as shown in the table. In any case, the behavior was almost the same (the DC gain of transfer functions is smaller than the reference, except for L).

I checked the IM transfer functions after the evacuation. The behavior reappeared in a vacuum. The DC gain of transfer functions is smaller than the reference, except for L.

[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.