[Kimura and Yasui]

 We installed an interlock circuit in the #-39 pumping unit at X-end on Apr. 24..
After installation, we confirmed that the interlock circuit works in the event of a power failure and that the shutoff valve between the dry pump and the TMP is closed.
After this operational test, the gate valve between the pumping unit and the duct was opened and the TMP of the #-39 pumping unit started pumping.

 We also installed blue and red double-sided magnetic sheets to indicate operating status of vacuum pumps.
The blue side of the magnetic sheet indicates " operation", while the red side indicates " shutdown".

> My guess to explain this result
A day later, I read this description again. But, I don't understand it. Please ignore this explanation.

I tried to recover OMC ASC with beacon. I scanned ETMX and found the current drumhead frequency is 23605.5 Hz. Also I performed phasing for 17 MHz demodulation with this frequency and phasing for 24 kHz demodulation with OMMT2 PIT excitation at 0.5 Hz. Then I decoupled OMMT2 and OSTM dofs. But I will try to close the loops yet because today is already late.

Details will be reported later.

[Tanaka, Ushiba]

Abstract:

Current OMC length fluctuation might be limited by sensing noise feedback through OMC PZTs.
So, it would be better to optimize OMC LSC to reduce the residual OMC length motions.

Detail:

Since OMC LSC residual motion was one of the large coupling path to the OMC transmission (klog30657 and klog30613), we evaluated current OMC length residual motion.
To evaluate the length motion, we used following two ways:
1. Length fluctuation by vibration of optical table
2. Length fluctuation by sensing noise through OMC LSC.

Length fluctuation by vibration of optical table

To evaluate the length fluctuation by #1 path, we measured transfer function from OMC geophone to OMC length fluctuation by white noise shaker injection.
We measured the transfer function of each 10Hz bands and combined it to avoid saturation.
OMC length was calibrated into displacement in the unit of meter by comering the condition with old optical efficiency measureent (klog30535).
Calibration factor of 5.1e9 cnt/m was obtained 300-cnt dither amplitude with 29mW OMC trans.
During the measurement, we used 30000-cnt dither amplitude with 30.5mW OMC trans.
Since the actuator efficiency of dither PZT was reduced by a factor of 10 due to the change of PZT driver for OMC DC PD protection (klog32549), calibration factor during the measurement is 5.1e9 *(30000/300)/10 * 30.5/29 = 5.4e10 cnt/m, which is 1.9e-11 m/cnt (note that current nominal OMC lock was done by 500-cnt dither amplitude, so calibration factor during the OBSERVATION state should be 1.1e-9 m/cnt).

Figure 1 shows the measured transfer function from optical table displacement measured by in-vac geophone to OMC error signals.
Bottom left and middle right show the TF from OMC geophone signals (calibrated into the unit of meter) to OMC error signals (also calibrated into the unit of meter) at 130-200 and 50-130 Hz, respectively.
Top right and bottom right shows the coherence between OMC error signals and geophone signals during the measurement.
By using these TF and geophone spectrum during the OBSERVATION state, we can estimate the OMC length fluctuation due to OMC stack vibration as shown in fig2 (blue).

Length fluctuation by sensing noise through OMC LSC

As discribed in the above, calibration factor of OMC error signals during the observation is 1.1e-9 m/cnt, so the current sensing noise of OMC error signals in the unit of meter can be plotted as shown in fig3.
Current UGF of the OMC LSC loop is about  9Hz (klog31780), so I made a OLTF model according to the filters implemented in K1:OMC-LSC_FB_FLT filter bank and calculated the OMC length fluctuation due to feedback of the sensing noise.
Figure 4 shows the result with OMC length fluctuation estimated from OMC geophone spectrum.
According to this calculation, current OMC length fluctuation seems to be limited by OMC LSC feedback noise upto 120Hz except for several peaks.
So, it might be worth trying to modify the OMC LSC loop to mitigate the OMC length fluctuation around 100Hz.

Note:

Currently, CARM noise coupling somehow increased and noise around 5kHz is larger than the best sensitivity.
So, reducing CARM coupling and improving the sensitivity around 5kHz also contributes to reduce the OMC length fluctuation through sensing noise reduction.

[Guo chin, Yuzu]
I made the histogram to count the phenomena before the lockloss between 2025/02/10 and 04/21.|
Recently, the number of the lockloss related to the IMC oscillation is the dominant phenomena. After 4/3, the lockloss related to the oscillation (saturation) of OMC drastically was mitigated.

This time, I focused on the lockloss from OBSERVATION_WITHOUT_LINES. Note that I counted the number manually. It's possible to shift the number a bit.

Important date for commissioning

• 2/19 : 10 W operation started (klog#32748) (klog#32756)
• 4/3: engaging whitening filters for GAS to mitigate the 20 Hz resonance kick by DAC noise (klog#33197)

[Guo Chin, Yuzu]
We compared the number of lockloss with the IMCL label between 1.2 W and 10 W operation. The frequency of occurrence was 4.5 times per day during 1.2 W and 1.3 times per day during 10 W operation, respectively. This number is smaller than the number in the case of 1.2 W operation (last Feb.).

Details

• For 1.2 W operation, we checked the data of 2025-02-10 14:19:50.312500 UTC ~ 2025-02-18 14:22:44.062500 UTC.
  • During this period, the number of the lockloss with the IMCL label was 3. The duration of OBSERVATION_WITHOUT_LINES (called silent run) is 57183 s.
  • The frequency of occurrence is evaluated as 4.5 times per day.
• For 10 W operation, we checked the data between 2025-04-06 06:02:46.937500 UTC ~ 2025-04-21 16:50:17.687500 UTC.
  • The number of lockloss with the IMCL label was 21 in this period. The duration of OBSERVATION_WITHOUT_LINES (called silent run) is 506818 s.
  • The frequency of occurrence is evaluated as 1.3 times per day. This number is smaller than the number in the case of 1.2 W operation (last Feb.).

Note that we collected the lockloss only from OBSERVATION_WITHOUT_LINES (called silent run).

My guess to explain this result

1. In Febrary, we didn't collect enough length of the silent run (~16 hours) with 1.2 W operation. As a result, we did't observe many lockloss (actually just 4 times) due to the oscillation of the IMC.
2. After 10 W operation, we performed the interferometer as the silent run and collected enough data (~5.9 days). As a result, we observed the lockloss due to the oscillation of the IMC, many times.

We will summarize the comparison ot the ampitude to trigger the lockloss of the IMC oscillation,

I also evaluated the projection from the Leg ACC to the OMC error (length)

Scanning plots for each frequency injection are here
https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=16649

scanning plots (Leg ACC -> Strain) for each frequency injection

https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=16648

I analyzed the same analysis using SEIS_Z or ACC_Z and compared the results for both 2025-04-20 and 2024-11-09 data.
For both days, the results of using SEIS_Z and ACC_Z are consistent.

By the way, even before the shroud installation (2024-11-09), the linear-like coupling and its siderobe were found. 
On 2024-10-16 (before the stack connection), a ground shaker injection was performed (klog31337), but almost no significant excess was found in the DARM. (due to the shaking amplitude not being enough?)

The reason of SR3 oscillation seems F1 GAS control.
Fgure 1 shows the F1 LVDT signals with nominal gain (blue), -3dB (green), and -6dB from the nominal (red).

Since -3dB and -6dB seems no significant difference, I engaged -3dB at FM7 of F1_DAMP_GAS filter bank.
Then, we can engage 2-stage dewhitening filters for SR3 tower as well as the other Type-B suspensions.

What we intslled this time are the devices in the red region on the attached design figure.

Tests we perfomered after the install:

• Made a very simple model with one filter bank.
• Input sin wave into AA and checked with ndscope on the stand-alone system.
• Output sin wave by the stand-alone system and checked the output from AI by an oscilloscope.

Pictures: link

[Tanaka, Ushiba, Komori]

Abstract:

We have started fine-tuning the beam spot position on the input test masses.

Details:

The current DARM noise around 100 Hz may originate either from jitter coupling caused by birefringence in the input test masses (ITMs) or from unexplained interference observed in the REFL PDs.
In both scenarios, adjusting the beam spot position on the ITMs could help mitigate the noise—either by targeting beam spots with lower birefringence or by approaching the condition of a critically coupled cavity.

In previous attempts, we tried to shift the beam spot by adding offsets to the BPC, but lock loss occurred before we could observe any meaningful change in the DARM sensitivity.
Possible causes included reduced PRCL control gain and misalignment to the OMC.
To address these issues, our new strategy is to apply BPC offsets during 10 W RF locking, perform initial OMC alignment with RF sideband, and then transition to the observation state.

As a first trial, we applied a negative IX BPC pitch offset to move the beam spot closer to the center of IX, which is favorable for reducing pitch suspension thermal noise.
With an offset of -3.0 (corresponding to +3.0 in IN1), the X-arm transmission increased by 1–2%.
We also measured the PRCL open-loop transfer function at the same offset and found that the gain increased by approximately 1-2 dB.
This suggests that the optical loss at this alignment point may be lower than in the conventional configuration.
This offset corresponds to approximately -0.5 urad in PR3 pitch, which implies a beam spot shift of roughly 0.5 mm—still relatively small.

We then applied an offset of -5.0, but the arm ASC likely began to oscillate around 1 Hz, leading to IFO unlock.
After recovery of 10 W RF locking, I temporarily reduced the DHARD and CHARD gains by half, but the same oscillation reoccurred.
We will need to address this issue in order to enable further beam spot adjustments.

Abstruct

Details

[M. Takahashi and H. Sawada] 
 Attached is photo of water leak in the water  cooling unit for Y-27 TMP  provided by M. Takahashi-san.
The leak occurred at the joint where the plastic piping was inserted.
The leak was temporarily stopped by redoing the joint connection of the leaking piping.
The leakage area is believed to be the result of aging. 
 Other water cooling units need to be inspected as well, as aging are possible.
The leaking water cooling unit was overhauled in 2023.

I attempted to further reduce the PR3 pitch noise by optimizing the control filter.
The results are shown in the attached figure (black: before this work, red: after this work).

The total RMS was slightly improved but remains nearly the same.
We are currently facing a trade-off between increasing the gain at the pendulum resonance (~0.8 Hz) and maintaining sufficient phase margin around 2 Hz.
I have left the filter configuration corresponding to the red trace in place.

To further suppress the PR3 pitch noise, we may need to retune the damping filter for the tower stage or consider changing the feedback topology—such as incorporating the WFS signal into the control loop.

[Tanaka, Ushiba]

We engaged dewhitening filters for Type-B tower.
2-stage dewhitening filters are engaged for SR2 and SRM.
1-stage dewhitening filter is engaged for SR2 because SR3 actuators are saturated if engaging 2 stages.

During the work, we found SR3 local control are oscilating (fig1), so it would be necessary to tune the local controls

[Uchiyama, Ushiba]

We installed resistance at coil driver output of IM/MN actuators.
After that, we compensate AE reduction at COILOUTF filter bank and confirmed that ETMY can reach LOCK_ACQUISITION without any problems.

[Uchiyama, Ushiba]

We installed resistance at coil driver output of IM/MN actuators.
To avoid affecting lock acqusition, 940 Ohm (470 * 2) is installed only for vertical actuators.

After that, we compensate AE reduction at COILOUTF filter bank and confirmed that ETMX can reach LOCK_ACQUISITION without any problems.

Here is the additional analysis by Guo Chin.

Preparation

He checked the 24 lockloss on April 2, 12, 13, and 14, excluding the state of WORKING_WITH_OBSERVATION_WITHOUT_LINES (upper half of the plot), and also he checked the 12 lockloss (lower half of the plot), which the LOCKLOSS guardian did not label.

He categorized the lockloss in terms of several conditions: (1) OMMT2 TRANS value > 200, (2) oscillation frequency is ~0.16 Hz, (3) DHARD feedback >= -600

The attached figure summarizes the categorization. Here, the notation indicates that O: OMMT2 over 200, H: with ~ 0.16 Hz oscillation, and D: DHARD over -600. Regarding the condition of the seismic motion, we checked the channel K1:PEM-SEIS_IXV_GND_X_BLRMS_100MHZ300 on the summary page.
 

Summary

• The seismic motion has some effects on the OMMT2 and DHARD at the time just before the lockloss.
• For strong seismic motion over 0.3 um/s, OMMT2 TRANS is likely over 200, and the lockless occurred. If the seismic motion reduces to 0.2 um/s, the possibility that the OMMT2 TRANS exceeds 200 is reduced.
• If the seismic motion is larger than 0.2 um/s, we can see that DHARD oscillates at ~0.16 Hz in about half of the events. In several cases, DHARD feedback signals exceed -600.
• If the seismic motion is less than 0.1 um/s, there is no clear relationship between OMMT2 TRANS value and DHARD feedback signal.

So sad news.... For your reference, a document used for the technical review for the OMC shroud can be found in JGW-T2415840.