The temps in the PSL room are cooler by ~ 0.1~0.3 C than before. So, anyway, I changed the target temp of the precision cooler from 20.90 to 21.10 around 20:32.

IM Temperatures are still decreasing. However, IY and EX temps decreased rapidly. So it is better to set some amount of current for IY/EX heater within one or two days.

Operation Shift Summary (Sept. 15, 2025)
Operator's name: K. Tanaka and N. Kimura

Shift time: 9-17 JST

Check Items:
VAC: No issues were found.
CRY cooler: No issues were found.
Compressor: No issues were found.
Temp check: No issues were found.
VIS GAS filter output check: Same as the sutiation of yesterday (k-log 35076), the F2 output of ITMX is still above 20,000 cts. At the  2:58pm, the parameter of this was 20997 counts.
We reported this situation to R. Takahashi-san (NAOJ) according to the manual.

IFO state (JST):
09:00 Start of shift, STANDBY remains
17:00 End of shift, STANDBY remains

This morning, I found that the PMC transmission power had often decreased (Fig. 1), and that the beam image on the PMC REFL camera had glowed at that moment (Fig. 2). This indicates that a mode hop occurred. This phenomenon seems to have lasted for the last three days, i.e. since last Friday (Fig. 3). Therefore, the current IR temperature does not seem suitable in terms of the mode hop.

Operation Shift Summary (Sept. 14, 2025)
Operator's name: C.Hirose

Shift time: 9-17 JST

Check Items:
VAC: No issues were found.
CRY cooler: No issues were found.
Compressor: No issues were found.
Temp check: No issues were found.
VIS GAS filter output check: Same as the sutiation of yesterday(klog35075), the f2 output of ITMX is still above 20,000 cts. At the 2:58pm, the parameter of this was 22149cts.

IFO state (JST):
09:00 Start of shift, STANDBY remains
17:00 End of shift, STANDBY remains

Operation Shift Summary (Sept. 13, 2025)
Operator's name: C.Hirose

Shift time: 9-17 JST

Check Items:
VAC: No issues were found.
CRY cooler: No issues were found.
Compressor: No issues were found.
Temp check: No issues were found.
VIS GAS filter output check: 
I found that the output filter of ITMX f2 exceeded 20,000 cts around 7:40 this morning.(FIG1)
Checking the time series of long period, I found that it had been drifting for the past month.(FIG2) I reported the matter to Takahashi-san.

IFO state (JST):
09:00 Start of shift, STANDBY remains
17:00 End of shift, STANDBY remains

Operation Shift Summary (Sept. 12, 2025)
Operator's name: Uchiyama, Miyakawa

Shift time: 9-17 JST

Check Items:
VAC: No issues were found.
CRY cooler: No issues were found.
Compressor: No issues were found.
Temp check: No issues were found.
VIS GAS filter output check: No issues were found.

IFO state (JST):
09:00 Start of shift, STANDBY remains
17:00 End of shift, STANDBY remains

(Endo, Miyakawa)

Since a strange temperature response happened on the IR laser after we changed the current from 2.0A to 1.55A. To change the temperature for green lasers by 1 degree, we needed 5 degrees change on the IR laser. To make sure, we measured the relationship between the crystal temperature of the master IR laser with 1.55A current and the frequency.

The result is shown in Fig.1. It clearly shows the graph shifted to the upper side, and it corresponds to roughly 5 degrees to obtain the same frequency. This shift can be understood as the measurement point of temperature is not exactly the same as the cavity length, which determines the laser frequency. Actual cavity length depends on the inside power, i.e. injecting LD current. Mode hop points seem to be unchanged.

[Kimura, Yasui, Fukumura and Nojiri]

Time: From 13;20 to 14:00

 We confirmed evacuation route from the Y-end mirror room, enter the mine through the Mozumi Mine Entrance.

During this confirmation process, Fukumura-san and Nojiri-san were briefed on the evacuation route.

Miyakawa, Endo, Tanaka

We measured the frequency responses of two EOM amplifiers in the EOM path of the IMC loop, in order to investigate the filter characteristics of each amplifier—particularly around 150 kHz, where a resonance pole and zero are observed in the IMC open-loop transfer function (OLTF).

### Measurement Procedure

Before the main measurement, we evaluated the effect of the BNC cables used in the setup and corrected for their response using a calibration function in Moku:Lab.  
We then measured the frequency response of each EOM amplifier—F10A and F30PV (both from FLC Electronics)—using Moku:Lab over the range from 1 kHz to 1 MHz.

### Results

Figures 1 and 2 show the measured frequency responses for the F10A and F30PV amplifiers, respectively.  
In this measurement, the gain of the F10A was fixed at ×10, and the gain of the F30PV was also set to approximately ×10.

The gain responses of both amplifiers were flat up to 200 kHz, and both showed a gain of approximately ×10.  
The phase shift at 120 kHz was at most a few degrees in both cases.  
No anomalous features were observed in either amplifier’s response.

Based on this, we suspect that the resonance pole and zero observed at 150 kHz in the IMC OLTF originate from the EOM itself rather than from the EOM amplifiers.

I turned off IM heater for cooling ETMY again.

I turned off IM heater for cooling ETMX again.

I turned off IM heater for cooling ITMX again.

I turned off IM heater for cooling ITMY again.

The temperature displayed on the controller is not the actual temperature of the crystal. It may be the temperature of the stage where the crystal is on.

The crystal is warmed up directly according to the current applied to LD and its part of heat contributes the temperature enhancement of the thermometer.
 

So the actual temperature of the crystal will be higher than the value of the thermometer. How much higher maybe depends on the heat generated in the crystal according to the current.

Unfortunately, 2A condition cannot be used because of glitches. So we can find acceptable condition between 2 and 1.55A or less as long as glitches won't happen.

Miyakawa, Tanaka

This morning, we found that the GRY absolute frequency appeared to be in a mode-hop region, because the Y-arm cavity transmitted power showed two distinct states.  
Figure 1 shows the trend of the GRY flash and the oplev signals related to the Y-arm. Although no suspension seemed to move, the normalized GRY flash level frequently switched between 0.8 and 1.0.  

Moreover, during today’s initial alignment, we noticed glitches in both GR transmitted powers (Fig. 2). Therefore, the current GR frequencies at their respective temperatures are not optimal for the arm cavity. We decided to adjust the temperatures of the IR and both GR lasers.

---

### Adjusting the IR main laser temperature

Before adjustment, the IR main laser temperature was 36.27 °C (Fig. 3). Due to the work from two days ago, both GR laser temperatures had been reduced.  
To raise them, we increased the IR laser temperature to 36.7 °C.  

According to Endo-kun’s slide (JGWdoc), when the IR laser temperature is ~36.7 °C, both GR temperatures should exceed 30 °C.  
Before adjustment, the GRX and GRY temperatures were 28.23 °C and 25.64 °C, respectively (Fig. 4). So we tried to raise both above 30 °C.  

However, we could not find any PLL error signals when both GRs were around 30 °C. When we decreased the temperatures again, we recovered the PLL error signals near their original values. This behavior seems strange and suggests that the GR frequencies may not have shifted significantly.

---

### Suspected temperature–frequency relation shift

We suspected that the relation between temperature and absolute frequency had shifted to lower values compared to Endo-kun’s slide. One possible reason could be the change in the IR master laser drive current from 2.0 A to 1.55 A.  

In this case, to push both GR temperatures above 30 °C, the IR temperature would need to exceed 40 °C.  
We tested this and increased the IR temperature to 41 °C (Fig. 5).  

However, even then, we only observed the PLL error signals when GRX and GRY were at 29.31 °C and 26.35 °C, respectively (Fig. 6). These values represent at most ~1 °C increases from their previous values.  

This indicates that the current relation between IR absolute frequency and temperature is completely different from the earlier result shown in Endo-kun’s slide.  
The reason is unclear, and we will need to remeasure the relation.

---

### ALS stability check

In this state, we engaged ALS_CALM to check the stability of ALS lock for both arms.  
There was no significant drop in transmitted power for either arm, except when SR3 fluctuated strongly or when the actuators saturated (Fig. 7).  

On the other hand, the PLLY occasionally became noisy (Fig. 8).  
The interval between noisy moments was several hours, and each noisy moment lasted only a few minutes.  

Therefore, while the current GR frequencies appear to be acceptable with respect to mode-hop, the PLLY stability is somewhat degraded. Still, it may be acceptable for lock acquisition.

---

Tonight, we left the Y-arm locked with both IR and GR to check long-term stability.

Operation Shift Summary (Sept. 10, 2025)
Operator's name: Uchiyama, Miyakawa

Shift time: 9-17 JST

Check Items:
VAC: No issues were found.
CRY cooler: No issues were found.
Compressor: No issues were found.
Temp check: (10:00) No issues were found. (16:00) FIELD_IYA had -0.4 degree change from 21.70 degree (10:00) to 21.30 degree (16:00).
VIS GAS filter output check: No issues were found.

IFO state (JST):
09:00 Start of shift, STANDBY remains
17:00 End of shift, STANDBY remains

Summary:

Considering only thermal transfer via fibers, it seems difficult to warm up sapphire mirrors above 160K.

Detail:

Summary of the physical parameter:

Radius of sapphire mirror: R = 0.115 m
Length of sapphire mirror: L = 0.15 m
Surface area of sapphire mirrors: A = 2*pi*R^2 + 2*pi*R*L
Emissivity of sapphire: ϵ = 0.8 (Chun-Yang Niu et al 2016 Chinese Phys. B 25 047801)
Number of sapphire fiber: N = 4
Radius of sapphire fibers: r = 8e-4 m
Crosssection of sapphire fibers: S = pi*r^2*N
Length of sapphire fibers: L = 0.35
Stepan Boltzman constant: σ = 5.67e-8 W/m^2/K^4
Thermal conductivity of sapphire fiber: κ =  (A Khalaidovski et al 2014 Class. Quantum Grav. 31 105004).

Calculation:

Since the temperature of sapphire mirrors were stable at 90K when radiation shields were cooled down, we can assume the thermal radiation from the atmosphire is roughly same as the thermal radiation of sapphire mirrors at 90K.
So, we can calculate the thermal injection due to the thermal radiation as
Qin_r = ϵσA*90^4 = 0.68 W.

Next, I calculated the thermal radiation from sapphire mirrors at 160K to evaluate how much heat flow is necessary to achieve 160K as follows:
Qout = ϵσA*160^4 = 6.8 W.
So, the heat flow to the mirror via fibers should be larger than Qout - Qin_r = 6.1 W.

Thermal resistance of sapphire fibers are described as
R = L/κS = 1.1e2 (κ=400) - 4.4e2 (κ=100) K/W
So, the temperature difference between IM and TM with the heat flow of 6.1W is 670-2700K, which is almost impossible to achieve.

Therefore, it is necessary to increase the heat flow via the thermal radiation to reach the mirror temperature more than 160K.

Crosscheck:

To crosscheck my calculation, I roughly scaled the temperature difference between TM and IM from the design value.
Final KAGRA design, heat absorption at ITMs are about 0.6W and temperature of sapphire mirrors are 23K if the IM temperature is 16K.
So, sapphire fibers can extract the heat of 0.6W with the temperature difference of 9K.
According to the thermal conductivity data, κ = 4000 @ 20K, so necessary temperature difference to extract the heat of 0.6W with κ = 100 and 400 is 360K and 90K, respectively.
Since our necessary heat extraction power is roughly 10 times larger, necessary temeprature difference can be expected as 900K (κ = 400)-3600K (κ = 100), which is almost same as what I estimated above.

So, it is necessary to increase not only IM temperature but also the surrounding environment to achieve the sapphire mirror temperature more than 160K.

Summary:

The previous defrosting was succeeded with the condition of all cryocooler stopped with 100Pa nitrogen introduction.

Detail:

According to the finesse data of Y arm, followings are the time we faced previous fronsting issue on ETMY.

According to the temperature data, IM temperature was 213K at 04:06:31, Oct 30, 2023 and 221K at 23:55:48, Oct 30, 2023.
So, it is very likely that IM temperature greater than 220K is neccesary for finesse recovery from water gas frosting.

In addition to this, two radiation coolers were stopped on Oct 24 (klog27339) and nitrogen gas was introdeced on klog27377.
I haven't confirmed the stop of duct shield coolers from the klog but it should be turned off before the nitrogen gas injection.

So , the previous defrosting condition seems that all cryocoolers were stopped and nitrogen gas was introduced upto 100Pa.

Fig.1  and Fig.2 show the partial pressure changes in IY and around BS. N2, O2 and Ar were emitted a lot. becaus of the warming up. However, no change in H2O.

Fig.3 shows EX. Also it shows the same tentency.

Fig.4 shows EY. No changes. Or already showed the same trend in the past because EY temperature increased first amond of these payloads.

Fig.5 shows IX. Almost same with IY and EX.

Fig.6 shows IMs temperatures. Anyway, IY temperature increeased with the fastest speed, and is expected to reach higher temperature than others. So we can recognize what will happen when the temprature increses more.

Water evapolation temperature depends on the surface smoothness? polished mirror and rather rough aluminum (Duct Shield). Or H2O is just easily absorbed on surrounding structure and does not reach the Qmass position?

Tonight, I left IMC with PROVIDING_STABLE_LIGHT to check the stablity.