## Abstract
DC readout will be back soon.
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## What I Did
After the Kamchatka earthquake settled down last night, I performed INITIAL ALIGNMENT.
We successfully locked the PRFPMI without PRMI ASC and BPCs.
This time, both arm transmitted powers reached 83 and 78, respectively—similar to the values observed during the successful PRFPMI RF lock last Tuesday.
Thus, I attempted to engage the PRMI ASC without any parameter changes, and the controls were successfully engaged.
Additionally, the BPCs could also be engaged.
One possible explanation is that PRMI ASCs—especially PR3 Yaw—depend on the initial alignment.
In short, the linear range of PR3 Yaw control is narrow, and engaging the control becomes difficult if the initial misalignment is large.
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### Improvements in Lock Stability
The 180 Hz peak observed when PRFPMI is locked has decreased to ~10⁻¹⁵ m/√Hz, likely due to the completion of the cooling.
No suspensions were kicked during locking.
As a result, the PRFPMI RF lock duration significantly improved to more than **1 hour**.
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### Increasing Input Power to 10 W
I attempted to increase the input power to 10 W.
However, I found that the HWP rotation script did not work due to today’s maintenance.
I asked Ikeda-san to resolve the issue, and he did (see details in the klog35101).
I then requested the `LSC_LOCK` guardian to proceed to the `INCREASING_LAS_POWER` state, but the attempt failed.
I found that the lock loss consistently occurred when REFL power increased beyond ~1.4–1.5 mW.
The CARM gain margin at 150 kHz appears to be only a few dB when the REFL power is ~1.2 mW, so the 150 kHz peak may cross 0 dB when REFL exceeds ~1.4 mW.
According to the script for the `INCREASING_LAS_POWER` state:
- Input power is increased by rotating the PSL HWP.
- REFL power is subsequently reduced by rotating the IFO REFL HWP.
- This procedure is repeated until the input power reaches 10 W.
- The threshold for switching the HWP is defined as REFL power reaching **1.3 mW**.
To avoid crossing the instability threshold, I reduced this REFL threshold to **1.1 mW**.
Also, the REFL power when the input reached 10 W was **4 mW**, which is higher than the nominal value (~3 mW).
I adjusted the IFO REFL HWP setting from **149° to 155°** after reaching 10 W.
Thanks to these changes, **PRFPMI lock with 10 W** was successfully achieved.
Afterward, I measured the CARM OLTF and confirmed that the UGF was maintained at **54 kHz** (Fig. 1).
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### Attempt at DC Readout Lock
Next, I attempted to lock the PRFPMI with **DC readout**.
However, the **TEM00 carrier power** currently appears too low.
Figure 2 shows the OMC transmission power during the scan.
The TEM00 carrier appeared at –25 s and had a power of only ~2 mW.
In the best case, power was still <6 mW.
I increased the **DARM offset**, and when the offset was tripled, the OMC locked with 15 mW.
After that, `LSC_LOCK` reached the `OBSERVATION_WITHOUT_LINES` state, but the IFO soon went down.
- **Fig. 3** shows the LSC error signals at the time of lock loss.
The immediate cause appears to be **OMC PD saturation** due to a **52 Hz oscillation**.
- **Fig. 4** shows that **MICH** was already oscillating at **17.9 Hz**.
We need to check the MICH OLTF. However, another earthquake occurred tonight and it's time limt. I decided to stop recovery work for today.
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### Note
- just memo: fig. 5 shows the time series of GR/IR trans. power of X arm and oplev signals of suspensions related X-arm. At -12 mins, the lock loss occured and then IFO tried to restore automatically. However, GR trans. power drastically decreased after the lock loss even though suspensions' alignment were not changed so much. However, judging from Tcam image, the beam spot on ITMX was shifted in horizontal direction. On the other hand, POP90 seemed to be not decreased. Therefore, the actual alignment of only ETMX Yaw was changed. The recorded ETMX Yaw is sum of Yaw and Length. So, if the Length feedback is too large, the Yaw alignment is different from the optimal one after the lock losses. In this case, you should adjust ETMX Yaw.
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I'm leaving with the LSC_LOCK Guardian set to PRFPMI_RF_LOCKED state. If anyone wants to work on the interferometer over the weekend, feel free to do so.
