[Tanaka, Ushiba, Komori]
Abstract:
We successfully implemented DC control of IMMT1 to prevent alignment drift.
We also adjusted several offsets—INP2 pitch, PRC2 pitch, and IY drive align P2L/Y2L—to reduce jitter coupling.
While this work led to a slight reduction in jitter, further noise suppression would require larger offsets, which currently causes lock loss of the interferometer.
Detail:
We performed two main commissioning tasks.
1. Implementation of IMMT1 DC Control:
Previously, IMMT1 pitch and yaw were not controlled at all, leading to alignment drift and, consequently, beam spot drift within the PRC.
In this work, we implemented DC control with fixed offsets (pitch: –45, yaw: –0.5).
Although the IMMT1 alignment does not appear to significantly affect DARM sensitivity at this stage, stabilizing it is important for long-term operation.
The control loops used for IMMT1 pitch and yaw during the lock acquisition phase are essentially the same as those used during the aligned state, but the gain is reduced by a factor of 100 via the newly implemented FM7 to avoid injection of the control noises.
The open-loop transfer functions are shown in Fig. 1 (pitch) and Fig. 2 (yaw), with UGFs of approximately 10 mHz for both.
To preserve the fixed offsets, we removed "IMMT1" from the oplev channel list recorded by the alignment recorder guardian during the IFO-locked state (Fig. 3).
Additionally, we changed the filter module that is activated during the transition to the lock acquisition state from FM8 (null) to FM7 (×0.01) (Fig. 4).
2. Offset Tuning:
We adjusted the following:
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INP2 pitch and PRC2 pitch
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IY drive align P2L and Y2L
We found offset values that result in lower jitter coupling compared to recent settings, although not yet matching the best sensitivity.
Current offset values:
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INP2 pitch: –1.5, PRC2 pitch: 0.12
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IY drive align P2L: –2.5, Y2L: –0.6
Further jitter reduction will be possible with larger INP2 pitch offsets, but such changes currently lead to lock loss of the interferometer.
For now, we have stopped increasing this offset.
We confirmed that the interferometer can reach the observation state with the new settings, but long-term stability should be monitored.
