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MIF (ITF Control)
kentaro.komori - 2:50 Friday 31 January 2025 (32517) Print this report
Stable DC readout came back and summaries of today's measurements

[Tanaka, Carl, Haoyu, Sugimoto, Oshima, Yamaguchi, Takidera, Sugioka, Komori]

We successfully restored stable DC readout control by addressing various issues and implementing solutions, some of which led to notable improvements, as outlined below.
The details will be posted later.

  • Improved GRY PNC, increasing the likelihood of successful IR handover (klog:32516).
  • Reduced PRCL 3F gain by a factor of 0.3, which likely enhanced the probability of successful PRMI 3F locking.
  • Reduced DSOFT yaw gain by a factor of 0.5, leading to more stable operation after PRFPMI RF locking.
  • Reduced PR3 pitch control gain by a factor of 0.3, preventing 4-Hz oscillation and ensuring stable DC readout.
  • Adjusted beam spot positions using the BPC offset, though this had negligible impact on DARM sensitivity.
  • Applied offsets to the IMMT alignment control, successfully eliminating some peaks in the 300–380 Hz range from the DARM sensitivity (klog:32518).
Comments to this report:
kentaro.komori - 12:34 Monday 03 February 2025 (32554) Print this report

[Tanaka, Komori]

Below we provide additional details on the observed issues, implemented solutions, and the current status regarding gain tuning of the following three components.
These adjustments were made last Thursday.

- PRMI 3F locking

We frequently observed oscillations around 200 Hz when engaging the PRMI 3F lock.
This scillation frequency is typically due to large mass feedback gain applied to the PRM.
To mitigate this, we reduced the gain by one-third (adjusting REFL_PDA2_RF135_I in the LSC sensing matrix for AIR 3F from 1.0 to 0.3).
As a result, the locking probability appears to have increased.

However, the FO lock still occasionally fails during the PRMI 3F locking phase.
One potential cause is the gain increase while the PRCL is not near the operational point (indicated by POP 90 being close to zero), leading to oscillations in the PRM.
We expect resolving this issue by replacing the REFL 3F PD.

Another potential reason for the lock failure is that the guardian forces the ALS lock to drop whenever the PRMI 3F lock attempt fails.
We can improve this behavior by modifying the guardian to retry the PRMI 3F lock even after an initial failure, rather than immediately releasing the ALS lock

- DSOFT yaw

After reaching the PRFPMI RF locked state, we observed oscillations in the DSOFT yaw, which subsequently caused IFO lock losses.
To address this, we reduced the DSOFT yaw gain by half.
This adjustment stabilized the IFO.
The gain reduction was applied by setting a factor of 0.5 at FM9 in the DSOFT yaw filter bank, which is engaged when the arm WFS is activated.

- PR3 pitch

We observed oscillations at 4 Hz in the PR3 pitch after reaching DC readout, leading to lock losses.
Reducing the gain of the PR3 pitch control eliminated the oscillations and stabilized the DC readout.
However, since this oscillation does not occur consistently, we have not modified the guardian settings or the corresponding values in the sensing matrix.
If the oscillations become more frequent in the future, we recommend reducing the gain accordingly.

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