KAGRA Logbook

VIS (IY)

dan.chen - 16:09 Wednesday 14 January 2026 (36130)

ITMY suspension adjustment

We adjusted NBDAMP filters and MN DAMP filters.
After that we kept the suspension at LOCK_AQUISITION state, but an oscillation at 2.19Hz occurred again.
So still we need more adjustment.

Filter adjustment:

NBDAMP L1
- Kept FM3.
- Tried modification for the filter, but it did not clearly outperform the original FM3.
- Need more checking.
NBDAMP T1
- FM3 and FM1 were not effective enough.
- Created and tuned FM5.
- High gain caused oscillation, so a moderate FM5 setting was adopted.
NBDAMP Y4
- No clear resonance peak observed. So we turned the loop OFF.
- Changed the guardian code.
NBDAMP L2
- FM1 showed better suppression.
- Switched to FM1.
NBDAMP L3
- FM1 performed slightly better.
- Switched to FM1.
NBDAMP L5
- Kept this setting. Did not change it.
NBDAMP T2
- FM1 provided stronger suppression.
- Switched to FM1.
NBDAMP T3
- Switched to FM1 from FM4.
NBDAMP P1
- FM1 gave the best suppression.
- Switched to FM1.
NBDAMP Y1
- Damping performance was already sufficient.
- No change.
NBDAMP Y2
- FM1 showed gain peaking.
- Created an alternative configuration with reduced peaking.
- Switched to FM4.
NBDAMP Y3
- Very unstable and easily kicked during measurements.
- Oscillation observed around ~1.7 Hz, making TF measurement impossible.
- Removed the loop from the Guardian.
DAMP L
- Checked behavior in LOCK_ACQUISITION and ISOLATED states.
- No issues found.
- No change.
DAMP T
- Updated the gain at FM1 by +2 dB.

Images attached to this report

Comments to this report:

dan.chen - 7:47 Thursday 15 January 2026 (36136)

After turnning ON Y4 filter, the oscilation observed yesterday looks dissapeare.

After more adjustment for Y4 filter, we measured the TFs again for L1, Y4, T1 which are close to the oscilation 2.2Hz.
All of them look OK.

Then, the TFs of MNOLDAMP floops were measured.
We did not change anything for it as they look OK and there was not mergin to increase the gain.

We will keep the suspension at LOCK_AQUISITION for a while to check the stability and noise.

Images attached to this comment

dan.chen - 10:12 Friday 16 January 2026 (36153)

We measured MN OLDAMP filters.

Red lines: today
Blue lines: 2024/8
Green lines: 2025/2

We measured these with LOCK_AQUISITION state.

Images attached to this comment

dan.chen - 11:01 Friday 16 January 2026 (36154)

We found the MN COILOUTF values are very high comparing to other suspension.
It can be caused by the loop of NBDAMP T1.
After the adjustment FM5 was used. If we use FM1 for the loop, the COILOUTF values are smaller a little.
FM5 = FM1 + 10dB.

As the suspension could be kept at LOCK_AQUISITION state for long time (>1day), I did not change it.

Images attached to this comment

takafumi.ushiba - 12:15 Friday 16 January 2026 (36156)

Even if LOCK_ACQUISITION can be kept, this issue should be solved because the output is continuously saturating.
So, some modifications of the NB DAMP T filter (and others if necessary) is necessary.

dan.chen - 13:07 Friday 16 January 2026 (36157)

We changed FM5 to FM1 in the T1 loop filter. As a result, the saturation disappeared.
The values of COILOUTF are now slightly larger than they were in O4c. So we decided to keep this situation.
The TF of this T1 loop when using FM1 is the attached figure.

Images attached to this comment