(Green)
We found that turning off FFU in the PR area caused quick large change of beam direction maybe in pitch in X arm.
(img1580, red: REFL PD Output, Skyblue: Green Transmitting PD Output)
Turning off FFUs above PR3 caused faster response for beam direction change than turning off FFUs above MCF.
However, higher mode resonance can be seen after beam direction change.
Turning on FFU can recover the beam alignment.
(img1587)
No idea at present about this mechanism.
Pressure change ? Temperature change ? Vacuum chambers, ducts and bellows fixing problem ?
On the other hand, turning off FFU in the BS area caused no serious change of beam direction in X arm.
This plot shows the drift of control signal of PR3 TM alignment servo, using the oplev as a sensor.
When the alignmnet went wrong (can be seen by the degrade of Gr transmission) by turning off the air filter, we can see that the control signal was drifting in one direction.
When the alignmnet comes back (can be seen by the recover of Gr transmission) by turning on the air filter, we can see that the control signal was drifting in the other direction.
This indicates that the oplev sensor is affected by turning on/off the air filter.
