4.3 - Vibrations of the mirror mounts due to temperature related to cavity power (the electrons machine is OFF)

1) to test this possible issue, we plan to make a run at low power in the FP-cavity, around 10kW, instead of ~90kW as usual.
for that, I will need to change the diffuser position on axis 18 to get the same error signal for the locking.
the initial axis 18 position is +11304

I used the Alphanov amplifier at its standard value : 33%

first, I did a step at 26kW in the FPC by tuning the CEP
at this power, I saw exactly the same kind of behavior with a sudden large high frequency noise which prevent the lock to run properly and which is impossible to compensate.
=> one just have to wait... until it becomes more stable.

then, I went to 13kW in the FPC by tuning the CEP.

• I did the same alignement optimisation than for high power to remove possible coupling to high order modes in the error signal
• I tuned the power in the FPC by changing the CEP => the equivalent LW of the cavity will be different => I got ~ 13kW
• I changed the diffuser position on axis 18 to -23079
• I tuned the PID parameters :

initial => final
P = 0.055 => 0.1
I = 0.0005 => 0.002
D = 0.6 => 1

I started to record a long trend of the FPC power => see attached picture

I still observed the ~20Hz noise and sometime some high frequency noise, but not at a level which prevents the locking system to work.
CONCLUSION :
at this power and with a larger LW (CEP is not optimized), the FPC seems much more stable without any lock loss during 1h !

2) then I will do a test by decreasing the Alphanov amplifier ratio to work at a lower input power but with an optimized CEP.
Now, I optimized the CEP and alignment and reduce the Alphanov amplifier ratio to 15% => 24kW power in the FPC.
on CH2, I got 60mV (when I got 250mV when I'm at 97kW in the FPC) => ratio ~ 4.2 => 97kW / 4,2 ~ 23 kW
I checked that the PID parameters are not over-valuated which can produce a power reduction and the CEP is optimum as well as the alignment.

Axis 18 position : -13188
PID parameters (P = 0.1 / I = 0.002 / D = 1.5)

I started to record a long trend of the FPC power => cf 2nd trend with gradually increasing of the power because of the drift of the CEP + feedback adjustments.

CONCLUSION :
at this power and with nominal LW (CEP is optimized), the FPC seems much more stable without any lock loss during 1h !

3) I found a stable position for FPC around 46kW

I used the recepie for 46kW => after some time, I observed a lot of high frequency noise which induces some lock loss.