We had a long run without any lock loss during ~1h with the 23kW recipie in the morning.
during the afternoon, we had some few lock losses but the FPC system was globally quiet and stable,
but at the end of the day, I observed more and more frequent high frequency noise which implies lock losses.

at the same time, Dalkia changed the temperature of the air flow in the bunker.
could it be the reason of the stability degradation or is it because the cavity is more unstable when hot ?

For ~ 23kW power in the FP-cavity:

• Alplhanov amplifier ratio : 15%
   
• Axis 18 position : -0016014 steps
   
• PID A (for laser/FPC lock) :

input : input a
P = 0.1
I = 0.0016
D = 1.4
Sign : positive
Sampling : fast
filter : off

• Search A :

criterion : input d
upper th. : 10V
lower th. : 0.03V
speed : 100V/s
relock mode : none
off mode : hold

• Output A :

range : 50%
offset : 5V

• PID B (for RF/FPC lock) :

input : input b
P = 5
I = 0.0001
D = 1
Sign : negative
Sampling : mid
filter : off

• Search B :

criterion : input c
upper th. : 0.45V
lower th. : -0.45V
speed : 1V/s
relock mode : none
off mode : hold

• Output B :

range : 20%
offset : 5V

For ~ 46kW power in the FP-cavity:

• Alplhanov amplifier ratio : 20%
   
• Axis 18 position : -0004710 steps
   
• PID A (for laser/FPC lock) :

input : input a
P = 0.06
I = 0.0007
D = 0.85
Sign : positive
Sampling : fast
filter : off

• Search A :

criterion : input d
upper th. : 10V
lower th. : 0.04V
speed : 100V/s
relock mode : none
off mode : hold

• Output A :

range : 50%
offset : 5V

• PID B (for RF/FPC lock) :

input : input b
P = 3
I = 0.00002
D = 2
Sign : negative
Sampling : mid
filter : off

• Search B :

criterion : input c
upper th. : 0.45V
lower th. : -0.45V
speed : 1V/s
relock mode : none
off mode : hold

• Output B :

range : 20%
offset : 5V

New locking parameters (I got 98kW for 33% amp ratio) :

Axis 18 position : +0009420
P = 0.05
I = 0.0005
D = 0.6

stability seems better than previous parameters.

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.
 

yesterday, we were able to store more than 98kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio

list of possible causes of the high frequency noise which makes the system loosing the lock => to be checked.
(if you have some new idea, I will edit the post)

1- Laser
   1.1- vibrations on the laser box coming from the optical table / housing
   1.2- vibrations coming from the Smaract translation stages (Frep or CEP) inside the laser box
   1.3- pressure noise coming from the weather (the laser cavity is sealed) on the laser box

2- Amplifier
   2.1- vibrations coming from the fans in the controler crate
   2.2- vibrations coming from the chiller (water cooling cavitation)

3- Beam propagation
   3.1- vibration noise coming from the housing / the table
   3.2- vibration noise coming from the motors of the injection mirrors

4- Fabry-Perot cavity
   4.1- pressure noise on the cavity vessels coming from the weather/air cooling
   4.2- vibrations coming from the rust on the mechanics
   4.3- vibrations of the mirror mounts due to temperature related to cavity power
   4.4- vibrations coming from the electron ring mechanics
   4.5- "vacuum" index variation coming from local ionization due to the electron beam
   4.6- vibrations coming from the far position (related to the middle range) of some mounts and due to the spring of the translations stage.

5- Feedback system / Electronics / CEM
   5.1- CEM votage noise on the laser or FPC PZT's

the 33MHz beating signal (phase) is used to start and stop automatically the lock on the 500MHz beating signal.

this 33MHz beating phase has a fixed range (typically +/-0.5V), so it is important to center this beating phase in the middle of its own range when the 500MHz signal is locked

=> tune the 33MHz phase in order to get ~ 0V on 33MHz beating signal when the 500MHz locking is ON.

this can be done by using the python script "Write_Phase_Rigol_33MHz located in the path /tmp_mnt/data/shared/commissioning_scripts/common
 

Today, I removed the NKT oscillator from the bunker to put it in the PLIC room.

I locked the FP cavity and obtained 97kW with 33% amplifier ratio, without any alignment, just by changing a little bit the CEP.

I finally got 93-94kW for 33% amp ratio.

I had to move the FP-cavity length to compensate the temperature change in the bunker.
doing that, I had the message "Low Limit SW pressed" when I tried to move P4z under -98 000 steps !!!
=> so, maybe this number is not the correct steps number and we are much closer to the end than we could expect.
=> could it be the reason for the 20Hz noise related to the too strong springs, as we already observed before ?

in the meantime, I changed the use of the P1z and P4z motors to not face the situation again.
so, P4z will be used to move to higher steps (even if its apparent position seems higher than P1z).
and P1z will be used to move to lower steps.

surprinsingly, the CEP position was almost already optimized...
 

after the new temperature setting for the bunker air cooling (winter to summer transition), the temperature drop from 24° to 20°C which induced a slight misalignment between the onefive oscillator and the fiber injection.
the amplifier photodiode PD_IN which checks the amplifier power injection drop also from 3mW to 2.885mW... hopefully, it is enough to start the amplifier without realignment.

but bad news, I don't see any transmission of the cavity... to be checked...

The temperature slitly increase in thomx Bunker from 23° to 24° during the last day.

The thermalisation of the CFP is very long...

In any case the operation of the CFP is so easy, it seems to be correlated to the weather... tody it's very nice as of hte rest of the week.

The stored power is easily 94 KW and there is not jump or delock problems.

The temperature of the bunker continiusly increase and now we observe some delocking du to high frq perturbations... may be the twe things are correlated.

today, after 1 week of vacation, I was able to easily lock the cavity with 92kW with 33% amp ratio without too much optimizations (no walking alignment).

could it be related to the nice weather we got during this week ?
as in the past we observed more noise when the weather was changing and we had very difficult locking procedure after a simple weekend...

today, we were able to store more than 97kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio

today, the alignment of FP cavity was so bad that I tuned the 01 mode by error (CEP and alignment)...  insead of the 00 mode !!!
I was able to reach ~30kW and was limited at this value, so I used the camera to check what was happening and saw the 01 mode.

so, I moved the lock on a 00 mode and redid a full tuning (CEP and alignment).
I got ~93kW => ok

today, we were able to store more than 95kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio

this morning, the cavity was pretty misaligned => basic alignment in y direction helps a lot => 93kW for 33% amp ratio.

the Rigol 33MHz generator phase was adjusted : phase ch2 = phase ch1 (40 deg) + 36 deg = 76 deg.

the phase between the 500Mhz ring RF oscillator and the electrons is measured on a BPM and is very stable after 2-3ms (<< 6ps, typically) !

in the same time, we can use the residual phase beating after our 500MHz mixer to estimate the phase noise coming from the laser part.
the rms noise of this signal is around 60mV for 1.25V peak (2.5 pk-pk) signal => ~ 50mrad rms

dt = dphi / 2pi * 2ns ~ 15 ps which is the noise measured between electrons and photons at the IP !!!

we clearly see that this rms noise level is correlated with the 20Hz oscillation observed on the FPC feedback signals.

BUT even when this rms phase noise varies, we DO NOT see any effect on the X-ray production at this time,
which means this is not yet a limiting factor.
this observation is confirmed by the X-ray production flux calculation for which the time jitter is "added" to the electron beam size parameter which seems to be dominant.

we installed a measurement at the IP with the BPM for ion cleaning for electrons and we used a fast photodiode at the output of the FP-cavity and measured with a fast oscilloscope the phase jitter between the 2 signals.
in red, the BPM signal
in blue, the photodiode signal.

on the right of the picture, this is the trend of the phase difference measurement.
in yellow, this is the histogram of this phase difference.

one measured 35ps of FWHM which is equivalent to 15ps rms for a Gaussian distribution.

from now on, one reverts the FP cavity motors direction to compensate the long drift from the begining of the project.
the motor MOT.06 will be used for the upward direction (presently -1 082 650 steps)
and the motor MOT.03 will be used for the downward direction (presently +37 170 steps)

after CEP and walking alignment tuning, I got quickly 92kW.

the new MOT.06 and MOT.03 positions are : -1 071670 steps and +34 110 steps.
the CEP motor is at +345µm