I put back and aligned the 1/2 waveplate only and I get 71kW max after its optimization and optimizing the CEP and alignment (without walking procedure).
this waveplate change a lot the reflected power seen on the CH2 of the scope.

to be continued this afternoon...

this afternoon, we put back the 1/4 and 1/2 waveplates and after a long alignment/angle tuning/CEP optimization process, we hardly got 80-81kW in the cavity for 33% amplifier ratio. the transmission photodiode is then ~ 7.5 divisions (500mV/division) on the scope.

BUT during the 1/2 and 1/4 waveplate angle tuning process, we clearly saw:
- a reflection signal level which is very sensitive to the input polarization
- a coupling which can be almost zero with ~70kW inside the cavity
- the more important: ~9.5 divisions (500mV/division) on the scope with only 70-75kW in the powermeter
        => which device should we believe ?

the problem is all these photodiodes (reflection, error signal, transmission) and powermeter are aligned with mirrors which seem to be sensitive to the polarization state of the beam... and for the "output" beam side of the cavity, we cannot remove the mirrors because of the mechanics of the cavity which prevent a direct view of the ports. for the "input" beam side of the cavity, the output port is used for the cavity beam size measurement with a Basler camera.

we have to think to how to solve this issue !

the machine people saw a correlation between BPM jumps and vacuum gauges peaks related to breakdowns in the beam pipe.

these breakdowns could produce some accoustic noise which could be related to our lock losses.

=> we tried to do a corrrelation between our lock losses and the vacuum gauge peaks => WE DON'T SEE any correlation !!!

today, we were able to store more than 93kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio

today, we were able to store more than 94kW in the FP cavity, always with the same parameter : 33% laser amplifier ratio

today, we had a strange effect :
both the laser and the FP cavity were locked on the RF frequency (500.10045 MHz), and we were producing X-rays but with a slowing (several seconds) fluctuation exactly as the electrons-photons phase was drifting and the fluctuation can goes to 0 Xrays produced even at the "right" phase.
but the ring people says there is no phase fluctuation in their measurements.
and I looked the baseline of the 500MHz beating (which is the synchronisation signal with electrons) and I didn't see any fluctuation either.

we tried 2 other ring frequencies : 500.10030 MHz and 500.1 MHz.
there were still X-rays production fluctuations but with a faster rate.
when we went back to 500.10045 MHz, we found back the same X-rays production fluctions at a slow rate.

to remove this effect, I had to add an integrator gain (I = 1e-6) in the Laselock in the FPC/RF loop.
this integrator was not used (I = 0) in the previous stable X-rays productions....

it means that we have now a slow phase fluctuation.
we have to check if we see these fluctuations in the 500MHz beating signal.
loosing the Xrays means we moved by the packet length dt ~ 50ps which is equivalent to 9° @ 500MHz of fluctuation.
with A= +/-1V signal, it would be equivalent to a drift of ~ A*(2pi*dt/T) ~ 157mV !!!
it is strange we didn't see it => to be checked !!!

conclusion : now, we have to put this I=1e-6 in the FPC/RF loop to get a stable Xray production.
but the locking acquisition works better with I=0.
so we need to remove it before starting to lock and then, when the lock is stable (but the X rays are poor), we need to put I=1e-6 => the Xray flux inscreases dramatically.

this afternoon, we succeeded to get a quite long Xray production trend around 15-20 minutes with a flux above 25k on i1.

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

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.

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.

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

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

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
 

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

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

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 ~ 66kW power in the FP-cavity:

• Alplhanov amplifier ratio : 25%
   
• Axis 18 position : +0018369 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

Today we did several long runs (~1h each) at ~23kW, 46kW, 66kW with different PID parameters which seems better.

P = 0.1
I  = 0.0015
D = 1.5

which implies a different fast loop gain.
For these new PID parameters, it was impossible to use 33% amplifier ratio => to much power on the PID at the diffuser limit (axis 18).
so, we added a NE02 optical attenuator on the mobile diffuser => we can't use the old recipies anymore.

With optical attenuation only

For ~ 92kW power in the FP-cavity:

• Alplhanov amplifier ratio : 33%
   
• Axis 18 position : +0003768 steps
   
• PID A (for laser/FPC lock) :

input : input a
P = 0.1
I = 0.0015
D = 1.5
Sign : positive
Sampling : fast
filter : off

• Search A :

criterion : input d
upper th. : 10V
lower th. : 0.05V
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

Today with Daniele, we did 6 long runs at different power (23kW, 46kW, 66kW, 73kW, 92kW, 92kW)

All the lock loss in between these several runs are due to FPC locking parameters change.
most of the few lock losses during the stable power duration, are due to 20Hz oscillation noise or because we forgot to center properly the PZT in its range (operator faults).

the 4 first runs (23kW, 46kW, 66kW, 73kW) are using the PID : (P=0.1 / I=0.0015 / D=1.5) without the additionnal optical attenuator placed after the diffuser.

the 5th run (92kW) is using the PID : (P=0.1 / I=0.0015 / D=1.5) with the additionnal optical attenuator placed after the diffuser.

the 6th run (92kW) is using the PID : (P=0.05 / I=0.0005 / D=0.6) and obviously a different diffuser position) with the additionnal optical attenuator placed after the diffuser.

surprisingly, we never saw any high frequency noise during the day !

to be noticed : the electron machine was OFF / the day was sunny without wind / almost nobody was working in the bunker.