Today with Ronic,

starting with major events that happened (Water circulation + ThomX valves )  

• Early in the morning a company worked on the water network, it seemed it was not restarted properly so there was no water circulation in the main ThomX pipes.
  • There was an error on the ThomX amplifier chiller , which was noticed at the end of the morning work , where we couldn't find the cavity resonance , could be due to the temperature increase of the amplifer.
  • Solved :  the issue was solved by restarting the water network and the amplifier chiller was restarted an no error found and temp. Stabilized around 25°
• Around 12h15 ThomX there was a power cut off for less than a second (micro cutt), which cased all the valves to close, the valve air compressor did not restart after the power cut off, we restarted it with Daniele in the evening with the help of Marie and the valves open around 5-6 bar , you will see then all the controllers green and the valves will open.

Results of the day :

• All the power supplies and function generators are under the table, nothing on the table (reduced noise on the signal)
• EOM of Oscillator Off (for now, might turn on if needed)
• Amplifier on 0% , output ~ 300 mW
• Locked on the first resonance of the cavity,  lase lock parameters to be optimized better
• Transmitted power increased to ~ 267.2 uW
• Average power inside cavity = 89. 06 W , peak power = 0.2 kW (pulse width = 12.6 ps , frep = 33.36 MHz )
• current effective cavity gain > 300 

Oscilloscope :

•  error signal, not shown as it was too noisy to have in the image
• blue : reflection (low bypass filter ), yellow : transmission (resistance of 100 k ohm added), green : piezo signal
• 1st image : showing lock and scan regions.
• 2nd image: time zoom on the locked signal 

oscillators controller / smartAct:

• Frequency and CEP control , the parameters in the photo attached are the best for the moment to see the smooth change of the resonance peaks when operating the motors, and we can stay FSR range and compensate the variation easily. 
• even though we stayed at closed loop (for easier adjustment), we still managed to lock , will try to switch off later to see if reduces a lot of noise. 

After removing the 2 generators from the optical table, the lock is much more stable and now, it is possible to lock on the main resonance with a poor CEP but with quite good stability.
the coupling is still very low ~ 5% for that CEP but if one improves it (CEP ->0), using the laser double-wedge motor, one clearly sees an improvement of the coupling... but at the cost of the lock stability.

the reason of the poor coupling is also because the laser amplifier is used at 0%, for which we know the part of the laser signal power, compared to the total power, is low.
(a part of the beam @1030nm is not propagating in the fiber core of the amp, and then, it cannot be coupled to the FP-cavity).

the fast lock loop on the EOM has been disabled for the moment.
it has to be installed back to improve the stability at a better CEP.

at present, the FP-cavity power is estimated at ~ 90W (~270µW in transmission of ~3ppm mirrors) for ~300mW of total power coming from the laser amp.

next steps :
- in Open Loop : check what is the best coupling we can observe for CEP=0 @ P ~ 10W (laser amp at ~ 25%)
- in Closed Loop : @ P ~ 10W => measure the best transmitted power after alignement/polarization/feedback adjust => ~ 3-10kW in the cavity ?

In the morning, we locked on the first resonance. With an increase on the amplifier power @ 20-25% and 30%, which reduced the noise

we manage to get a coupling when improving the CEP up to 55 - 60 % coupling : put there is still a drift on the CEP

The controller for the oscillator frep and CEP produces a lot of noise, even if it is disconnected. we need to switch it off to remove its effect (Kevin will order a new one)

The image attached is of the oscilloscope lock on the first resonance @30% amplifier power ~ 10 W injected into cavity,

transmitted power ~ 36 mW (cavity average power 12 kW)

cavity effective gain > 1200

in the afternoon, we installed and aligned a gentec power meter we can monitor from the computer. (will need to buy a permeant one for ThomX)  

there was an issue with the laselock USB connection, yet to be solved.

and the control of the computer.

The computer is possible to connect from the control room,

we are able to run remotely the amplifier, cavity motors, oscillator motors, lase lock (note: there is an issue with the keyboard, we are not able to use it with remote access!!!!!!!!)

Note on Amplifier: On Friday, Daniele and Ronic faced again the temperature issue that came up before. it is due to the fluctuation of the diode temperature.

It was fixed today by Ronic and Daniele. it seems there are three diodes for monitoring the temperature, only one was connected, and it had issues, with some directions from alphanov. 

Ronic just removed the defective diode and soldered (connected) a different one. The amplifier should work without this issue. 

now we start searching for X-rays after locking the cavity remotely  

this morning, I tested the laser+amplifier @ 30% lock on the FP cavity with and without Smaract motors.

I recorded the PDH error signal during a lock:
- blue   : with Smaract motors controller powered ON but motors are stopped
- yellow: with Smaract motors controller powered OFF

with Smaract motors controller powered ON and motors stopped, one can see a group of resonances around 10kHz (8 - 11 - 14kHz) which disappears when the controller is powered OFF.
one can see also a group of resonances around 25-30kHz for which some peaks desappear when the controller is OFF but most of them are still there... could it come from noise on the Onefive laser PZT ?
one can see also a noise reduction at low frequency with a corner frequency around 17kHz, which could be the Unity Gain Bandwidth of the feedback loop on the laser PZT (fast feedback loop on EOM was disconnected)
=> to be confirmed

*************************************************************************************************************************************************************

I was able to lock with a decent noise on transmission and reflection signals @ Pin=17W (30%) of input power and with a coupling ~ 20%.
I measured 31mW in transmission => Pcav ~ 10.3kW (T ~ 3ppm)
as T1=115 ppm and F=30000, the cavity gain is T1*(F/pi)^2 = 10.5k,
so, the FP cavity power should have been 17W * 20% * 10,5k = 35.7 kW !!! (maybe the formula is wrong if the coupling loss comes from the CEP detuning effect)
=> we have to check the incoming power and the formula !

so, maximum expected power in FP-cavity could be 70W * 100% * 10,5k * (10.3/35.7) = 210 kW !!! :-(

*************************************************************************************************************************************************************

I was able to redo the lock easily in remote in the control room (with the Smaract motors controller OFF).

I checked with the Matlab code below the CEP detuning effect (2nm sech² spectrum... not exactly the same as in ThomX)
@ CEP = 0 => coupling = 100% and Gcav = 10.5k
if all the coupling loss comes from the CEP detuning effect :
@ CEP = pi/5 => coupling = 20% and Gcav = 2.14k (~ 10.5k x 20%)
so, it does not matter if the coupling loss comes from the CEP detuning effect or from beam mismatch or misalignment.

=> we should have more power at 20% coupling, not 10kW but 35kW !!!
=> we have to check the real input power !

we checked yesterday morning the real input power @ 30% for the amp => it is 16W in agreement with the previously measured values

on friday 23/06 afternoon, after scanning the hexapod z-axis, we observed a cut of the power stored in the FP-cavity (with constant input power/coupling) at both ends of the scan, due to the losses of the beam-pipe aperture.
we placed the hexapod exactly in the middle of this range ~ z=-1.68 mm (which is not the middle of the maximum range of the hexapod), we opened the slits on the X-table and we found the first signal.

after scanning, shuting the laser beam and electron beam ON and OFF, we confirmed this signal was coming from X-rays.

on monday 27/06, roughly in the same condition (25kW of power stored in FP-cavity), we scanned properly the same z-axis with the hexapod and we measured the pico-ampermeter current related to the produced X-rays to get the approximated size of the laser and electron beams at the IP position : see the raw data.

Daniele did the data analysis : see the attached analyse_profil_vert_X-1.pptx file
the total rms size (sigma of the Gaussian profile) is 100µm.

this morning, we measured the FP-cavity waist size by measuring the waist size of beam at the focal distance of a lens used after the spherical mirrors.
we measured the rms size of the FP-cavity mode to be 60µm in agreement with simulations, which leads to 80µm rms for the electron beam waist size.

today, we observed on the photodiode used in reflexion of the cavity, that its voltage level stops increasing after 50% of power for the laser amplifier when the cavity is not locked (the FP-cavity is just a reflective mirror).
the photodiode itself is not saturated (low power sent after a wedge and an absorbing filter) and the reading is done on 50 ohms.
so we are investigating the reason of this "saturation".

1) we did a beam power measurement just after the 2 mirrors, right after the amplifier, with the "big" powermeter:

power ratio        Power (W)
10%        =>  0.9
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  33
60%        =>  41
70%        =>  48
80%        =>  55.5
90%        =>  62.2
100%      =>  68

plot in attached file

2) we did the same measurement after the periscope:

power ratio        Power (W)
10%        =>  0.82
20%        =>  7.7
30%        =>  15.5
40%        =>  24.2
50%        =>  32.6
60%        =>  39
70%        =>  44
80%        =>  47.4

we clearly see a power reduction from 50% and beyond.
with the viewer we saw 2 effects : a slight clipping in the telescope and some speckles on the periscope mirrors mainly.
the laser is also slightly shifted on the 2nd mirror after the amp.
it is possible the laser beam moved due to thermal effect => we will need to realign properly the whole injection line and be aware about mirrors and lens cleaning !

3) we did some measurements using 2 wedges with the "small' powermeter in reflection of the cavity.
we remove the small aperture half-waveplate to see the effect and we used a lens to focalize the beam:
the half-waveplate has an effect on the reflectivity of the wedges, this is the reason of the difference in the measurement.

                                                               Power (µW)
power ratio        with lambda/2           w/o lambda/2             w/o lambda/2+lens
10%        =>           12.4                           4.8                               5.1
20%        =>            132                            48                                51
30%        =>            262                            96                               101  
40%        =>            370                            137                             147   
50%        =>            420                            158                             175 
60%        =>            430                            165                             181

we observe a clear "saturation" after 50%-60% which is very similar to what we observed with the photodiode in reflection when the cavity is not locked.
the small iris used in front of the injection window of the cavity is a good "candidate" for clipping the beam at high power (we observed it was still quite hot after stopping the beam).
=> we have to redo the measurements after removing it.

Today, we locked the cavity with input power @ 30% for the amp => we got 40kW with only 30% of coupling (and a bad lock => we could have more power inside cavity).

P@30% = 16W
Coupling = 30% => 4.8W of input power => Measured Gain = 8300

several times already, when the electrons ring injection kicker is running @10Hz, we observed peaks at the same frequency on reflected and transmitted photodiodes,
at the exact moment when the kicker is activated. we have in addition, USB communication issues with the Alphanov amplifier... the connection is lost several times.
sometime, the connection is retrieved, sometimes not... one has to stop the Alphanov application and restart it => the laser power get down to 0 % !!!

one way to get rid to this problem could be to add a "metallic sock" around the USB cable, to connected to the ground to remove the external charges.
we could ask also for a kicker shielding as this noise could perturbate several systems in the bunker.

presently, the voltage on the laser PZT is 0-10V, but during the several runs we did, this voltage is too low to compensate low frequency fluctuations:
we need to change often the motors position implying a cavity lock loss.

we have to check if running the laser PZT with the HV output of the Laselock is possible without too much additionnal noise.
=> it can be done simultaneously with the fast feedback loop implementation, which has to be done.

Yesterday, I checked the 2 output HV channels of the LaseLock : they work properly but the noise is bigger than the low voltage outputs
~ 1mV rms on 0-10V outputs
~ 8 mv rms on 0-100V output

on the August 30rd and 31th, a global power shutdown was forseen for ThomX.

to prevent damage on equipments, I switched off all of them (and disconnected power cables from the wall plugs).

yesterday, after the week-end, all the equipments have been restarted and the cavity as been locked properly.

the power coupled to the input fiber of the amplifier has decreased a little bit from 3.8mW to 3mW during summer.
=> we need perharps to do some alignment on the Schaftner-Krischoff mount.

today, to match the ring frequency at 500.25MHz (15th laser harmonic), we did a frequency shift on the laser using the Smaract motors (not too fast, ~2µm/s, to prevent laser modelock loss)
and we "followed" this shift on the FP-cavity using the FP-cavity motors.

we did several steps during the operation, to control the alignment, coupling and transmission.

finally, we locked again the laser and FP-cavity at 17kW for 30% of input power.
the coupling was quite low ~10%

today, we tried to lock the FP cavity with the Smaract motors ON (with option -LV).
we know the Smaract controllers produce some noise and the lock is very bad or impossible when the controller in ON (whatever the displacement mode is : closed or open loop, or piezo scan).
then we need to do a fast feedback loop on the EOM inside of the Onefive laser.
the problem is we cannot fill directly the error signal (~ 300mVpp) as the signal level is too low to produce some effect.

1- we tried to use the AC-coupled homemade amplifier alone but the output range is too low (+/- 3V)

2- we tried to use the M250 video amplifier for EOM with AC-coupling but the output range is still to low (+/-30V ? => to be confirmed)
but we saw an improvement in the locking.

3- we tried to combine AC-coupled homemade amplifier + M250 video amplifier for EOM with AC-coupling.
we are able to lock (~ 30% coupling) but the lock quality is very poor : we clearly see that we oscillate around the maximum of the Airy peak.
we tried several combination of the global gain (fast feedback + Laselock) using the diffuser, of the Laselock PID parameters but it seems we are not able to lock properly.

we measured a global delay of this double amplification stage of 80-90ns for the homemade amplifier and 150ns for the homemade+M250 amplifiers.
this delay is compatible with ~ 500kHz BW for the feedback => it seems it is not the reason...

we measured also the linearity of the homemade amp => there is ~ 30dB between a frequency and its 1st harmonic even at low signal...
the amp scheme is not very linear.
in comparison, the HV M250 amplifier exhibit > 80 dB of linearity !
we will try to remplace the homemade amp by a commercial FEMTO amplifier to imrpove the linearity and see if it improves also the lock.

we measured also a 100Hz AM modulation on the output signal of the homemade amp+M250.
=> we can try to work in differential (HV+ - HV-) to see if it helps to remove this modulation.

previously, we were using a spare Smaract MCS controller to drive the Onefive linear stages (without -LV -low vibration- option and using Ethernet),
during the repair of the initial MCS controller with -LV option.

this morning, it has been remplaced by the initial OEM Smaract  MCS controller integrated by Onefive (with -LV option and using USB).

it worked fine with the Precision Tool Commander 2 software !

today, during an attempt in improving the CEP with the Smaract controller CH2, we lost the laser modelock.

at this point, we decided to reference the Smaract CH2 for CEP :
now, 0 mm is roughly the middle of the motor range.
the endpoints of the motor are : +/- 5.9 mm
once we found back the laser modelock, we searched for the region where the modelock was still effective : +/- 1.8 mm

today, after the several unsuccessful attempt yesterday to get an improvement in the lock,
we decided to "rebuild" the error signal electronics block by block and step by step :
the scheme is basically :
- DET36 photodiode
- followed by a 10MHz low pass filter to remove the laser frequency harmonics and keep only the modulation frequency at 8.4MHz.
- connected to a FEMTO HPVA AC-coupled 40dB gain amplifier with 50ohms in parallel on its input (which is connected to the photodiode).
- connected to a Minicircuit mixer which is also demodulated by the generator at 8.4MHz
- followed by a 1.9MHz low pass filter to remove image frequencies

when this signal is sent to the Laselock box, the lock of the cavity is possible but very noisy.
we need to put a large D values in the PID to maintain the lock at the price of oscillations and gain loss ! :-(

when this signal is connected also to the M250 video EOM amplifier (which is 50 ohms), but this amplifier is not used,
we suddenly got a much better lock (see the attached pictures), certainly due to the 50 ohms connected to the input of the Laselock system => to be verified.
one could have some noise current at the Laselock input which produces less noise at the output when the input impedance is 50 ohms, instead of the several kohms when the input is unloaded...

then, we were able to get a stable lock at ~40kW with 30% coupling and 30% of amplifier :
see this post with the same values : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/227
but now, the lock has been done WITH ALL the motors controllers ON !!! :-)))
now, we can try to improve the lock with the fast feedback loop.

and the only real problem is the mode degeneracy we need to block with the L-shape.

this morning we locked the laser and the FP-cavity with ~ 40kW => some HOM appeared "randomly" depending on the power.

we tried to play manually on the L-shape arm :
- first, we moved horizontally to put the L-shape in the beam axis
at some point, we saw the power level divided by ~2, we stopped and came back to restore the full power.
- then, we tried to move vertically.
whatever the direction, we were not able to see a clear cut of the beam.
BUT "strangely", at some point, the PZT drift followed the direction of the motion and was not really depending on the "cooling" or "heating" process when the lock stops or restart.
"strangely" again, at the endpoint, the power dropped by ~20-30% after a delock but it was impossible to restore the power when we put the arm at the initial position.
=> we had to adjust the FP-cavity alignment to restore the power !!!
=> we had the feeling that moving the arm could have misalign the cavity axis !!! :-(
=> we have to discuss with Yann and get the CAD files of the cavity to have a better understanding...