08/02/19 - End of the cleaning process.

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.

The circuit breaker on ThomX cavity drops @1.5-1.8kW when we turn on 1/2nd stage of the amplifier or 3rd stage of the amplifier.
Amplifiers are now connected to K-BOX UPS.

We are buying a clamp ammeter for further investigations.

Mail de Bruno Mercier du 18/10/2018 :

Un petit CR de la journée:
Nous avons effectué une détection de fuite sur l'ensemble des cavités optiques. Une fuite très importante a été localisée au niveau de la bride de la Pompe ionique (PI) située sur la deuxième cavité optique par rapport à l'entrée de la salle blanche.
Manu a démonté la PI , la fuite provenait du fait que le joint n'était pas tout à fait centré. Après un remontage de la PI, une nouvelle détection a été faite:
détection d'une fuite sur la bride avec les sorties Sub-d M4 après resserrage c'est OK.
Détection d'une fuite sur la bride de sortie DN200 avec  hublots de la cavité 1 après resserrage c'est OK.

Demain et le début de la semaine prochaine sera consacré à la mise en place de l'étuvage, puis de la descente en pression avec pompage ionique.

bonne fin de journée,
Bruno

https://atrium.in2p3.fr/0dded51a-1cb0-43e6-ae0d-626cd5db7078

a lot of schematics regarding the FP cavity are stored by the mechanic group on the Atrium repository.

Observation made with IR Beam Viewer: 

The observation of P1 and S3 simultaneously while locking shows much higher diffusion on the ULE mirror. The diffusion on the two other ULE mirrors (S2 and P4) shows similar diffusion as S3.

Attached is the current setup in ThomX

Coatings reflectivity curves and datas for HR Saphir, HR Suprasil and HR ULE.

From 7th january to 11th january.

this morning, I shut off all the equipements (especially the computer) except the ones powered by the UPS : laser controller, laser peltier controller, laser motors controller.
and I disconnected the cables from the power plugs of the wall (to prevent any possible overvoltage).

Jean-Noël Cayla will need to shut off the electrical shelf (the one powering the UPS) during the afternoon, I will check at this moment if the laser controller is still properly powered thanks to the UPS.

this afternoon, I checked visually what happens when the AC power of the UPS is broken down => the laser controller has still its "emission" button ON... no AC interruption.
so, I guess the UPS switched properly the output AC line from the input AC line to the internal batteries.

this morning, with Daniele, we checked the amplifier power, right after the CVBG (the power meter has to be placed on a metal plate above the large table hole), and just at the input of the FP cavity, after the 1/4 and 1/2 waveplates (the power meter has to be placed on flat beam dump+ Thorlabs beam dump + V metallic mount to be at the right height).

we compared with the power measured the 9th of september : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/356

the 3 collumns are the measurement close to the CVBG on 9th of september / same position today / just befor the FPC today

amplifier ratio (%)                 power after compressor (W)                 (new) power @ CVBG   (new) power @ FPC      ratio FPC/CVBG power (%)
10                                               0.91                                                  0.93                                    0.83                                       89.2
20                                               8.6                                                    8.6                                      8.0                                         93.0
30                                              16.7                                                  16.9                                    15.8                                        93.5
40                                              25.5                                                  25.6                                    24.0                                        93.7
50                                              34.5                                                  34.6                                    32.5                                        93.9
60                                              42.5                                                  42.5                                    39.5                                        92.9
70                                              50.0                                                  50.0                                    45.0                                        90.0

we observed with the viewer the beam on the powermeter.
at 70% of amplifier ratio, the beam size is as big as the powermeter detector.
then, the 93% to 90% transport efficiency drop could come from this "too small" powermeter detector.

we observed also at this power ratio (70%) that the 1/4 and 1/2 waveplates were not perfectly centered and we burn a part of the platic mount at this power.

we did a long term run (25 mn) with the powermeter located at the FPC position at 33% amplifier ratio.

the initial power was 18W .
then, it goes to 18.5W in 10 minutes, then 5 minutes later, it goes to 18.3W and stay stable until  the end the run.

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 !

Direction of HR face of P1 and P4 have been checked, there was no problem. The mirrors are in the good direction and the coating is on the good face.

Once P1 and P4 outside of the cavity, we checked the coating position by sending the HeNe laser with an angle. There was two reflections and the most intense was the one closer to the normal to the mirror.

On P4 we noticed a spot which seems to be in the coating (does not move while sending nitrogen).

Yesterday Ronic and I worked on the high laser power measurements with the Vertical set-up of the Focus Tunable Lens. We went upto 38W@4 dpt at the begining and then also changed the focal power at 38W.

Observation:

1. We observed nice rings @38W 4dpt, maybe it hits some optics we have to check.

2. We also observed flactuations of power for two adjacent index of the beam profiler when we increase the laser power, which we need to understand.

3. We could see that the vertical deformation is not present for this set-up, we could remove the effect of gravity by this set-up.

The cavity is right now at 6.5e-8 mbar (turbo pumps on for 1 night and ionic pumps for 3h), primary pumps and turbo pumps are cut, ionic pump are on and the pressure level is still going down. 
Therefore there aren't any big leak in the cavity and we can expect a final vacuum level close to e-9 mbar.

Several experiments have been done in order to check the potential source of dust in the ThomX cavity. For each step, we look at a clean surface, put the cavity under vacuum, go back at ambient pressure and check back the "clean surface".

1st: Primary pump on
2nd: Primary pump + Turbo pump on
3rd: Primary pump + Turbo pump + Ion pump on
4th: Primary pump + Turbo pump + Ion pump on + 10 "open/close" displacement of the pneumatic valve
5th: Primary pump + Turbo pump + Ion pump on + 1 full range displacement of P1 z motor
6th : Primary pump + Turbo pump + Ion pump on + RGA

None of these experiences have shown good correlation with dust generation. 
The RGA has shown reasonable values.
Conclusion is that dust was already in the cavity and was only displaced by all combined motors, pneumatic valves, cavity opening and closing movements.