Last Friday, we tried to increase the FP-cavity power by just increasing the amplifier ratio from 35% to 70%.

It took a bit longer than expected because it started with the surprise of finding our entire setup powered off…
The power had tripped, and the switch that allowed us to remotely reset the power supplies had been damaged.
The switch was bypassed, and the instruments restarted → OK.

As usual, the locks were pretty unstable (as has been the case for several weeks), and we couldn’t hold them for more than 10–20 seconds…
So I removed the HV amplifier between the LaseLock and the laser PZT.
Since the Smaract MCS2 controller is much less noisy than the MCS1, we can use it directly without unlocking the cavity or going through "piezo-scan" mode.
So basically, having a larger range on the laser PZT is less useful.
Result: perfectly stable locks in the short term (as before installing the amplifier) AND in the long term as well… no more random unlocks!!!
So the amplifier must have been picking up parasitic noise that was being reinjected into the loop and causing regular unlocks… it works much better now! :-)

Actual power ramp-up was done entirely from the control room:

Previously measured relation between amplifier ratio and input power to the cavity:

Amplifier ratio =    10     20     30      40      50      60     70 %
Amplifier Pin =     0.8     8     15.8     24    32.5    39.5  45 W

We performed 7 measurement points:

1. Amplifier ratio = 35%
   Vr unlocked = 248 mV
   Vr locked = 103 mV
   Coupling = 58.5%
   Stored power = 90 kW
   Vpdh = 50 mV rms
    
2. Amplifier ratio = 40%
   Vr unlocked = 282 mV
   Vr locked = 130 mV
   Coupling = 54%
   Stored power = 97 kW
   Vpdh = 52 mV rms
    
3. Amplifier ratio = 45%
   Vr unlocked = 311 mV
   Vr locked = 152 mV
   Coupling = 51%
   Stored power = 103 kW
   Vpdh = 55 mV rms
    
4. Amplifier ratio = 50%
   Vr unlocked = 328 mV
   Vr locked = 171.5 mV
   Coupling = 47.7%
   Stored power = 103 kW
   Vpdh = 51 mV rms
    
5. Amplifier ratio = 55%
   Vr unlocked = 353 mV
   Vr locked = 204 mV
   Coupling = 42%
   Stored power = 96 kW
   Vpdh = 51 mV rms
    
6. Amplifier ratio = 60%
   Vr unlocked = 375 mV
   Vr locked = 227 mV
   Coupling = 39.5%
   Stored power = 95 kW
   Vpdh = 53 mV rms
    
7. Amplifier ratio = 70%
   Vr unlocked = 396 mV
   Vr locked = 272 mV
   Coupling = 31.3%
   Stored power = 82 kW
   Vpdh = 53 mV rms

Normally, the unlocked Vr voltage should be proportional to the incident power on the cavity.
But it clearly isn’t at all!

We need to verify that this photodiode remains well aligned as power increases.
Or whether we might be clipping on a lens edge as the power increases.
Or whether the power still follows the amplifier ratio/power relation measured several months ago.
To be checked during the power ramp-up from the bunker.
We didn’t go above 103 kW!!! :-(((

I think the main reason is that we are probably hitting a mount edge, and the coupling drops so quickly that it dominates
→ so we absolutely need to redesign a proper telescope.

We also took beam images for different Pin values, but since the power in the cavity barely changes, it’s not very informative.

The last plot shows that the signal from the reflection photodiode indeed corresponds to the incident power in the cavity.
There is a strong chance we are hitting something when changing the power… to be checked in the bunker!!!

Have a nice weekend

Daniele & ronic

today with Alice and Daniele,

we checked the distance of the optical setup :

from the amplifier output (the fiber output) to the 2nd lens (+200 mm) :
131.5+41+17+28 cm = 217.5 cm

from the 2nd lens (+200 mm) to the FP cavity window :
133+97+70+14+22 cm = 336 cm

1st lens : -100 mm
2nd lens : +200 mm
distance between them : 11 cm

we checked also the alignment of the different iris and waveplates of the optical setup :
we had to realign a little bit the 2 waveplates.
after the realignment, we obtain the 96-97kW in the FPC for 33% of amplifier ratio !

new power increase after alignment of the waveplates and iris.

amplifier ratio (%) => FPC cavity power (kW) => FPC cavity coupling (%) => Vr unlock (mV)
33               => 98                      => 57              => 260
35               => 101                     => 56             => 280
40               => 107                    => 53             => 320
45               => 109                    => 49             => 354
50               => 114                    => 48             => 381
55               => 114                    => 44             => 403
60               => 110                    => 40             => 420
70               => 100                    => 31             => 453

We still observe a discrepancy between the measured and estimated input power (using the unlocked reflection photodiode) as the amplifier ratio increases.
But this time, the estimated FPC power is also badly estimated.
So, it is possible that the reflection photodiode is operating in a nonlinear regime, as 300 mV across 50 Ohms is equivalent to 6 mA, which may be too much to remain in the linear regime.
It could be interesting to reduce the photodiode's power (add an optical filter) and use a 1k-Ohm load impedance.

I installed a stronger optical filter on the reflection photodiode and loaded it on 1kOhms.

70% amplifier ratio => 575 mV => 575 µA => no more photodiode linearity issue

amplifier ratio (%) => Vr unlocked (mV)
10 => 14
20 => 156
30 => 295
40 => 412
50 => 491
60 => 545
70 => 575

to be compared to : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/378

=> still a big discrepency => could we have some clipping in the reflective wedge ?
=> I tried in realigning the wedge but got roughly the same result.
=> could it be the size of the PhD (DET36 presently) ?

=> we should also check the amplifier power at FPC input before increasing power

this morning with Daniele, we set the amplifier ratio to 33% => 97kW

then, we set the amplifier ratio to 50% => 124 kW (position of the diffuser on Axis 18 : +11775)

we changed the waveplates axis => no effect

we changed the position of the L-shape :
- lower button / clockwise / 1/20 of a turn (one has to decrease the laser motor position to compensate) => no effect but the misalignement is very sensitive for this axis

- upper button / anti-clockwise (the Lshape position is getting down) / we went to the limit (not very sensitive in alignment) => no effect ! (no beam cut nor HOM apparition)

=> conclusion : now, the L-shape is badly positionned (it's off the beam) but it cannot be the reason for a lake of power in the cavity.

PS : the lock seems very stable for long term (~ 130kW)

Last friday, with Daniele, we did some realignement on the 2nd CVBG of the compressor : the beam shape seems better, now.

this morning, we checked that the injection path (periscope and injecton mirrors) is not sensitive to the polarization state of the beam.

we checked also the power at the input of the FPC for 70% amplifier ratio : 40W !

so, with 100% coupling, we can only expect 400kW in the FPC !

after trying to increase the power and after moving S3 to try to change the size of the beam on the mirrors, the FPC power suddenly droped.

so, we need to remove M1 to check if it has been damaged.

we placed an iris in reflection of the FPC to keep the alignment for the new mirror M1.
the alignment has been done with an iris and the amplifier at 10% (to get a quite round beam) and the beam profiler.
once the iris is almost closed, the alignment is good, using the diffractions rings.

this morning, with Daniele, we took some images of the M1 mirror HR surface :

1- large image span of the mirror surface (~center) before being cleaned :
one can see a lot of spots

2- zoom in on some spots in the middle of the mirror.
Daniele saw 2 suspicious spots among many.
difficult to be sure if it's only some dust or if it's some damage

3- large image span of the mirror surface after being cleaned with the spin coater machine
only the 2 suspicious spots remain, all others have been washed.

4- zoom in on the 2 suspicious spots
if one compares with the image before cleaning, one can see the spots are much larger

5&6- zoom in on the 2 suspicious spots separately after a 2nd cleaning process with the spin coater,
it seems the spots are even a bit larger than before

conclusion : it seems that these spots are presently showing some damage on the mirror surface.
it is strange that there are 2 in the same time.
one possible explanation is that, as we had to move the z-axis motor before, and we know that the mechanics are rusty, it's possible that it released some metallic particles on the mirror surface.
and 2 were in the beam area.... 

This afternoon, we installed a new M1 mirror, the only one for which the box was not already opened.

as the mirror should be "brand new", as the LMA has packaged it, we decided not to clean it with ultra-pure water and to install it out of the box.

we realigned it with the iris previously installed for that purpose.

primary and turbo pumping are in process. one has to wait until the vacuum level is low enough to start the ionic pumping and to reopen the isolating valve.

let's cross our fingers...

This is a long time issue for the ThomX amplifier : on the 4 pumping diodes available, the 1st diode has a higher temperature than the others around 40°C (see the picture).
the problem comes from the TEC which is not activated (see diodes parameter files in execel format). the related software windows are shaded.

I phoned to Jerome to ask him if one can securely activate the TEC, and he answered "yes".
but once the TEC is activated by loading parameters and modifying the line of the TEC activation, the temperature does'nt fall down as it seems the TEC does'nt work properly.
maybe it is not connected properly ? to be checked...

the result is, as the diode temperature is too high, an alarm is fired and the diode is deactivated... impossible to increase the current.

one has to deactivate the TEC and shut down electrically the amplifier to go back to the starting point.

Conclusion :
- the TEC of the diode 1 does'nt work properly.
- one can't activate it otherwise an alarm is fired and the diode is disabled.
- one should check the TEC connections in the amplifier

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.

Power during ~20h, power after turn on, spectrum image and excel datas, polarisation and pulse length.

Powermeter measurments were done with a /10 filter.. 3 mW means 30 mW

Important: Onefive does not automatically locks after being turned on.

one rotate a lambda/2 before a PBS and meausure the refected power:

Pmax=35.6 mW

Pmin=107 µW

Ellipticity ~ 3 per mille

* les vérins de 20mm (axe Z) ont été testés sur table grace à une rallonge SUB-C - DB9, au chassis ISP et au soft SUPERVISEUR écrit en labwindows CVI par Didier.

pour rappel, la course soft va de  -1 650 000 à +1 650 000 pour 20mm de course, soit 0.6mm pour 100 000 de course soft.

ils ont été réglés en positions minimale (-1 650 000) pour que Yann puisse les intégrer facilement dans le chassis mécanique.

* une fois installés dans la capsule, les signaux passent par le feedthrough de la capsule.

MAIS cette capsule a vocation a être intégrée au chassis mécanique, intégré lui-même dans une enceinte et les signaux passent alors par un autre feedthrough, celui de l'enceinte.

Il faut donc IMPERATIVEMENT ce feedthrough (ou l'effet miroir qu'il produit) pour tester correctement les vérins dans leur capsule !

Après avoir utilisé la chaine : driver ISP - cable ISP DB9 - Feedthrough enceinte DB9-DB9 - cable perlé DB9-SUB-C - Capsule SUB-C,

l'ensemble de tous les vérins 6mm et 20mm ont été testés => ils fonctionnent TOUS.

Les vérins 6mm ont TOUS subis un HOME. cette commande cherche une fin de course du vérin et le place ensuite en position médianne.

Au niveau soft, la course d'un vérin 6mm va de -22 000 à +22 000. Après un HOME, chaque vérin est postionné à la valeur 0.

Précision : le HOME sur les vérins de 6mm place AUTOMATIQUEMENT le vérin en position médianne après avoir atteint une fin de course (FDC)

Pour les vérins de 20mm, la commande HOME place le vérin sur sa FDC positive : +1 650 000 mais NE DEPLACE PAS ensuite automatiquement le vérin en position médiane.

Il faut le faire de façon manuelle.

Tous les vérins 20mm vont subir un HOME et être placés manuellement en position 0.

Il faut ensuite leurs envoyer un offset +825 000 ou -825 000 selon les vérins pour que la cavité ait une conformation rectangulaire.

(actuellement elle ne l'est pas à cause de cales de longueurs différentes selon les axes pour anticiper les effets thermiques à compenser ultérieurement).

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

Mail de Yann du 12/10/2018 :

Je vous fais le bilan des actions effectuées :

1/ La mécanique a été remontée avec changement des billes céramiques, soufflet défectueux, ect...

2/ Test et mise en place des vérins en position

3/ nettoyage des chambres et insertions de la méca

4/ connectorisation des câbles perlés et test des vérins par le soft

5/ fermeture des brides

6/ nettoyage du marbre et de la zone

On est donc prêt pour les test de vide et d'étuvage.

RAPPEL pour tous :

Des nouvelles combinaisons propres sont à disposition (on ne sort pas du flux avec la combi !)

Mettre MASQUE / CHARLOTTE / COMBI / GANTS !

Ne pas rentrer de CARTONS ou autres sources de pollution.

NETTOYER les outils ou objets à rentrer dans la zone

RANGER après les manips.

Mail de Yann du 8/10/2018 :

Bonjour,
Tous les vérins sont maintenant OK ! Merci Ronic.
La cavité a été réglé en mode nominal (tilt à +2/-2 et Z à +5/-15 ou -5/+15).
Demain avec Christopher on remet tous dans les enceintes à vide.
Du coup, Patrick tu pourras ré-connecter les câbles céramique (à partir de demain apres midi).
On est dans les temps pour des tests de vide et d'étuvage a partir de lundi prochain.
A+
Yann.