Christophe MICHEL told us more precise informations about spin coater cleaning.

They always clean with pur water and cotton tipped sticks.
First a spray of water, then they put the wet cotton tipped sitck onto the mirror from the center to the edge ~x3 while continuing to spread water. They finish by putting more water without the cotton tipped stick.

WARNING: WATER HAS TO BE CHANGED EVERY DAY 

They said they only use clean water but in case of grease/oil onto the mirror they would use first acetone then clean water.

Output power of the Onefive, and power after injection and different components have been measured.

Coupling ~= 90%
Each of our two EOM roughly have the same loses ~3dB

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 !

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.

We've added a clamp in the 3rd stage to prevent the pump fiber from moving and tilting (it was tilting to the left).

Today, the lock of the FPC was particularly bad (maybe the worst ever seen), with a lot of high frequency noise.
the lock was impossible during several minutes !

we tried to switch off all the equipments of the machine, one by one without any effect on the lock.

at the end, we looked at the accelerometer installed inside the housing to check if there was some correlation.
and for the 1st time we clearly saw a 100% correlation beween the accelerometer signal with a noise oscillating above +/-300mV
but we didn't find the origin of this noise.

At the end of the day, we found out what was the origin of this noise : some road renovation work with jackhammer and road roller just at the entrance of the "Igloo".

this origin was 100% correlated with a large increase of the accelerometer signal.
but we clearly saw that the FPC is much more sensitive than the accelerometer... the signal can have a small increase or just one peak and the cavity lock is lost.

then, we can make the assumption that all the "high frequency noise" which produces some lock losses could come from acoustic noise due to the road traffic or from the equipments in the bunker itself.

After this observation, we did a test with Daniele to try to correlate the road traffic in front of the Igloo with the lock losses observed on the FPC.
We didn't see any clear correlation. Cars or buses are not the direct origin of the FPC lock losses... only when some heavy load is hitting the building, we observed a clear correlation.

So, we still observed a lot of FPC lock losses when the day is windy... 
One possible cause could be the large door of the igloo (~20 m²) hitting the Igloo when the day is windy.
I installed the accelerometer on the rail on the bottom of the large door to see if there is any correlation.

we did some tests with Marie to make some vibrations on the door.
below, I exhibit some pictures of the scope with the accelerometer plugged on it without or with noise...
this level of noise is not able to make the FPC loosing the lock.
even when saturating the noise signal on the accelerometer, the FPC is not loosing the lock all the time.

then, when a day is very windy, it's possible it could have some effect, but for a normal day, it's seems very doubtful that is the reason...

This morning, I installed a high-voltage probe (1:1000) on the AC-line to see if one could detect a correlation between lock losses and AC-line voltage variations.

I observed many lock losses but without any variation or correlation of the AC-line signal (scope set in "peak detect" with "envelope" arithmetics).

see attached figure.

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

FSR is 33.347 MHz @1e-8 mbar

this morning, we locked the laser on the FP-cavity with about 48-49kW stable in the FP-cavity (for 32% of amplifier ratio).
and then we locked the FP-cavity on the RF reference (500MHz beating lock + 33MHz beating for starting lock search).

the starting lock search voltage has been chosen between -20mV and +20mV.

then we lock with an uncertainty of 180°... but we can check the lock phase with the "Synchro oscilloscope" (192.168.229.21).
if necessary, we can stop the lock and let the phase drift and then relock with the right phase.

Kevin did a buckets scan (30ns by 2ns steps) to find the right bucket and also a fine phase tuning scan (2ns scan).
unfortunately, the fine phase was close to 0, so the Laselock did the fine phase adjustement by changing the SetPoint of the RF loop to : -720mV

at this moment, we got relatively stable X-rays at an approximated flux of 1e10 photons/s

we still need to optimize phase, table position and improve the jitters...
 

Yesterday, we've observed the first degeneracy on ThomX cavity at ~115 kW stored. The shape of the stored power on the oscilloscope is really close to what has been published in the D-shape mirror's paper ("tek0000" transmission in yellow, piezoelectric activator in green, error signal in blue).
This mode could be a 8.0 or 9.0. 
We have also observed a change of the angle of the mode (the pictures "degenere", "degenere2" and "degenere3" were taken by order). The only changing parameters are the alignement and the length of the cavity. 
The last thing we noticed is the shape of the mode which could not be an hermite-gauss mode (Bessel mode ?).

Aurélien, Manar, and I spent more than 2h trying to start the software communicating with the Alphanov amplifier controller.

each time, we had a problem with the software, asking to switch OFF and ON the controller before being able to switch the amplifier diodes ON.
we switched OFF and ON many times without any success.

in the end, Aurélien called Guillaume from Alphanov... and without changing anything, it worked... strange!

one possible problem could be the correct detection of Frep of the seed laser (OneFive).
as we didn't check the signal coming from the seed laser, it could be the reason... to be confirmed.

the present status for the controller is:
- the power connector (on the rear side) is ON
- the green relay (on the rear side) is ACTIVATED
- the key (on the front side) is OFF
- the emission button (on the front side) is OFF

the normal procedure to start the controller is:
- switch the front side key ON
- start the software (possible error msg asking to switch OFF and ON the power button: don't do that)
- switch the emission button on the front side (which is red) ON
- switch the preamplifier button ON
=> all the software LEDs should be green and the PD_PULSE window should indicate 133.33MHz
otherwise, try a RESET on the software and restart the procedure (and pray).

at the end of the day, we successfully switched ON the preamp and increased the Power adjustment around 20% to get something about 10W on the big PowerMeter placed at the output of the amplifier.
=> we need another day of practice to be more confident with the software!