for this 20Hz oscillation, could it be a natural resonance of the optical table placed on its feet ?

typical mass of the table m ~ 8T

feets :
young modulus of steel E ~ 210 G N/m²
length L ~ 1m
diameter d ~ 10cm
rigidity for 6 feets => k ~ 6. pi.d²/4L E ~ 9.9 G N/m

rough estimation of the oscillation frequency : f = sqrt(k / m) / 2pi ~ 177 Hz

if the rigidity of the real moving feets is lower (~ by a factor 80) than if they were made of plain steel, we are not so far.
to be continued...

 

Today, we restarted the FP-cavity locking, after 3 weeks without using it.
those 3 weeks have been used to set the machine ring at the new (better) frequency : 500.067MHz (corresponding to a change of -183kHz)
or 33.3378MHz in fondamental frequency (corresponding to a change of -12.2kHz)

the power supply of FP-cavity photodiodes was OFF maybe due to some electrons losses.
we had to switch it OFF and ON to restart it.

the computer was restarted and we had to restart the applications and set their parameters.
in attachement, the parameters of the Kangoo software.
we had several "warnings" from the laser amplifier software => the amplifier stopped
but maybe it's because of the electrons not sent in the dump which pertubate the monitoring signal levels of the amplifier => to be confirmed

we had to do some alignment.
with 33% of  amplifier ratio, we had 49kW inside the FP-cavity.
we tried to move a bit the L-shape mirror but without any significant effect to improve the intra-cavity power.

we didn't start the laser and FP cavities length increase procedure.

surprinsingly, the beating frequency between laser cavity and 33.3378MHz is only 140Hz and not 12kHz as expected.... to be investigated
 

this morning I solved the problem of the 33MHz frequency beating which should be ~12kHz between the generator (at the new frequency 33.3378MHz) and the laser cavity frequency (still at the old frequency which is 33.35MHz)...
the generator providing the 33MHz frequency has 2 channels, and only one was set at the proper frequency !

so, I set both channels at  33.3378MHz and now I measure properly ~12kHz of frequency beating.

now, we can move the laser cavity and the FP cavity in confidence.

this morning, I finished the FP-cavity alignment with the new ring RF frequency.
I get back 50kW @ 33% amplifier ratio.

I'm also able to lock very smoothly on the 500MHz ring RF frequency.
so, we are ready to produce X-ray.

warning : apparently, when one changes the 33MHz generator frequency with the "Nicolas" script, it does not change both channels of the generator, which is mandatory for us.
=> to be fixed : freq(CH2) = freq(CH1)

new optimized lock parameters in attached file (Kangoo parameters)

no more "Alphanov amplifier warnings" when one works without the ring machine => it is important/mandatory to send the electrons in the dump.

 

previously, we have connected motor M4 with an IcePap controller.
see this post : https://elog.lal.in2p3.fr/FPC/THOMX+commissioning/278

due to the backlash behavior of the FP-cavity motors, it is impossible to work in both directions on one motor when the FP-cavity is locked.
so, we decided to use also the motor M1 to work in the other direction.
then, normally M1 will work in one direction and M4 in the other direction.

motor M1 (or P1Z) which was on the axis 9 on the ISP controller has been assigned to OC/OP/OCH.01-MOT.06

motor M4 (or P4Z) which was on the axis 12 on the ISP controller has been assigned to OC/OP/OCH.01-MOT.03

this double motors scheme works good.

if motor P4 is used by increasing value, the FP-cavityPZT signal (pink signal on scope) goes HIGHER
if motor P1 is used by increasing value, the FP-cavityPZT signal (pink signal on scope) goes LOWER

!!! CAREFUL !!!
after each session, it is mandatory to put the motors back to their initial position to avoid FP-cavity misalignment.
on P1 : - 900 000 steps
on P4 : - 360 000 steps

at the begining of each session, it will be mandatory to avoid any backlash for the motors :

- for the P4 motor : one can go 2k steps DOWN, then 2k steps UP to go back to -360 000 and use the steps UP without any backlash
- for the P1 motor : one can go 2k steps UP, then 2k steps DOWN to go back to -900 000 and use the steps DOWN without any backlash

today, I used the P1 motor to do some steps DOWN to find back the main resonance at the begining of the session.

The temperature slitly increase in thomx Bunker from 23° to 24° during the last day.

The thermalisation of the CFP is very long...

In any case the operation of the CFP is so easy, it seems to be correlated to the weather... tody it's very nice as of hte rest of the week.

The stored power is easily 94 KW and there is not jump or delock problems.

The temperature of the bunker continiusly increase and now we observe some delocking du to high frq perturbations... may be the twe things are correlated.

after the new temperature setting for the bunker air cooling (winter to summer transition), the temperature drop from 24° to 20°C which induced a slight misalignment between the onefive oscillator and the fiber injection.
the amplifier photodiode PD_IN which checks the amplifier power injection drop also from 3mW to 2.885mW... hopefully, it is enough to start the amplifier without realignment.

but bad news, I don't see any transmission of the cavity... to be checked...

I finally got 93-94kW for 33% amp ratio.

I had to move the FP-cavity length to compensate the temperature change in the bunker.
doing that, I had the message "Low Limit SW pressed" when I tried to move P4z under -98 000 steps !!!
=> so, maybe this number is not the correct steps number and we are much closer to the end than we could expect.
=> could it be the reason for the 20Hz noise related to the too strong springs, as we already observed before ?

in the meantime, I changed the use of the P1z and P4z motors to not face the situation again.
so, P4z will be used to move to higher steps (even if its apparent position seems higher than P1z).
and P1z will be used to move to lower steps.

surprinsingly, the CEP position was almost already optimized...
 

Today we do XRays for experiment.

The laser start to 33% at 93 KW but Pgain = 0.033

The laser is locked so stable for the moment. We have Xrays and the system is stable.

I have started a trend of laser power.

Some high freq perturbations are visibles but no delock.

It's clear that when the charge vary a lot in the ring we see more high frq perturbations!!

Each time we inject electrons we see high freq pert!!! But the delock are rare!

When electrons are lost we see high freq perturbations!!

New PID parameters

P=0.03

I=0.0005

I2=0

D=0.5

Axe18=+0.0016014

We have done a vertical scan and de e-beam seem to be very very large!! >1mm

The e-beam is not well adjusted but the lock is really much better!

We do good measurement of X fluorescence spectrum of Prostate, ThomX is becoming more and more interesting Xrays source!!

Daniele

There was some ceramic wires close to the beem path right before P4. We moved them away (we can imagine they could move again with vibration).

Last Friday and this morning, with Bruno, we close the ring/cavity valves and put some dry air in the two vessels through a filter to avoid turbulences.

Now, the two vessels are at ambient pressure and ready for opening (to remove the mirrors).

One can let the ambient pressure in the vessels as long as we want, the ring/cavity valves have been chosen to be compatible with the high vacuum in the ring and the ambient pressure in the vessel for a long time.

looking in the vessel of the FP cavity using a UV lamp shows a lot of dust... more dust than outside the cavity !!!

here is picture where one can see a cleaned zone using a finger (with gloves).

question : where does come from this dust ?

2 possibilities :

- large and small belows which could trap the dust and release it during the pumping process

- ionic pumps could release some clusters when they are working.

we plan to clean as effectively as possible the inner surfaces, to close bellows, make woking ion pumps to check if it released again some dust.

we plan to use Electrically Tunable Lenses EL-10-30-C NIR to build the ThomX telescope but they need to be compatible with the powerful laser beam.

for CW regime, on the datasheet (see attached file), the optical damage threshold is 10 kW/cm².
with 70W laser power and 1mm² beam surface (which is much smaller than the real beam size), we are at 7kW/cm².

for pulsed regime, with Daniele, we can imagine that the inner material is water (n ~ 1.3 in the datasheet).
One found a paper about laser induced electric breakdown in water (see attached paper) which give a breakdown field of about 1e8 V/m (see Fig. 1 of the paper) for 7ns pulse width.
with 70W average power, 33MHz repetition rate, 10ps pulse width 1mm² beam surface, one has an intensity of ~ 2e11 W/m² (or 2.7 GW/cm²)
I = E² / 377 => E = 8.7e6 V/m which is also much smaller than the breakdown value.

from the fig. 4 of the paper, it seems the breakdown threshold increases a lot for shorter pulses from 30 GW/cm² (~10ns pulses) to ~150 GW/cm² (~10ps pulses)
so, one can expect a good behavior in pulsed regime too.

can anyone check the validity of these rough calculations ???

Today with Daniele, we tuned the potentiometer after the Laselock which drives the analog BW of the PID on the laser PZT,
and we tuned also the fast loop gain (with the rotary potentiometer dedicated to it) => the PID parameters have changed.

so, the recipies has changed => to be updated

the global observation with the x3 amplifier on the laser PZT channel, is the system is more noisy.
the maximum power for 33% amp ratio is now ~93kW instead of 97-98kW.

we tried this afternoon to do a long run with the RF and the FPC loops activated, but it seams that a lot of high frequency noise is present.
is it coming from the new parameters feedback setup or because of the electron machine which is ON ?

We openned the cavity, removed S2 and measured trans of P1 and same for S2.

Pin ~= 3.91 mW
PS2 ~= 20 nW --> ~~ 5 ppm !!!
PP1 ~= 5.23 uW --> ~~ 1337 ppm !?!?!

P1 is the Sapphire mirror. 3mm thick. Tomorrow we'll measure its trans again, being out of the mount for constraint issues.

After verification, transmission of P1 correspond to ~150ppm.

The 1337ppm come from a beam leakage. Reflection from P1 goes to the input window which reflects again a part of the beam directly into the cavity between P1 and S3 then between P4 and S2.

P1 transmission out of the mount (means angle of the mirror is not the real angle):

Pin = 3.2 mW
Pout = 545 nW
Offset = 68 nW

Transmission = (545-68)/3.2e6 = 149 ppm