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Hey Andrew! Sorry for the late reply!

 

After reading the suggestions I just realised that I forgot

another modification that was done before I started....

 

Below the the crystals in the pictures I sent we installed

some ceramic balls to thermally isolate the crystals from

the mechanical support.

 

That’s cool. I’ll forward this note to one of our engineers…

 

I agree with James Holton, if the energy change is comming

from the second crystal you will see a massive drop in

intensity....we actually never observed this, only thermal

drifts correlated with thermal movements on the second

crystal.

 

Yes, you were right. According the tests I’m performing since Monday what is taking place is exactly the mentioned above. According to an experiment in this morning, the observed decrease in the normalized intensity would respond up to 0.5eV, far from the 3eV I mentioned in my last messages.

 

I hate to add to your workload but we have a very nice setup

here for calibrating monos. Our engineers installed a retractable

Pt foil that can be inserted after the mono. Just after this there is

a diode. You can then simply scan the Pt edge and look for the

inflection. Very simple and very practical for diagnosing mono

problems and much simpler than aligning the beamline just

for Se scan. For more info see

J. Synchrotron Rad. (2006). 13, 227-238.

 

I remember this article, at that time this system took my attention, it would be really useful. However we were developing a quadrant BPM (J. Synchrotron Rad. (2000). 7, 61-68) to keep track of the beam movements at the experimental hutch and today I’m able to use one of its foils (Ge) to calibrate energy.

 

Since last year I have used a typical XAFS setup where I keep a Se foil as the absorbing medium. By keeping the mono around the inflection I’m able to keep track of up to 5eV changes.

 

Maybe tomorrow I’ll have access to some temperature measurements from inside the mono. In 2 weeks we have another experiment planned which consists in run the beamline with just a small fraction of the beam. With that we expect dramatically minimize thermal effects associated to the beam and better understand the contributions coming form the storage ring.

 

Well, thank you one more time for the tips and I hope to keep you all informed of the advances and drawbacks.

 

Ow, and of course, please, thank Travor in my name.

 

Lucas.

 

 

 

Best of luck,

 

Andrew

 

 

 

----- Original Message -----

From: [log in to unmask]">Andrew Mccarthy

To: [log in to unmask]">[log in to unmask]

Sent: Saturday, July 26, 2008 11:49 AM

Subject: Re: [ccp4bb] Beamline Stability Issues

 

 

Hi Lucas,

 

Ok, the evidence does suggest a real energy drift related to the

heatload on the second crystal.

 

I forwarded your email to one of our mechanical engineers to see

if he has some ideas.

 

We never saw this problem with our Khozu double crystal mono.

We only saw intensity drifts due to the thermal heatload on the

second crystal. We did three things to reduce these.

1. Installed a compton scattering sheild on the second crystal.

2. Tandem cryo cooled the first and second crystal.

3. Mounted a piezo feedback system on the second crystal.

(see attached figure of the ID14-4 Khozu monochromator)

 

Best of luck,

 

Andrew

 

 

----- Original Message -----

From: [log in to unmask]">Lucas Sanfelici

To: [log in to unmask]">'Andrew Mccarthy'

Cc: [log in to unmask]">[log in to unmask]

Sent: Friday, July 25, 2008 8:21 PM

Subject: RES: [ccp4bb] Beamline Stability Issues

 

Hey Andrew! Thanks for answering!

 

Does someone have experience in minimize energy instabilities in
beamlines?

Are you sure it's an energy instability and not an intensity one?

It's quite rare to see energy instabilities from a well calibrated

monochromator. All monochromators should be in a closed

loop, if there really are energy instabilities, then it's either 

mechanical (the encoder or crystal is loose) or the closed loop

parameters are wrong for the rotational motor and the mono

is slipping out of the closed loop window.

 

Yes, I’m sure. I’m certain most of this problem arises from thermal issues in the monochromator. For example, we control the temperature of our 1st crystal, but for the 2nd one there is kind of control, the tunning between them is not closed-looped through a MOSTAB or something, there is no kind of shielding for the Huber. Thus, I have several reasons to be concerned!


MX2, our new beamline devoted to MX experiments, are facing problems
with energy drifts. As far as we could notice, theses drifts are results
of the contribution from several sources - possibly electron beam
movements, heating of optical elements, etc...

Could well be, but these will result in intensity instabilities as opposed to

energy ones

 

Yes, this is evident in my data.


LNLS is a 2nd generation machine with 4 straight sections available for
insertion devices. MX2 is a 2T wiggler-based beamline and produces a
peak flux of 10^11 photons/s.


What I'd like to know, before start performing calculations, how far
should I expect the heating of a non-cooled 2nd crystal affects energy?
Does someone know cases of a few eVs drifts?

The second crystal will have no effect on energy, it will have a major

impact on intensity. We had extreme difficulties in stabilising and sheilding

our Khozu second crystal from thermal drifts. We eventually gave up and

installed a channel cut mono. The much simplier design is far more easy

to work with....

 

Luck you! I’m in the start of the journey! I guess I’ll have to try fix that first!  :( 

Andrew, what kind of approach did you try to stabilize 2nd crystal?

 

If you know someone else who could contribute in someway to this problem, please tell me.

 

Thank you one more time,

 

LS