Dear Lucas
It looks to me that the angle of the VCM and therefore of the reflected pink beam which has double the angular change may be your major contributor. This could be coupled and complicated by any vertical movement in the source. Things I would look at are the motors on the kinematic jacks and the stability of the VCM pitch system, whether or not you have linear encoders and if the motors are heating up with a holding current (if you have good gearboxes on the jack motors you can deactivate the motors). Secondly Temperature stability on VCM substrate may be a factor; I would be monitoring the water temperature closely and the performance of your water chillers. In the DCM you could be suffering a significant heating problem with all the motors in there if the motors have gearboxes or lead screws with significant mechanical advantage the I would kill all the motors except Bragg. The Bragg needs a position feedback loop directly on the Bragg axis not on the motor upstream of the gearbox as you would be subject to backlash. We suffered DCM instability from motor heat in our initial commissioning. We solved it by having the beamline control software automatically deactivate each in vacuum motor after each move. With a wiggler beam I would guess that Compton scatter will be a problem. I would be considering retro fitting Compton shielding for crystal 2 Again temperature stability in the DCM may be a factor; I would be monitoring the water temperature closely and the performance of your water chillers particularly if you are rejecting heat to the hall air. Another factor could be vibration from the coolant flow that changes with time, or temperature. Hall temperature, which you have no control over is definitely a suspect, do you see less drift now in the winter than in January (summer)? Good luck Best regards Julian Dr Julian Adams| Principal Scientist - Protein Crystallography | Australian Synchrotron t: +61 3 8540 4232 | f: +61 3 8540 4200 | m: +61 4 2382 0866 [EMAIL PROTECTED] | www.synchrotron.org.au Australian Synchrotron, 800 Blackburn Road, Clayton VIC 3168 -----Original Message----- From: CCP4 bulletin board [mailto:[EMAIL PROTECTED] On Behalf Of Lucas Sanfelici Sent: Tuesday, 29 July 2008 4:38 AM To: [email protected] Subject: [ccp4bb] Beamline Stability Issues Hey James! Man, you did really organize my thoughts! Weeks ago I did the calculations you mentioned but without paying attention to certain points. After I read your message the # of tests I intent to do almost double! With regards to mono heating I'm particularly worried about the 2nd crystal, which has no shielding, no cooling and no feedback! However, this will be probably a minor effect since the features of the energy and position drifts is much correlated to room temperature! In addition, I heard sometime from the machine guys the temperature homogeneity inside the tunnel is not good - several degrees instead of .xºC! By the end of last year we measured temperature along 10 points of the beamline and we did not noticed any appreciable temperature difference (see attached figure) upon the supports of a same optical element, at least not that could lead to something near of 40urad(3eV @ 12657eV). You definitely took my attention when you mentioned the motors influence. We have a temperature sensor only for the goniometer motor (Huber) and it says the motor is kept around 40ºC when steady. In order to minimize gradients inside the mono we increased the temperature of the 1st crystal cooling water to the same 40ºC, but it seems not to result in any change! Do you have to say about heating on the 2nd crystal? Is your 2nd crystal cooled too? What kind of control system did you implement? It seems the best way to track is really attempt to angles coupled by temperature gradients!... I enjoyed the laser pointer idea. In the case I don't used it right now, I'm sure it'll be useful in my life someday! :) Thanks one more time for the valuable help, Lucas. -----Mensagem original----- De: CCP4 bulletin board [mailto:[EMAIL PROTECTED] Em nome de James Holton Enviada em: sábado, 26 de julho de 2008 17:38 Para: [email protected] Assunto: Re: [ccp4bb] Beamline Stability Issues As I recall, your setup down there is similar to mine (ALS 8.3.1), so perhaps I can be of some assistance. There are only two things that can cause changes in energy: d and theta. This is because lambda=2*d*sin(theta). If the change is entirely due to thermal expansion of the mono crystal, then the fractional change in wavelength or energy (3 eV is 0.03% of 12657 eV) must equal the fractional change in cell edge. 0.03% may not sound like much, but the thermal coefficient of linear expansion for silicon is only 2.6e-6 / K, so this change represents a shift in crystal temperature of 121 C. I doubt your cooling water is getting that hot. This leaves theta (or rather 2*theta) as the culprit, and for this change in energy you are looking for a tilt of 0.005 degrees or 0.1 millirads. This is a change of 100 microns over 1 meter, which represents a temperature difference of 8 C if you have two steel support legs (expansion coefficient: 12e-6 /K) 1 meter tall and 1 meter apart. This is not too hard to imagine. If your supports are made of aluminum (TCE = 23.5e-6 /K), then a 3 C change in one leg of a square will make a 0.1 mrad tilt. Good news is, this change in tilt must be happening either inside (or underneath) the mono or somewhere upbeam from it. Focus your attention there. As Liz pointed out, temperature fluctuations can always be a headache, and it is important to remember that your x-ray beam is not the only source of heat you need to think about. Motors generate heat (such as the ones in your mono). I too saw a lot of drift in the second crystal of my Khozu monochromator when we first got it, but this turned out to be due to heating of the "Z2" motor. This motor only moves when you change energy and at first I thought it was just bad luck that we were seeing big drift during every MAD data set. Nowadays we only move this motor if the beam is in danger of walking off the edge of the crystal (~2 keV moves), and the mono is remarkably stable now. Another things you might not expect is computers. I recently narrowed down a large (1 mm) beam drift problem on ALS 8.3.1 to tomography reconstruction jobs at 8.3.2. They had bought a new computer and happened to accidentally point its exhaust fans at the downbeam support leg of my M2 mirror tank. The extra CPU heat from the tomography jobs was enough to heat the support by ~10 C, which moves the beam at the sample position (10 meters away) by 1 mm. Took me a while to figure that one out. Since your fluctuations are not correlated to ring current, I am willing to bet that heat from the x-ray beam is not your problem. More likely a different source of radiation (with lower photon energy, but higher total power) is to blame. Your energy drifts seem to start at sunrise and then turn around at sunset? Although it is tempting to just tell your facility that they need to buy a better air conditioner, a more cost-effective solution will probably be to figure out exactly what is moving and do something to fix it. It is still possible that you are doing something every morning that starts the drift (such as moving a lot of motors to optimize the beam). Measuring the temperature of the optics support legs is always useful, but you can also get a lot of mileage of of a laser pointer, a mirror, a piece of paper and a pen. Rig the laser pointer to a remote power source (you don't want to have to touch it to turn it on), attach it to something "stable" (like the shield wall) and bounce the laser off a mirror mounted on your mono tank, and back to a piece of paper near the laser pointer. Use the pen to mark where the beam falls at different points of the day. You can catch very small angles this way, and you might get lucky. Good luck, -James Holton MAD Scientist Lucas Sanfelici wrote: > Hello all! > > Does someone have experience in minimize energy instabilities in > beamlines? > > 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... > > 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? > > Thanks in advance and regards, > > Lucas Sanfelici > Physicist > > Brazilian Synchrotron Ligth Source- LNLS (www.lnls.br) > Diagnostics Group > PO Box 6192 Postal Code 13083-970 > Campinas-SP Brazil > Phone: +55-19-3512-1153/1152 Fax: +55-19-3512-1006 > E-mail: [EMAIL PROTECTED] >
