Tuning the beam for the general user or when switching from SAXS to PX.

The general idea is to maximize the flux that is getting to the sample. Briefly the basic steps involved are:

  1. Switch the experimental table in the hutch from SAXS to PX.
  2. Center the crosshair in the BLU-ICE video tab with the direct beam.
  3. Align the phi axis with the newly determined beam center.
  4. Tune Theta2, M2 tilt, and Chi2 to maximize the flux onto the sample.
  5. Check the beamstop.
  6. Collect great data.




1. Switch the experimental table in the hutch from SAXS to PX.

Enter the hutch and crank the table from SAXS position to PX position. You will have to remove the pin located under the table and use the hand crank that is sitting next to the table. It is not so important to be precise with the table positioning. The most important thing is to make sure the table is cranked all the way towards you, and that you can insert the pin into the bracket under the table.

Place the YAG alignment jig on the goniometer stalk and exit the hutch. You will use this in a few steps to align the beam and the phi axis.

The positions of many of the motors are different between PX and SAXS so it is important to restore the motors to their last PX state. James Holton and Ken Frankel have a script for doing this.

Open a terminal window on graphics3 and change directories into /data/calibrations. Run the following command.

./restore_PX_Si.com

Switch the video channel to PX BPM (Beam Position Monitor). This is a video image of the x-rays on the back of the experimental shutter. You should see the beam. If there is no light on the back of the shutter then you may need to manually adjust theta2 (or theta2 PZT), M2 Tilt, and Chi2 to get enough beam on the shutter so that you can proceed to the next steps. If you DO have beam then skip ahead to step 2.

Make sure that the PSS shutter is open.

Switch to the beamline computer via the KVM switch just above teh PSS chassis.

Check the current at I mono In. This should be reading about 5-6000. If you are not getting light into the monochromater then call for help.

The next diagnostic is I Mono Out. This is often the culprit. The theta2 (and theta2 PZT) motor control the tilt of the second crystal in the mono. If it is not parallel with the first crystal then no light will exit the monochromater. The I Mono Out ion gauge should read 0.2- 0.6. If it is lower than 0.2 you should manually jog theta2 PZT by very small steps (how small?). Keep one eye on the video monitor and the other on the I Mono Out signal. If you are going the right direction you should see the I Mono Out signal start to increase. Eventually you will see the x-ray beam sweep from top to bottom or vice-versa across the video screen. jog theta2 PZT in very small increments until the beam is roughly in the middle of the YAG on the back of the shutter.





2. Center the crosshair in the BLU-ICE video tab with the direct beam.

Make sure you are on graphics3 and go to the BLU-ICE control window. If it is not open check the other three desktops. If no BLU-ICE is running then go to the first desktop and type go in a terminal window. This will launch BLU-ICE.

In the Low mag camera align the fiber on the end of the alignment jig. The rotate phi until you are looking at the jig from the side. If the ±10° buttons are too large to get the YAG edge on then type: phi.com by 1 to jog the phi motor by 1° at a time.

With the YAG edge on rotate 90° and move the YAG so that the crosshairs/bounding box are roughly within the flat portion of the YAG crystal.

manually open the experimental shutter by pressing the "shutter" button located at the lower right corner of teh BLU-ICE control window. You should see a glowing circle on the YAG where the direct beam is hitting. Because one side of teh YAG crystal is painted black if you don't see the fluorescent circle then try rotating the alignment jig 180°.

move the stage to High magnification. The direct beam will very likely be out of focus. Type focus.com to automagically focus the direct beam on the YAG. This usually works, but if you do not have a nicely focussed round beam you can do this manually quite easily by typing sample.com ±0.05 in a terminal window. This essentially moves the alignment jig toward or away from the camera and you can achieve a nicely focused image this way.

Go to the Hutch Tab in the BLU-ICE control window and insert the Aluminum or Copper filter to attenuate the beam. This will allow the find_beam.com script to more likely work. Now type find_beam.com in a terminal. This should move the crosshairs to the center of the beam. At this point you also want to make sure to define the bounding box for the direct beam on the video screen using the four dials on the Colorado Video box located under the monitor.

Now close the experimental shutter.





3. Align the phi axis with the newly determined beam center.

Return the stage to the Middle magnification position, and center the glass fiber located on the end of the alignment jig. Now return to High mag again and attempt to align the tip of the fiber as best you can. If it is not possible to center the phi axis (and it most likely won't be as you have just moved the white crosshairs to coincide with your new beam center) then type find_axis.com which should suggest a new position for the stage motor. cut and paste the suggested values into your termianl window and hit return. Attempt to align the fiber tip again. Wash, rinse, and repeat unti you are happy that the phi axis is properly centerd on the crosshairs.





4. Tune Theta2, M2 tilt, and Chi2 to maximize the flux onto the sample.

This is the easiest step. type tuneup.com and wait a few minutes for teh script to automagically tune Theta2 (or Theta2 PZT), M2 Tilt, and Chi2. The script will display several plots of the various motor positions and diagnostic signals. They should all be roughly parabolic such as this:

[pic]

In the end the tuneup.com script will finish and will report a value of how well the tuneup worked. Hopefully it reports between 80-110% of max. Depending on the divergence settings you have selected things can be a little higher or lower.





5. Check the beamstop.

The last thing to do before collecting data is to make sure the beam stop is still properly aligned. Go to teh Collect Tab in the BLU-ICE control window and take a 1 sec. shot. Zoom in on the direct beam and make sure all is hunky dory.

[pic]

If not either call for help or follow the beamstop alignment protocal [link] .





6. Collect great data.

If you need help with this step then just ask one of your friendly beamline staff members.