Alignment of the vacuum chamber to the beam

These are the procedures to align the cryo STXM ``from scratch''. When the cryo STXM comes back online after a break, of after a shutdown where the top flange had been removed, it is highly recommended to follow these procedures.

The optical axis of the cryo STXM is defined by the center of the entrance port (2-3/4'' flange which connects to the beamline) and the VLM port (4-5/8'' flange with coated viewport on the downstream side of the vacuum chamber). The position of the scanning stage, sample, zone plate and OSA stage, anticontaminator and detector are all referenced to this axis. Therefore, the first step in aligning cryo STXM is to bring the axis of cryo STXM as defined by those two flanges on line with the beam axis.

1. Align the vacuum chamber in z.

   It is important that the sample stage is centered on the ports through which the sample and the anticontaminator are inserted. Alignment in z can be controlled in the following way:

   • Check whether the bellows connecting the scanning motors to the vacuum chamber are straight in the z direction. They may be bent in x direction (inboard/outboard) which does not matter for the z alignment.
   • Look through the little view port above the air lock onto the scannning stage and make sure that the corresponding holes in the stage are centered on the respective flanges, i.e. that the holder axis is not too far up- or downstream. This is especially important for tilting the sample for tomography!

   If the scanning stage is not centered on the sample / AC port, then move the chamber in z until it is aligned. Moving the chamber in z is accomplished by turning the corresponding screws on the kinematical mount on the upstream leg of the vacuum chamber. Lock the legs by tightening both z screws when the chamber is in the right position.

2. Align the upstream port of the vacuum chamber.

   Since the installation of the STXM IV upstream of cryo, the cryo entrance window has been moved from inside the cryo chamber to inside the STXM IV chamber (i.e. upstream of V11i). It should therefore probably be called ``STXM IV exit window''. Make sure that no beam can be seen on the phosphor around the exit window. Move the Korde photodiode that is mounted downstream of the cryo entrance port (but upstream of the cryo optics) into the beam. (This is the ``upstream Korde photodiode'' or uKPD.) Maximize the flux on that photodiode by moving the chamber in x and y. Compare it to the flux in the beamline as read on the Shutter/Photodiode. When the beam is in the right position, lock the screws by tightening them with respect to each other. Confirm that the beam position is still correct after tightening the screws.

3. Align the downstream port of the vacuum chamber. This will results in a small change on the upstream side as well and requires successive readjustment of the upstream port.
   • Close valve V9i, V10i and V11i.
   • Vent the vacuum chamber.
   • Look through the VLM viewport towards the entrance window and make sure that there is nothing in the way which could obstruct the beam (like zone plate, OSA, sample, wires, or forgotten screwdrivers).
   • Open the viewport door on the downstream inboard side of the chamber.
   • Place a phosphor covered glass slide inside the chamber just in front of the VLM port. You might want to use a piece of (clean) aluminum foil to lightly press it against the viewport.
   • Place a glass slide on the outside of the viewport. Mark the center position of the flange on this glass. DO NOT WRITE ON THE VIEWPORT! (It has an expensive antireflective coating for use of ther VLM in the cryo STXM.)
   • Close the vacuum chamber and pump it down.
   • When the pressure is good, open the valves to the beamline. Check whether there is beam through the STXM IV exit window on the uKPD.
   • Place the phosphor on the detector stage into the beam axis. You might have to move the detector stage vertically. Make sure you get beam on the phosphor.
   • Move the phosphor out of the way and get the beam on the phosphor that you put into the VLM port.
   • Move the chamber in y by turning the leg, and in x by turning the screws on the kinematical mount. If the center of the port is far away from the beam position, the entrance window might be moved so much that you lose beam. Therefore, when the spot on the phosphor gets dimmer, realign the entrance window.
     When the beam is in the center of the VLM port, lock the screws.
   • Go back and realign the upstream window if necessary.
   • Vent the chamber, take out the phosphor slide, close the chamber again and pump down.