Beamline Optimization

The optimization of the SER-CAT undulator (22ID) and bending magnet beamlines is performed by using the SERGUI hutch page (shown below).

Wavelength Section

The wavelength section provides feedback (in blue) on the current x-ray energy, wavelength, and the undulator Gap Energy (ID only). The energy and wavelength also have desired fields, which are used to change the beamline energy. The energy (wavelength) of the delivered x-rays can be changed by entering a value in the desired field and hitting a carriage return. A large number of motors may be required to move, so be patient and let the system move to the new energy (wavelength).

22ID

(1) The value of the gap energy for normal operation (below 13.0 keV) should be ~ 140 eV larger then the beamline energy. This offset is inherent in all APS undulators and reflects the errors in calibrating the position (gap) of mechanical jaws of the undulator to delivered x-ray spectrum.

(2) If the beamline energy is larger than 13.0 keV, the system will automatically move the undulator to use the third harmonic and the Gap Energy will be 1/3 of the beamline energy.

(3) If the energy is moved below 8.0 keV, the vertical focusing mirror will change lanes, moving from a palladium coating to a bare glass coating. This is done to improve throughput as well as reduce any harmonic component of the delivered x-ray beam. Conversely, if the energy change is from low to high energy and the new energy is above 8.0 keV, then the mirror coating will be changed back to palladium. The mirror lane change motion is done slowly, so this type of energy change will take longer then normal.

22BM

When the energy is moved more then 1,000 eV the sagittal focus motor needs to be reset. The setup focus gui can be opened from the main sergui setup pull down menu.

After the energy change has completed, push the reset button. The terminal window that you started sergui in will report back the status of the reset. This process can take a few minutes depending on what the original value of the focus.

IMPORTANT: The beamline should be optimized after any large energy change.

Optimize Section

The optimize section provides the user the capability of optimizing 3 beamline parameter:

(1) Monochromator Tune

(2) Vertical Beam Position

(3) Horizontal Beam Position

Monochromator Tune: The beamline energy is selected by use of a double crystal monochromator (cryogenically cooled Si (220) on 22ID and water cooled Si(111) on 22BM). The optimize tune button will perform a rocking curve of the second crystal; in which the Bragg angle of the second crystal is “rocked” about its current position to maximize the delivered flux. If the “rocking curve” does not measure the peak and 2 half heights, the system will automatically repeat the scan. This procedure does not expose the sample to x-rays as the detector is upstream of the experimental timing shutter. The system will automatically perform a fit to the measured data and move the monochromator tune to the optimal value.

Vertical and Horizontal Beam Position: Once the beamline energy has been changed, the delivered beam position at the sample position could have also moved. The vertical and horizontal beam optimizations will change a number of beamline conditions to perform these scans:

(1) The attenuation will be set to 30%;

(2) The sample z position will be moved 300 micrometers out of the beam to minimize any x-ray exposure to a mounted sample;

(3) The beam line slits will be set such that the scanning direction is small (50 micrometers) and the off axis is large (1 mm);

(4) Open the experimental timing shutter.

After the beamline has been properly set, the system will scan the slits to measure either the vertical or horizontal beam profile. The scan may be repeated if the peak and 2 half-heights are not recorded. The system will perform a fit to the measured data and move the slits to the center of the x-ray beam. After a successful scan, the system will return all of beamline components back to their pre scan positions.

NOTE: The sample position will track any change in beam position, as identified by the difference in the old and new positions for the vertical and horizontal slits. It is always a good idea to mount a phosphor and follow the procedure in OPTIMIZING SAMPLE ALIGNMNET to ensure that the delivered x-ray beam is properly aligned.

The feedback button has not been activated and is not functional.

Shutter Section

The shutter section allows the user to open or close the experimental timing shutter.

Attenuation Section

This section identifies if any filters are in the beam by identifying the current value of the transmission (which will turn red if not at 100%). The desired field allows a user to insert filters into the beam as a percent transmission. The filters are discrete foils, and as such, not all desired transmission values can be obtained. The system will insert the filter combination which will come closest to the desired transmission.

Slit Section

The current values of the vertical and horizontal slits are shown in here in blue. The desired field allows the user to change the slit size by entering a new value and hitting carriage return.

View Section

A live streaming video feed from a camera in the experimental hutch is shown in this section. The controls are active and the camera can be moved and have its zoom modified from outside of the hutch

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We welcome your questions and comments. Send e-mail to: SER-CATwebmaster@anl.gov