Beam drift: what to do
Sometimes the parallelism between the two crystals of the monochromator can be partially lost. This causes the beam to move vertically and the intensity of the beam to decrease.
To recover the lost intensity, you can do one of the following procedures:
- The simplest way to optimise the silicon pitch value it with the specific script.
Open a terminal (right click on the screen and then "open terminal")
then type the following command
si-pitch
The outcome will be automatically logged to the logbook. The script occasionally fails due to the very slow responses from the diode mover. Simply re-run the script in such cases.
2. In alternative, you may find the optimal value manually as follows:
From the Beamline Browser (sunflowers window), start the Beamline controls and, from there, click on the light blue button "IN" in the Diode2 section
This will put a diode in the beam path, therefore the reading of the BMS will go to zero. This diode will be used to monitor the beam intensity shortly after the monochromator.
From the BL browser, open Diode (Current Trends):
In the TrendPlot window that will open, double-click on Channel 1 to make this line disappear (you want to follow Channel 2 only!). Then, right-click on the plot and set a forced reading period to 300 ms:
From the BL browser, under the Optics section, double-click on Monochromator Si111: this will open the following panel:
In the panel, make sure that Pitch is selected at the bottom.
Make sure that the step size is 0.0002, then click on the right arrows while following the reading of the Channel 2 in the TrendPlot. If the signal increases, keep going —slowly, step by step— until it starts to go down. Then move a step back (left arrows).
If the signal decreases, use the left arrows and keep going—slowly, step by step—until it starts to go down. Then move a step back (right arrows) and you're done.
Remember to remove the Diode2 from the beam path. To do so, use the OUT button in the Beamline controls window (see point 1).
