Four-quadrant photodiode: measure of pointing and intensity stability

A four-quadrant photodiode is set at the entrance flange of the DiProI vessel. Under moderate FEL intensity conditions (average pulse intensity 10 µJ), it can provide an accurate shot by shot measurement of the FEL intensity and pointing instability. This will ensure a smooth treatment of data in post processing analysis, keeping into account the beam-transport losses.  
As an example, the panel on the right shows a laser-induced fast demagnetization curve on Co/Pd multilayer film, presenting an improved signal to noise ratio after normalization with the four-quadrant photodiode signal instead of the I0 monitor gas detector placed 60 m upstream the photon beam transport.
Magnetic scattering normalization example

Four-quadrant photodiode and signals

Last Updated on Friday, 18 October 2019 15:16