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CiPo - Beamline

Switching Mode

Because of the specificity of the CiPo Beamline to measure the dichroic behavior of materials, it was very important to provide a mode of operation that allows to minimize uncertainty introduced in the dichroic spectra by the random errors of monochromator repositioning. This is especially required at the edges of absorption of materials, where rapid change in the signal intensity may introduce false dichroic signal when scanning sequential absorption spectra alternating the right- and left–hand status of polarization of the photon beam. Handness switching of circular polarization is often important in the case of  XMCD measurements in magnetic materials where the reversing of the magnetic field in not always reproducible, but it is crucial in the case of PECD in which the effect is very small.
The EEW could produce alternatively a right- and left-handed circularly polarized radiation at a frequency of 0.1 or 0.05 Hz operating in wiggler or ondulatror mode. A square waveform is generated to drive the horizontal current power supply allowing to switch between +260 A and -260 A, producing the change of the polarization handness. After a switching time, a definite polarization state remain in the plateau of the square wave, during this steady state period the measurement occur before changing to the new handness and starting a new set of measurements. The sequence may be repeated for several times until the desired signal to noise ratio is reached. The timing of the measured signals to the polarization state is controlled by a suitable reference status voltage level from the EEW power supply. The measured signal from the sample is normalized to the incident photon flux detected through the photocurrent emitted by the last mirror gold surface or by a photodiode.
 

Last Updated on Wednesday, 13 February 2013 16:18