The next generation of photon beam position monitors for undulator beamlines

The detection of the real photon beam trajectory as delivered to the users, by a third generation SR facility, is of paramount importance for the effectiveness of the most sensitive experimental measurements. Photon beam position monitors (PBPM) have been developed for this purpose. The most common is the so-called blade-type-monitor which is based on the photoemission principle. Unfortunately when it is used into an undulator beamline, it suffers from a background contamination due to the dipole radiation. So, a more complex device has been successfully developed at Elettra for counteracting this effect. The novel detector is based on spectroscopic techniques and collects signals essentially due to the undulator radiation only. In fact the residual error due to the dipoles is less than 0.1%. Moreover it is easily tuneable on any desired working energy. The commissioning measurements have been carried on at ID4.2 and ID7.2 beamlines with undulator fundamental energies varying from few eV to 1 keV. A complete report about the results is presented on the present paper while the future perspectives of this detector are also fully discussed.