Experimental Investigations on Geometrical Resolution of Optical Transition Radiation (OTR)

Optical Transition Radiation (OTR) provides an attractive mothod for diagnostics on electron/positron beams of small dimensions at high energies (GeV). However, some limits on the geometrical resolution at very high energies have been often discussed in the literature and a minimum value given by 'γλ' has been invoked. In order to bring an experimental contribution to the problem, systematic measurements of electron beam profiles, in the energy range of 1-2 GeV and at optical wavelength between 400 and 700 nm, have been carried out at the Orsay 2 GeV Linear Accelerator. OTR emitted from an alluminium foil at a 30° incidence angle was collected by a two-lens telescope and recorded by an intensified CCD camera. The OTR beam profiles were compared to the profiles obtained by a SEM Grid having a resolution better than 0. mm.After a theoretical introduction presenting the different resolution limits, which can be invoked with diffraction phenomenon, and a presentation of our calibration procedure, the experimental results are presented and compared to these limits. They show that the resolution OTR measurements is definitely better than the already invoked gl -limit. For a small size electron beam an rms beam width as small as 170 mm, comapred to gl = 2.5mm (E=2 GeV and l = 650 nm), has been measured. Our theoretical analysis also provides a more precise evaluation of the resolution power of OTR, which depends on the sensitivity of the detector; this could be of interest for much higher energies.