Otr Polarization Effects in Beam-profile Monitors at the Fermilab A0 Photoinjector*

Optical transition radiation (OTR) imaging for transverse beam-size characterization is a well-established technique at many accelerators including the Fermilab A0 photoinjector (A0PI) facility. With low beam energies of 14-15 MeV and emittances of 3 mm mrad, one observes beam sizes of 0.8 to 1.5 mm (sigma). However, the use of 50-μm wide slits to sample the beam’s transverse phase spaces significantly alters the required resolution of the converter screen and imaging system. Slit-image sizes of about sigma =100 μm or less are observed, depending on drift distance and beam divergence. Moreover, the OTR polarized component perpendicular to the narrow beam dimension systematically gave us ~20-μm smaller projected image sizes than those with the total OTR intensity. This is one of the first reports of this polarization effect at such a low-gamma regime (~30).