Spatial resolution limits of YAG:CE powder beam-profile monitors at the Fermilab A0 photoinjector

The A0 photoinjector (A0PI) facility at Fermilab has an ongoing proof-of-principle experiment to demonstrate the exchange of the transverse horizontal and longitudinal emittances. This experiment relies on measurements of the transverse emittances and longitudinal emittance upstream and downstream of the exchanger beamline. At several locations along the accelerator beamline, YAG:Ce powder scintillator screens are used to determine beam size, divergence when the screen is downstream of a set of 50-micron-wide tungsten slits, and energy spread when used in an electron beam spectrometer. We have recently performed direct comparisons of beam image and slit image sizes using both the optical transition radiation (OTR) screens and the YAG:Ce screens. For micropulse charges of 250 pC and with beam energies of 15 MeV, we systematically observed larger beam image sizes with the YAG:Ce screens than with the OTR screens. We deduced a YAG:Ce screen spatial resolution limit of sigma = 140 to 180 microns. This term is large enough relative to the actual beam sizes and slit image sizes to be needed as a correction term subtracted in quadrature.