Characterization and monitoring of transverse beam tails

Low emittance electron beams accelerated to high energy in a linac experience transverse effects (wakefield, filamentation, optics,...) which produce non-Gaussian projected transverse beam distributions. Characterizations of the beam shapes are difficult because the shapes are often asymmetric and change with betatron phase. In this note several methods to describe beam distributions are discussed including an accelerator physics model of these tails. The uses of these characterizations in monitoring the beam emittances in the SLC are described here. First, two dimensional distributions from profile monitor screens are reviewed showing correlated tails. Second, a fitting technique for non-Gaussian one dimensional distributions is used to extract the core from the tail areas. Finally, a model for tail propagation in the linac is given