End fields in the harmonic generation superconducting FEL at BNL-NSLS

A study is presented of the end fields in the harmonic generation FEL radiator. This wiggler magnet consists of 6 contiguous 25 cm long steel yokes wound with superconductor, each with 18 mm period, 0.54 T field and 8.6 mm gap. Measurement results and analyses are given for third and sixth order binomial entrance excitations. It is shown that, when the steel cores are driven into saturation, the binomial winding is no longer steering-free, and that higher order entrance windings produce less electron beam steering than do low order windings.