Floating wire measurement of transverse magnetic field errors in a planar free-electron laser wiggler

The first use of a floating wire technique for measurement of erroneous magnetic fields perpendicular to both the electron beam and the alternating field of a planar free-electron laser wiggler is reported. The magnitude of the measured error fields compares favorably with independent measurements made using a Hall effect probe. The accuracy obtained appears to be sufficient to allow the wiggler to be tuned to eliminate the error fields, and thereby to suppress deleterious large amplitude betatron orbits of the e beam.