Deviations in the electron beam trajectory through the planar wiggler of the Israeli Electrostatic Accelerator FEL were found to be primarily caused by small variations in the strength and angle of polarisation of lateral focussing bar magnets which are positioned on both sides of the wiggler, and provide a quadrupole guiding field on axis.

The field of the wiggler on axis was measured using a Labview controlled automated system built in our lab, based on a 2-axis Hall Effect magnetic sensor driven by a stepper motor. Polarisation field components of the individual focussing magnets were measured separately. Then, using an algorithm, the focussing magnets were paired, such that their non-uniformities were utilised to not only cancel out each other's error, but also to cancel out the field errors on axis due to variation in strength and polarisation angle of the wiggler magnets.

The quality of the predicted electron beam transport was evaluated by 3-D simulation with the General Particle Tracer code which allowed the input of all the measured fields.

We suggest this method as a simple scheme for providing lateral focusing in linear wigglers in general, and for improving the transport parameters of imperfect wigglers.

Acknowledgement

We thank S.B. van der Geer and M.J. de Loos of Pulsar Physics for programming assistance with GPT.