Design and magnetic measurements of bi-harmonic undulators for free electron laser

In this paper we describe the design and measurement studies of table top bi-harmonic undulators for higher harmonic lasing in FEL. The bi-harmonic undulator are fabricated by placing shims at suitable locations in the undulator gap of a six period planar undulator. An analytical formula is given for the shim counting and positioning along the length of the undulator and field integrals from the Hall probe data are presented for the third, fifth, seventh and ninth harmonic of the bi-harmonic undulator