Half wave injector design for WiFEL

Seeded FELs will require exceptional beam quality. The Wisconsin FEL (WiFEL) requires average current of 1 mA, peak currents of greater than 1 kA with less than 1 mm-mrad transverse slice emittance and 10^-4 δp/p at the undulator. To perform bunch compression after the injector without allowing micro-bunching requires very smooth bunch energy and density profiles with relatively low compression ratios. An injector which uses a low frequency, superconducting half wave resonator gun combined with self-inflating ellipsoidal bunches[1] to meet those requirements is described. The superconducting radio frequency TEM-class cavities have been in use for more than 25 years and because of their potential for flat field profiles, are desirable as electron gun structures. A Superfish[2] model and field map of the specific gun cavity is presented. ASTRA[3] simulations from the cathode to 120 MeV are provided, along with a description of the technique used to emittance compensate the space charge induced energy chirp while maintaining the peak bunch current.