BRIGHT ELECTRON BEAMS AND SMITH-PURCELL FREE-ELECTRON LASERS

We present further developments to the theory of Smith-Purcell free-electron lasers [1] (SPFELs) and characterization of a blunt needle cathode electron source in use to test the theory. The theory of SPFELs has been refined to include the effects of resistive losses on the evanescent surface wave supported by the grating and reflections of the wave from the ends of the grating. Losses are included directly in the grating dispersion relation and the reflections appear in the boundary conditions for the growing wave. Based on earlier work in sharp needle cathodes [2,3], an yttrium metal blunt needle cathode has been developed for the purpose of driving a SPFEL device. Space charge expansion of the beam in the transverse direction and aberration in the electron optics place limitations on the useful beam that can be generated. Both experimental and simple analytical characterizations of these limitations are presented and considered in light of the requirements of the SPFEL. SPFEL THEORY Smith-Purcell (SP) radiation is generated when an electron beam passes close to a grating. The virtual photons of the field of the electrons are scattered by the grating, and the wavelength SPλ observed at an angle θ is