Simulation of an iris-guided inverse free-electron laser micro-bunching experiment

This paper presents a detailed computational examination of various physical effects that enter into an innovative approach to inverse free-electron laser (IFEL) acceleration and microbunching experiments, involving use of irises to guide the high power laser beam. In IFELs, there is a great advantage to using long wavelength, and thus diffractive lasers, which are also quite high power. As this scenario presents challenges to the final focusing optics, one must consider guiding, which for present schemes is either too lossy (in metallic guides), or incapable of supporting high fields (as in dielectric guides). Hence we are driven to examine an alternative scheme, that of using the effects of diffraction off of periodically placed metallic irises which have an inner diameter in a relatively low field region. We present below a computational analysis of the wave dynamics associated with the laser beam in this scheme. We then proceed to integrate this type of circularly polarized electromagnetic radiation field into a self- consistent simulation of beam dynamics inside of a helical undulator under construction at the UCLA Neptune Laboratory inverse free-electron laser. With this integrated tool, we then study the degree of microbunching bunching at the laser optical wavelength induced in a relativistic electron beam. Finally, we study the propagation of the beam after the IFEL interaction, including beam self-force (single component plasma) effects, to predict the level of microbunching at the fundamental (laser) frequency and its harmonics that are observed at a detector using coherent transition radiation.