Speaker
Description
We study undulator radiation observed away from the beam axis for terahertz (THz) free-electron laser (FEL) beams. Earlier work on a single short bunch relates the continuous off-axis spectrum to the longitudinal bunch profile , without a discrete angular ‘resonance’ tied to micro-bunch harmonics. For a pre-bunched pulse-train beam, the bunching factor develops narrow spectral lines at the fundamental frequency and its integer harmonics. Off-axis phase matching selects a specific resonant polar angle for each line, producing pronounced, spatially separated ring-like intensity patterns in the coherent far field. We utilize the non-averaged 3D FEL code PUFFIN to simulate single-bunch radiation—reproducing non-integer harmonic patterns consistent with analytical Lié-nard-Wiechert (LW) results—and to demonstrate the expected off-axis resonance for pulse-train beams.
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