Simulation of prebunching in free-electron lasers

Prebunched beams are used in many coherent radiation sources. If the beam is bunched on scale lengths comparable or shorter than the desired wavelength, then the resonant wavelength is excited without a drive signal and grows faster than exponential. A free-electron laser with a prebunched beam combines the best characteristics of amplifiers and oscillators. In comparison with oscillators, no drive signal is needed and the wiggler is short. In comparison with amplifiers, no resonator is needed. Simulation of enhanced bunching has been demonstrated using the MEDUSA simulation code (IEEE J. Quantum Electron. 36 (2000) 275; Nucl. Instr. and Meth. A 475 (2001) 381; Nucl. Instr. and Meth. A 475 (2001) 118) for optical klystrons (Nucl. Instr. and Meth. A 475 (2001) 381) and high-gain harmonic generation (Nucl. Instr. and Meth. A 475 (2001) 118). In the latter case, modulation of the beam in one wiggler was enhanced in a magnetic chicane prior to injection into a second wiggler. We describe modifications to MEDUSA to simulate prebunching, and the application of the code to study requirements and limitations on the process.