ANALYSIS AND OPTIMAL-DESIGN STUDY OF TAPERED FEL AMPLIFIERS ON THE BORDER OF RAMAN AND COMPTON REGIMES

Detailed processes of FEL activity in a focusing planar wiggler with l inear tapering were studied with a 3D non-linear simulation code. The parameters in the simulations were chosen in accordance with the (non- tapered) experiment at JAERI. Many tapering patterns were examined for various frequencies of the input rf, wiggler field gradients, startin g points of the tapering, particle intensities, strength of subsidiary axial field for focusing, etc. An optimal design of the tapering styl e, resulting in maximum FEL power, was thus determined semi-empiricall y as a compromise regarding the available wiggler field. The longitudi nal space-charge force was included in the simulation to test the effe ct on the tapered FEL process, over a wide range of the parameter spac e. The physical processes involved in the radiation enhancement with t he tapering wigglers is also discussed.