IREB transport in a focusing wiggler and FEL experiments at JAERI

An experiment of Raman mm wave FEL was carried out using a planar focusing wiggler and an additional axial magnetic field. An IREB (0.9 MeV, 2 kA, 100 n s) was generated by a field emission gun drived with inductive acceleration units and flowed into the wiggler. The beam transmission up to 85% through the whole wiggler length(1.5 m) was realized at its best, though the axial magnetic field was kept very low. The particle orbits in the wiggler were to be in the "group I". There appeared a pattern showing effect of higher modes of the resonance between the electron cyclotron frequency in the axial field and the frequency associated with the wiggler field. The results were compared with the simulation made by a newly developed code. Strong superradiant emission was found several sharp peaks with a very narrow width, separated by 6-8 GHz in the frequency spectrum in the range between 30-65 GHz. The radiation power as well as the spectrum were correlated to the radial distribution of the particles in the cross section of the beam. At the amplifier mode of the FEL operation, rf pulse of 45 GHz was injected at the entrance of the wiggler, and amplified along the beam flowing in the wiggler from the power level of 100 W at the input, to finally 6 MW at the saturated state, at a spatial growth rate of 50-56 db/m. The amplification process observed was also consistent with the analyses by 3D simulations.