Microwave radiation from a tunable circular free-electron laser

Abstract

Measurements of coherent microwave emission from a rotating relativistic electron ring (200 kV, 100 A, 200 nsec) interacting with an azimuthally periodic wiggler magnetic field are reported. Tunable radiation is observed between 12.5 and 18 GHz at power levels of ∼5 kW. The predicted interactions occur near the crossing points of the TMlm electromagnetic waveguide modes, ω2=k2∥c2+ω2c(l,m), and the beam modes, ω=(l+N)Ω∥+k∥v∥, where ωc is the waveguide cutoff frequency, Ω∥ is the electron cyclotron frequency, k∥ is the axial wavenumber, v∥ is the axial drift velocity, and N is the number of wiggler periods. These observations are consistent with single-mode operation in the TM01 coaxial mode.