Cyclotron emission from intense relativistic electron beams in uniform and rippled magnetic fields

Millimeter wave emission from intense, relativistic electron beams (V ≃ 1 MV. I ≃ 5 kA) propagating in uniform and rippled magnetic fields has been studied experimentally at frequencies ranging from 8 to 140 GHz, For beams propagating in a uniform guiding magnetic field, the measured spectra correspond to those predicted theoretically for the unstable cyclotron maser (gyrotron) modes. Superimposing a periodic transverse magnetic field (a wiggler), the power in these modes is enhanced by one to two orders in magnitude, due to the additional transverse electron velocity acquired by the electrons. In our system the wiggler-enhanced cyclotron maser instability dominates over other radiation mechanisms, such as the collective (Raman) free electron laser instability, which has not been detected.