Coherent radiation from spatiotemporally modulated gyrating electron beams

An electron beam from a cyclotron autoresonant accelerator passing adiabatically along a down-tapered axisymmetric static magnetic field can propagate as an axis-encircling helix. This helix rotates temporally at the fundamental pump frequency of the accelerator, and varies spatially with an axial pitch that depends upon the difference between the pump frequency and the local gyrofrequency. We show that the pitch can thus be freely adjusted so as to allow both frequency and wave-number matching between the beam and a guided fast wave at each harmonic of the pump frequency. As a result, power transfer from beam to wave can occur in a process that is first order in the wave amplitude. Examples of a harmonic converter based on this concept are given for 94- and 1000-GHz sources employing magnetic fields no stronger than 10 kG. Since the electrons can maintain phase coherence during the interaction, one can speculate that the power transfer can be fairly efficient. Practical advantages of this harmonic converter for high-power applications may include alleviation of mode competition, direct output coupling, lower wall heat loading, and simpler beam collectors than for corresponding cavity gyrotron oscillators and noncryogenic magnets.