Competition between fundamental and second-harmonic operations in a submillimeter wave gyrotron

Competition between electron cyclotron fundamental (f=fc) and second-harmonic (f=2fc) operation in a submillimeter wave gyrotron is described. Even when the magnetic field intensity is adjusted to the optimum value for second-harmonic operation, we can get a pure mode only for small beam currents. As the beam current is increased, excitation of the fundamental appears and eventually suppresses the second harmonic. The observed competition between the fundamental and second harmonic is compared with a computer simulation.