Excitation of backward waves in beam tunnels of high-power gyrotrons

Summary form only given. The gyrotron is an important source of high power coherent radiation in 100GHz-200GHz range, which is based on the interaction of a spiraling electron beam and a mode of a resonant cavity. As applications call for higher power, higher beam currents are required. Under some circumstances the higher current can lead to an unwanted interaction of the beam with a backward wave in the beam tunnel before the cavity. This interaction causes energy spread in the beam and degrades gyrotron operation. Therefore, it is desirable to learn how the excitation of backward waves in the beam tunnel can be suppressed. The starting conditions for excitation of backward waves in the beam tunnel between the electron gun and the cavity of a high-power gyrotron are studied here. In the beam tunnel, the guiding magnetic field and the tunnel wall radius vary along the axis, so the theory is essentially the small-signal theory of a gyro-backward-wave oscillator (gyro-BWO) with tapered parameters. The operation in both regimes, far from cutoff and close to cutoff, is analyzed under the assumption that there is a lossy material on the wall. We have made numerical analysis of the condition of excitation of backward waves in the beam tunnel of a 170GHz gyrotron. Our results show the level of wall losses needed to suppress backward wave oscillations as a function of beam current and wall taper angle.