Destabilization of backward waves by space charge in gyrotron beams

Summary form only given. The beam tunnels of high power gyrotrons can become unstable to the excitation of gyro-backward waves, degrading beam quality and lowering the efficiency of the desired, operating mode. Efforts are usually made to suppress this instability by disrupting or damping the backward propagating electromagnetic mode. We study here the effect on start oscillation currents of the coupling of the backward wave to the forward propagating electrostatic cyclotron maser instability. Normally, in the analysis of the gyro-bwo an absolutely unstable mode is formed due to coupling of the backward propagating structure mode and the forward propagating beam perturbation. When space charge is included the forward propagating signal also acquires gain due to the negative mass effect, and this results in a lowering of the start current threshold for the gyro-bwo instability. The start current is minimal when the cyclotron resonance condition is maintained over the beam tunnel. This may take place when an axial wavenumber of unstable perturbations on the beam corresponds to the tapering of the magnetic field. This implies the modes will be sensitive to the magnetic field tapering and to the spread in electron axial velocity. We study the effects of space charge and velocity spread on a representative beam tunnel, assumed to have a smooth but tapered wall. We find that in the absence of space charge to start currents are in the range of 100-200A. However, with space charge included they drop to 50-70A. Start currents increase when velocity spread is included, roughly doubling when an 8% transverse velocity spread is included. Different options for suppression of this instability will be discussed.