Simulation study of fast and slow wave cyclotron instabilities

Summary form only given, as follows. The fast-wave and slow-wave cyclotron instabilities have been studied in both an infinite plasma and waveguiding structures. In the later case, the fast-wave instability forms the basis for the gyro-TWT and CARM, while the slow-wave instability was exploited in a device called the slow wave cyclotron amplifier (SWCA). Most linear theories and simulation codes model the gyro-TWT, CARM, and SWCA by a perturbation approach, namely, the electron beam is treated as a perturbation to the waveguide mode whose transverse profile is prescribed to be that of an empty waveguide. This approach is rigorous only when the electron beam and the electromagnetic wave are in cyclotron resonance. When this condition is apparently violated, the perturbation theory still predicts a growth rate for the fast wave and even generates a new branch of slow-wave mode with broad band growth in a fast wave structure. In this study, we examine the validity of the cyclotron instabilities in these non-resonant regimes with the MAGIC code which evaluates the transverse as well as longitudinal field structure self-consistently in the presence of the electron beam. Cases under study include the CARM amplifier where the beam resonance line intersects the waveguide dispersion curve at two points. The wave behavior at frequencies between the two intersections will be examined and results compared with those obtained with a particle tracing code whose validity in this regime is in question. The other case concerns the slow-wave branch of modes predicted by the perturbation theory. Its existence and properties will be clarified with fully consistent particle simulations.