Beam drift and diffusion on linear beam HPM

Summary form only given, as follows. Observations show that the ubiquitous pulse shortening in HPM devices arises from formation of plasma, electron streaming, high-E-field breakdown and beam disruption. Linear beam devices exhibit all these mechanisms; in particular, beam disruption by E/spl times/B drifts in the strong microwave fields and diffusion in turbulent electric field appears common. We present models for both drift and diffusion. We argue that since drift velocity is proportional to microwave power, microwave energy is fixed for a given device. Since source geometry is usually determined by resonance conditions, energy is fixed for each class of source. Thus drift due to microwave power leads to microwave destruction and high power microwaves are essentially self-limited in energy. We present theory that predicts that beam diffusion due to strong Langmuir turbulence in the background plasma may be the cause of microwave cutoff. Even 10 kV/cm microwave electric fields may cause high rates of beam spreading. We compare theory prediction with experimental measurements of both drift and diffusion.