The characteristics of high power RF sources are subject to increasingly demanding requirements, and, precisely for this reason, much important progress is made in this field. These klystrons are used mainly in two fields : radars and scientific instrumentation. Particle accelerators create the needs for higher and higher peak or CW power levels, and also high efficiencies, in order to optimize the costs of installation and utilization. Plasma heating tubes benefit from progress in these areas, but an additional requirement of tolerance of high output SWR is imposed because of the plasma which is a very instable, fluctuating load.