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Slow-wave structures in microwave tubes

In the early period of microwave tubes, a large effort was devoted to the research for new types of interaction resulting in new types of tubes. This field of activity is nearly exhausted, the last successful entry being the gyrotron, which goes back to the sixties. The emphasis is now rather on the analysis and improvement of the various parts which make up a microwave tube: this is the case for cathodes, guns (gridded or ungridded), single-stage or multi-stage collectors, etc. Among these, slow-wave circuits which interact with the electron beams are particularly important. They basically determine the microwave performances of the tube. They are the object of the present paper whose scope will be limited to either new or not too well-known ideas and techniques.