Pre-bunched beam devices - Efficient sources of UHF and microwave power

Forming bunches of electrons corresponding to an input rf drive signal is basic to all microwave devices. How the bunch is formed has been the subject of decades of development. TWT's, klystrons, cross-field devices, and gyrotrons are examples of devices which form discrete packets of charge by allowing electrons to catch up with one another while drifting under the influence of dc and rf accelerating fields. Triodes and tetrodes are examples of devices where the beam is turned off and on at the cathode surface, thus forming discrete bunches which can then interact with rf output circuitry. Photo emissive devices can also form packets of electrons by modulating the impinging light. These last two methods are now being given a fresh look by members of the microwave community in the light of new emerging technologies.