High power microwave tubes for scientific instrumentation

Particle accelerators, thermonuclear fusion machines, such as Tokamaks, and many free electron lasers are currently using high CW or pulsed power microwave tubes. We will briefly describe the characteristics of these tubes, but will point out the technical reasons why much better performances cannot be obtained. These reasons have several origins: heat transfer and cooling (cavities, windows, collectors), cathodes (current loading), breakdown or arcing (DC and RF). However, for the future machines now under consideration, output power requiremenst are at least one order of magnitude above: 1 GW peak for 50 nanoseconds in X-band, 1 or several MW CW in C-band... Several technical innovations have been recently introduced, to reach these goals. But also some fundamental principles have to be slightly modified or even changed.