Microwave Power for Thermonuclear Fusion Machines

Present experimental machines used in research on controlled thermonuclear fusion require additional power of a few or many megawatts for a few seconds. Although experiments with energetic neutral beams are progressing satisfactorily, this technique may not be feasible in large machines. RF waves can also be used, in several frequency bands, and plasma physicists are working to solve the problems of coupling the power to the plasma and dissipating it there. The general problems of generators are briefly outlined, and those of the RF-window and the collectors of the electron tubes are discussed in more detail. The present possibilities of amplitrons and klystrons are indicated. These tubes are capable of producing the frequencies corresponding to the so-called lower hybrid resonance (LHR) in the plasma. Also discussed are the ways of increasing the efficiency of klystrons above 65%. The gyrotron, a millimetric-wave tube that can excite the electron cyclotron resonance (ECR) is presented and discussed in a similar way. It is apparent that technical difficulties are associated with the plasma at lower frequencies and with the electron tubes at higher frequencies.