Theory and design of cyclotron-wave traveling-wave amplifiers

The energy exchange mechanism and the axial beam velocity spread induced by RF interactions are examined. Analysis of the energy spread in a realistic model of the beam with nonzero size and nonzero space-charge density shows that higher maximum efficiencies at higher power is attainable (with collector depression) in cyclotron-wave amplifiers than in synchronous-wave amplifiers. A one-watt, and a ten-watt cyclotron-wave amplifier at 3 GHz, utilizing bifilar helix circuits, scaled from a 2 kW, 5 GHz design, were constructed. A computer was used to design a pitch taper for the circuit of the ten-watt tube to maintain synchronism between beam and circuit waves so as to enable the extraction of more energy from the beam. Test results indicate the possibility of attaining efficiencies of the order of 70% in a 2 kW, 5 GHz cyclotron-wave amplifier. Because of the relatively low interaction impedance available for transverse-wave interactions, it is estimated that gains of the order of 1 dB/inch are possible.