Energy exchange instability in CFA noise generation

The dispersive nature of the slow wave circuit used in crossed-field amplifiers (CFA´s) allows a type of instability to occur in which a power flow perturbation traveling at the group velocity along the circuit can grow as it is passed by and fed by the space charge spokes, which travel at the slow wave phase velocity past the perturbation. Using a 2 1/2-D particle in cell simulation code to simulate an entire CFA, we have demonstrated this effect, which may be contributing to noise generation in these tubes. Both forward wave circuits and backward wave circuits are shown to produce the same effect; in the backward wave case, the growing perturbations are broader than for forward wave circuits because of the larger number of spokes passing by the perturbation during its trip from the input end to the output end of the tube.<>