Some considerations for reduction of noise and instability improvement in high-power crossed-field devices

The maximum potential of the high-power crossed-field devices has been restricted because of the presence of instability in these devices without any RF drive. The experimental investigation of the crossed-field instability as found from various tubes is briefly reviewed. It is believed that this instability is due to interaction of the slow cyclotron wave with the fast forward circuit wave. The theoretical work presented here refers to the interaction of both the slow and fast cyclotron waves with either of the circuit waves. The tuning characteristics of the interacting (oscillating) frequency due to slight changes in beam velocity resulting from changes in sole-to-anode voltage or magnetic field are derived. It is found that there is a good correlation between the theoretical and experimental results only for the case of interaction of the slow cyclotron wave with the fast forward circuit wave. For other interactions the tuning characteristics have opposite signs. In order to achieve a stable and less noisy operation of the device it is necessary to avoid the interaction of the cyclotron modes with the circuit wave. This limits the usable bandwidth which is plotted for different operating parameters. The electronic efficiency depending upon these operating parameters is also discussed. The problems in the gun design are reviewed. It is estimated that the conventional Kino gun may be used without any loss in bandwidth and efficiency up to frequencies in L band provided design procedures outlined in the paper are followed. For higher frequencies one has to use modified guns unless a reduction in bandwidth and efficiency can be tolerated.