Feedback-induced noise in crossed field devices

A guiding center fluid model involving a single RF mode of slowly varying amplitude and phase is used to study the long-time behavior of a distributed emission, reentrant crossed-field amplifier. The space charge recirculation through the drift space provides the main coupling mechanism between the output and the input. The output fluctuations among successive tube passes in the RF power, phase shift and hub density generate a narrow bandwidth around the carrier. By fixing the external control parameters and varying the secondary emission coefficient or the recirculating hub charge fraction, the pass-to-pass fluctuations exhibit transitions from steady-state to periodic limit cycles and finally to unpredictable, chaotic behavior. It is shown that the development of chaotic output patterns is connected with slow, large-amplitude hub density oscillations during a single pass. Analysis shows that the nonlinear and time-delay effects characterizing the hub density evolution cause unstable, oscillatory behavior. The influence of excess space charge accumulation on the tube intrinsic noise is discussed.<
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