Stability of Crossed-Field Amplifiers

TL;DR

This paper investigates the stability and mode transitions of Recirculating Planar Crossed-Field Amplifiers (RPCFAs), revealing how operating conditions influence amplification, driven oscillation, and self-excited oscillation modes, with implications for high-power microwave devices.

Contribution

The study provides a detailed analysis of RPCFA stability, mode transitions, and the effects of geometric and operational parameters on oscillation behavior, including a cold tube analysis and phase characterization methods.

Findings

RPCFA is stable with moderate gain under certain conditions.

Reducing AK gap or increasing current induces transition to oscillation.

Self-excited oscillations can involve slower electrons interacting with RF modes.

Abstract

This research examines the stability of crossed-field amplifiers (CFAs) and characterizes their different modes of operation: amplification, driven oscillation, and self-excited oscillation. The CFA used in this paper is the Recirculating Planar Crossed-Field Amplifier (RPCFA), which is a high power (MW) pulsed (300 ns) amplifier that operates around 3 GHz. Initially, the RPCFA is shown to be a stable amplifier with moderate gain (5.1 dB), but by either reducing the anode-cathode (AK) gap spacing or increasing the driving current, the amplifier operation transitions from amplification to oscillation. Depending on the operating conditions, these oscillations are either driven by the input RF signal or self-excited. These self-excited oscillations can have a lower synchronization phase velocity than the maximum velocity in the electron beam, implying that slower electrons within the Brillouin hub can interact with electromagnetic modes on the RF circuit. A cold tube analysis of the RPCFA shows that the Q-factor of certain modes on the RF circuit varies significantly when the AK gap geometry of the RPCFA is altered which leads to a discrete shift in operating frequency. The operation of the RPCFA close to Hull cutoff is found to share some key features of magnetically insulated transmission line oscillators (MILO) that could also explain the dramatic frequency shift. Instantaneous phase analysis by Hilbert transforms can be used, in conjunction with the frequency and output power analysis, to determine the onset of the transition from amplification to oscillation, and to characterize the oscillation.