Initial numerical results of a frequency agile, high-power, X-band BWO

This paper presents results from an initial numerical study into the design of a high-power Backward-Wave Oscillator (BWO), with dynamic tunability over the range ~9 – 9.6GHz. The source was modelled using the Particle in Cell (PiC) code KARAT, with predicted output powers lying in the multiple 100’s MW range and pulse durations of ~100ns. The electronic efficiencies were 10-20%, dependant on the operating conditions; i.e. whether the applied potential was effectively DC or “swept” in amplitude (and so resonant frequency) over the duration of the pulse.
Such a source has direct application in plasma research, for example in plasma heating, where this is achieved via coupling of energy from an incident RF wave to a resonant instability (or instabilities) within the plasma volume; the exact resonant frequency of which may only be known to within a certain bandwidth, indeed it may vary within this bandwidth over time. The tunability of the proposed BWO (especially when operating as a swept source), along with the high-powers predicted across the operational bandwidth, therefore present the potential for better characterisation of a given instability (via sampling of the pulse after propagation into the plasma volume) and for more efficient heating of the plasma, as the spread of energy in the pulse across the Fourier domain, may be adjusted via profiling of the applied source potential.
Further to this, the dynamic tunability of the source allows for exploitation of frequency-swept, fully-passive, pulse-compression techniques. Here achievable compression is a function of the pulse bandwidth, the pulse duration, and the profile of the “sweep” in frequency throughout. Optimum performance is obtained with these are closely matched to the dispersion characteristics of the pulse-compressor. The University of Strathclyde has experimentally verified the operation of a passive pulse-compressor, operating in the range 9 – 9.6GHz, showing compression factors of ~24; i.e. presenting the potential for an uplift in the output power of the BWO into the multi-GW range.