Three dimensional theory of helix PASOTRON, a plasma-filled, backward wave oscillator

PASOTRONs (Plasma Assisted Slow wave Oscillator) are new high power Cherenkov sources, working without the heavy magnetic field solenoids. Instead of the strong magnetic field, electron transportation in these devices is provided by plasma ions, which compensate the self space charge forces of the beam. The absence of strong guiding magnetic field, gives the electrons the freedom to move transversely by the RF fields besides the usual axial motion in these type of devices. The transverse motion greatly alter the operation of the Cherenkov device. The field intensity of the synchronous space harmonic is concentrated around the SWS. So, transverse motion can be beneficial in electron wave interaction, as electrons experience this strong field as they move transversely towards the SWS. This stimulated interests in theoretical analysis of these devices.; For symmetrical slow wave structures (SWS), i.e. corrugated waveguide, employed in traveling wave tubes (TWT) and backward wave oscillator (BWO), filled with plasma, it is shown in this study that the operation of these devices can be enhanced by adding a small magnetic field. The small magnetic field helps avoid interception, while maintaining the preferable transverse motion.; A 3D “amplifier” model describing the steady state operation of the helix PASOTRON BWO is presented. The results showed that electrons injected inside the helix are those that contribute most to the device efficiency over those electrons injected outside the helix. It is also shown that by reducing the beam size, high efficiencies up to 55% can be achieved. Such high efficiency, which is unachievable in conventional BWOs driven by linear electron beams, can be explained by a favorable effect of the transverse motion of electrons.; Temporal study of the helix PASOTRON BWO is also presented. It is shown that for zero reflection device, there was no automodulation oscillations. This is attributed to the 3D electron motion together with the 90° phase difference between the radial and longitudinal field components of the synchronous harmonic. For the non-zero reflection case, the reflection phase greatly affects the device operation.