Plasma effects in a travelling wave tube

The effect of a plasma on a travelling wave tube comprising a sheath helix is studied. The plasma, taken to be (i) strongly magnetized or (ii) unmagnetized, allows a larger beam current to pass owing to charge and current neutralization. The beam excites an azimuthally symmetric slow wave via Cerenkov resonance. In the case of a strongly magnetized plasma when ωp>ω (where ωp is the plasma frequency), the amplitude of the axial electric field peaks on the axis and the phase velocity is suppressed, whereas for ωp<ω the field amplitude peaks at the helix surface and the phase velocity is enhanced. As a consequence, when a beam is placed close to the helix, the growth rate increases with ωp as long as ωp <ω and decreases with ωp when ωp>ω. In the case of an unmagnetized plasma, the mode is more strongly localized near the helix and the growth rate increases with ωp.