Excitation of Hybrid Electrostatic-Electromagnetic Modes in a Plasma-Loaded Sheath Helix

A plasma-loaded sheath helix supports lower hybrid modes that extend to the outside vacuum region. The phase velocity of the mode is significantly retarded by the presence of the sheath helix, making it liable to excitation by a low-energy electron beam. The extent of the mode outside the plasma is also reduced by the helix. An analysis of lower hybrid mode excitation by a beam propagating in the vacuum region reveals that nonlocal effects significantly reduce the growth rate. If the instability is taken to saturate via particle trapping, it acquires a large amplitude, giving oscillatory velocity large enough to lead to nonlinear effects.