Plasma sheath and presheath waves: Theory and experiment

Abstract

A theoretical and experimental study of characteristic waves propagating along a wire immersed in a weakly ionized, isotropic plasma is described. The theoretical model uses the hydrodynamic equations with a sheath profile composed of uniform plasma density steps. Boundary conditions at each step boundary and at the wire surface complete the model. The sheath wave characteristic mode is studied, and also a new family of characteristic waves named “presheath waves” is discovered and investigated. A novel experimental setup is described, one that facilitates the observation of these waves by means of impedance measurements. The measured sheath wave complex propagation constant agrees very well with theory for frequencies below approximately half the plasma frequency. It is shown that the “series resonance” found on small probes is directly related to the cutoff frequency of sheath wave propagation. A family of presheath waves is observed experimentally for the first time through a set of resonances that occur close to the frequencies predicted, above the sheath wave cutoff frequency.