THICKNESS OF THE ION SHEATH AROUND A CYLINDRICAL ELECTRODE IN A PLASMA

The results of an experimental study of the thickness of the ion sheath set up at the interface between a cylindrical electrode and the plasma in which it is immersed are presented. A point-like, movable, electron-collecting cold probe is used for exploring the sheath region. The cylindrical electrode is biased negatively, and a low-frequency low-voltage ac signal is superimposed on this dc bias. The modulation of the electron density within the sheath, induced by this ac signal, manifests itself by a modulation in the current measured by the electron-collecting probe. The ac component of this current has a maximum peak value around the sheath-plasma boundary. The position of the point-like probe (relative to the cylindrical electrode) where the ac current is maximum is determined with a high degree of accuracy by means of a precision optical theodolite. Reliable data for the sheath thickness and its dependence on the absolute value of the applied dc bias could thus be obtained.