ABSTRACT:
Sheaths are universal features when plasmas come in contact with solids. They balance the fluxes of fast electrons and slow ions to keep the plasma neutral. Sheaths have been studied since the early days of plasma physics, yet some aspects are still not resolved. For example the sheath can become unstable and oscillate near ion and electron plasma frequencies. Examples of sheath instabilities will be given for plasma “bubbles”.
These are spherical plasmas embedded inside an ambient plasma, separated by a biased grid. The grid has two adjacent sheaths one of which becomes unstable, e.g. near the ion plasma frequency when biased negatively. The instability mechanism and properties will be explained. One interesting property is that a sheath is a highly nonlinear oscillator, which produces harmonics and sub harmonics and can oscillate at multiple frequencies. The present research has been focused on the basic physics rather than on applications.
