The transition from free to ambipolar diffusion under non-local conditions
- Univ. of Wisconsin, Madison, WI (United States)
We have performed a kinetic analysis of the transition from free to ambipolar diffusion in a weakly ionized bounded plasma. A coupled set of equations was solved including the spatially inhomogeneous electron Boltzmann equation in the two-term approximation, Poisson`s equation for the space charge and the continuity equation for ion density in a cylindrical geometry for two different axially uniform discharges: (i) a positive column (PC) of a DC discharge and H an inductively coupled plasma (ICP). A PC plasma is sustained by the axial electric field E{sub o} which does not depend on the radial position. The ICP is sustained by the azimuthal component of the inductive electric field which is radially nonuniform. The heating fields E{sub o} and E{sub {theta}}, necessary to sustain the discharge, have been found self-consistently as functions of the plasma density. A nonlocal theory of a bounded plasma, has been developed for a rather high plasma density (ambipolar regime). In this regime, the majority of the electrons are trapped in a potential well, and give a minor contribution to the fluxes. The electron flux, which equals the ion flux, is mainly due to free electrons which are capable of escaping the discharge. The free electrons constitute a small fraction of the ensemble in the ambipolar regime and have typically been neglected in most publications. The depth of the potential well together with the amplitude of the heating field and relative fraction of free electrons change rapidly with the electron density when the radius of the tube is of the order of the electron Debye length. The mechanism of discharge sustenance and the formation of the nonlocal EDF under these conditions will be discussed on the basis of self-consistent discharge modeling.
- OSTI ID:
- 213047
- Report Number(s):
- CONF-950749--
- Country of Publication:
- United States
- Language:
- English
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