City Research Online

Abstract

When a plasma is bounded by an electron-emissive wall, the sheath which forms in its vicinity will accelerate any released electrons into the plasma and this will give rise to the beam-plasma instability. The development of this instability has been modelled using a 1-D particle-simulation code developed for this purpose.
When simulating the instability the sheath region is not represented in the code and the parameters for the sheath and associated secondary-electron beam have been obtained from a separate calculation. Equations, generalised to include current flows to the wall under equilibrium conditions, have been derived for this purpose and their solutions obtained. The thermal fluctuation energy of the most unstable mode in the sheath has also been estimated.
The secondary-electron beam-plasma instability develops with initial linear growth of a spectrum of electrostatic waves. The spectrum eventually becomes narrow enough to be considered a single wave which ultimately grows large enough to trap the beam electrons, whereupon the growth saturates. Thereafter the wave amplitude oscillates as the trapped electrons bounce back and forth in the potential well. Neighbouring waves, phase-mixing and particle collisions all have the capability to smear out the beam orbit, thus destroying the wave amplitude oscillations and, depending on the conditions, this can cause the particle distribution to approach that of a BGK mode. When no current is flowing, and assuming the maximum secondary-electron flux, the distance from the sheath to the trapping region is of the order of 100 Debye lengths for all real plasmas; the stability of the sheath itself would appear to remain unaffected by the nonlinear development of the instability. When the sheath carries a current, however, the sheath stability and distance to the trapping region may be modified.
While pursuing the equilibrium calculations a hypothetical plasma system was discovered for which the choice of boundary condition was not clear. The particle-simulation has revealed that such a system adopts the configuration which minimises the wall-potential relative to the midplane. It has also been demonstrated that choosing the equality in the Bohm criterion for sheath formation corresponds to an approximate minimisation of wall-potential relative to the midplane for a plasma bounded by two

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QC Physics
Departments:	School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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