AVS 51st International Symposium
    Plasma Science and Technology Wednesday Sessions
       Session PS2-WeM

Paper PS2-WeM11
Particle-in-Cell Simulation of Ion Extraction Through a Grid

Wednesday, November 17, 2004, 11:40 am, Room 213B

Session: Plasma Sources
Presenter: S.K. Nam, University of Houston
Authors: S.K. Nam, University of Houston
V.M. Donnelly, University of Houston
D.J. Economou, University of Houston
Correspondent: Click to Email
Ion extraction from a plasma through a grid finds applications in ion beam and neutral beam sources. The flux, energy and angular distributions of ions (or neutrals) extracted from the plasma are of primary importance in such applications. These quantities depend critically on the shape of the meniscus (plasma-sheath boundary) formed over the surface topography of the extraction grid. For example, when the sheath thickness is comparable to or smaller than the grid hole size, the sheath tends to conform to the surface topography of the grid (plasma molding) resulting in divergent beams. A self-consistent Particle-in-Cell (PIC) simulation of ion extraction from a plasma through a grid was developed. Emphasis was placed on extracting ions, which are as mono-energetic and as directional as possible. The effect of control parameters affecting the sheath thickness (plasma density, electron temperature, sheath potential) relative to the grid hole size (and aspect ratio) was studied. For a hole size of 0.5 mm, the FWHM of the ion angular spread was about 4, 10, and 20 degrees for sheath thickness of 4.3, 2.6, and 0.7 mm, respectively (sheath potential of 50 V, electron temperature 3 eV). Ion flux uniformity was very high for the thick sheath case (within a 3%) but degraded gradually for the thinner sheaths due to stronger ion divergence. Conditions which allowed the extraction of an ion beam with specified energy and angular spread were determined. The influence of hole-hole interaction was also studied. Work supported by the National Science Foundation.