AVS 46th International Symposium
    Plasma Science and Technology Division Monday Sessions
       Session PS-MoP

Paper PS-MoP34
Grid Interactions with a High Density Plasma Source

Monday, October 25, 1999, 5:30 pm, Room 4C

Session: Poster Session
Presenter: J.E. Johannes, Sandia National Laboratories
Authors: J.E. Johannes, Sandia National Laboratories
T.J. Bartel, Sandia National Laboratories
C.K. Kim, University of Houston
D. Ecnomou, University of Houston
Correspondent: Click to Email
Plasma interaction with a grid/screen is important in a number of applications. In neutral beam etching, for example, a grid is used to neutralize ions and generate collimated beams of energetic neutrals for anisotropic etch without charge damage. Neutron generators, used for neutron activation analysis in downhole logging, use a grid to define the Child-Langmuir surface where electrons are shielded, to extract an essentially pure ion beam for acceleration to a target. Finally, many ion sources and satellite thrusters are based on extraction of an ion beam from a plasma through a grid. The plasma conditions and the grid hole aspect ratio determine the neutral, ion, or plasma transport through the grid. Plasma-grid interactions are investigated for a range of plasma densities (1x1017-1x1019 #/m3) using particle based plasma simulation tools, and results are compared to experimental data. Icarus, a 2-D transient Direct Simulation Monte Carlo (DSMC) code and Mercury, a computationally fast, steady-state particle code are used to investigate the plasma behavior in front of and behind the grid. Electrons, ions and neutrals are treated as particles and an explicit Poisson solver, using the boundary element method, is used to compute electric fields. Simulations of both hydrogen and argon plasmas in an inductively coupled source were performed to generate boundary conditions for the grid. The grid hole diameter ranges from 30-1000 microns with a grid thickness of 250 microns. Simulation results are compared to measured ion fluxes and angular distributions collected over a range of grid hole aspect ratios. The optimal interaction parameters and hole aspect ratio required to minimize ion escape through the grid for neutral beam applications are discussed. Plasma-grid interaction calculations were also performed for higher density plasmas (1x1019 #/m3) typical of an arc discharge to investigate optimized hole aspect ratios for ion extraction for neutron generator applications.