Title page for ETD etd-06262005-233343


Type of Document Master's Thesis
Author Brieda, Lubos
Author's Email Address lbrieda@yahoo.com
URN etd-06262005-233343
Title Development of the DRACO ES-PIC code and Fully-Kinetic Simulation of Ion Beam Neutralization
Degree Master of Science
Department Aerospace and Ocean Engineering
Advisory Committee
Advisor Name Title
Wang, Joseph J. Committee Chair
Scales, Wayne A. Committee Member
VanGilder, Douglas Committee Member
Keywords
  • plasma simulation
  • beam neutralization
  • electron dynamics
Date of Defense 2005-06-02
Availability unrestricted
Abstract
This thesis describes development of the DRACO plasma simulation code. DRACO is an

electro-static (ES) code which uses the particle-in-cell (PIC) formulation to track plasma

particles through a computational domain, and operates within the Air Force COLISEUM

framework. The particles are tracked on a non-standard

mesh, which combines the benefits of a Cartesian mesh with the surface-resolving power

of an unstructured mesh. DRACO contains its own mesher, called VOLCAR, which is also

described in this work.

DRACO was applied to a fully kinetic simulation of an ion-beam neutralization. The

thruster configuration and running parameters were based on the NASA's 40cm NEXT ion thruster.

The neutralization process was divided into three steps. Electron dynamics was studied

by assuming an initial beam neutralization, which was accomplished by injecting both electrons

and ions from the optics. Performing the simulation on a full-sized domain with cell

size much greater than the Debye length resulted in a formation of a virtual anode. Decrease

of the cell size to match the Debye length was not feasible, since it would require a

million-fold increase in the number of simulation nodes. Instead, a scaling scheme was devised.

Simulations were performed on thruster scaled down by a factor of 100, but its operating

parameters were also adjusted such that the produced plasma environment did not change.

Loss of electrons at the boundary of the finite simulation domain induced a numerical instability.

The instability resulted in a strong axial electric field which sucked out electrons from the beam.

It was removed by introducing an energy based particle boundary condition.

Combination of surface scaling and energy boundary resulted in physically sound simulation results.

Comparison were made between the Maxwellian and polytropic temperatures, as well as between simulation

electron density and one predicted by the Boltzmann relationship.

The cathode was modeled individually from the beam by introducing a positively charged collector

plate at a distance corresponding to the beam edge. The local Debye length at the cathode tip

was too small to be resolved by the mesh, even if mesh-refinement was incorporated. Since the

simulation was not concerned with the near-tip region, two modifications were performed. First,

the a limiting value of charge density at the tip was imposed. Second, the cathode potential was allowed

to float. These two modifications were necessary to prevent development of a strong potential gradient

at the cathode tip.

The modified cathode model was combined with ion injection from the optics to model the actual

beam neutralization. Three configurations were tested: a single thruster, a 2x2 cluster with individual

cathodes and a similar cluster with a single large neutralizer. Neither of the cases achieved

neutralization comparable to one in the base-line pre-neutralized case. The reason for the discrepancy

is not known, but it does not seem to be due a loss of electrons at the walls.

The difference could be due to limited extent of the modeled physics.

An additional work is required to answer this question.

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