Investigation of electrospray thruster in cone-jet mode using molecular dynamics simulation

In this thesis, a 3-D molecular dynamics (MD) simulation of an electrospray thruster combined with a finite element method (FEM) and a finite difference method (FDM) was developed to simulate the movements of ions or charged molecules in the electric field. This electrostatic ion thruster is operated by a high electric field generated by an extraction ring at a negative potential and uses liquid gallium and EMIM-BF4 as the ionic source and platinum as the capillary wall. The aim of this project is to simulate an electrostatic ion thruster operating in a cone-jet mode and to characterize its performance under several different operating conditions. Liquid gallium at 320 K was modeled as the liquid metal ionic source using a modified Lennard- Jones 12-6 potential, which takes account of the effect of electrons. The potential model was confirmed by finding the radial distribution function that matches the experimental value. For EMIM-BF4 the coarse-graining potential model was used instead of the all-atomic potential model in order to lessen the computational cost. The platinum capillary was simulated by a three-zone wall model with a Langevin thermostat capable of controlling the heat transfer between liquid ionic sources and the platinum capillary. To study the effect of the wettability between liquid ionic sources and the platinum wall, the interaction potential was approximately modeled based on the Lennard-Jones 12-6 potential. This potential model has factors to control the strength of the attractive and the repulsive interaction between liquid atoms and platinum atoms. The flow rate of liquid ionic sources in the platinum capillary, the critical variable for the stable current jet, was generated by the FPM (Fluidized Piston Method) method. To investigate the motion of an ion jet in the long term, it is necessary to supply the platinum capillary with liquid atoms continuously without physical violations. To do this, the creation part of the Grand canonical ensemble (muVT) in Monte Carlo was employed. The excess chemical potential that is necessary in this method was estimated by the Widom insertion method. Poisson's equation was solved using a finite element method (FEM) and a finite difference method (FDM) for the electric potential and the electric field between the platinum capillary and the extraction ring with and without the space charge. This simulation was carried out under the assumptions that, since liquid ionic source and platinum are perfect conduction materials, there is no the electric field in the platinum capillary and that the space between the two electrodes is a vacuum where the relative dielectric constant is unity. MD simulation showed the formation of the Taylor cone at the tip of the platinum capillary and the motion of ions and ion clusters. The variations studied in this research are the inner radius of the extraction ring, the operation voltage, the separation between the two electrodes, the flow rate, the wettability, and the temperature of the platinum capillary. The results are presented in terms of the total current and the current density and the average velocity of liquid ions passing through the extraction ring. The comparison with Child's law and scaling laws shows that this simulation is valid.