Solution of the neutrals species in a weakly ionised plasma by means of the SIMPLE algorithm
Masters Thesis Presentation by A. Zorzetto
Time: Fri 2021-08-20 13.15 - 14.00
Participating: Alberto Zorzetto
In recent years, the Helicon Plasma Thruster (HPT) has become one of the most promising technologies of in-space electric propulsion. T4i - Technology for Propulsion and Innovation S.P.A. is one of the leading companies working with this new type of systems, and their thruster, REGULUS, is the first HPT ever to be operated in orbit. To better assess the performance of the motor, the company has developed, in conjunction with the University of Padova and the University of Bologna, a numerical tool called 3D-VIRTUS (3-Dimensional adVanced fluId dRifT diffUsion plaSma solver), which simulates the plasma dynamics in the production stage of the thruster. The model describes the species present in the plasma (electrons, ions, excited and neutrals) by means of a fluid approach, as the plasma density in this part of the motor is in the order of 10^17-10^18 m^(-3). Particularly, the tool considers the Drift-Diffusion (DD) approximation instead of the full set of Navier-Stokes equations. Unfortunately, for typical discharges applied to HPTs, this assumption is accurate only for the electrons species, but not for the heavy species in the plasma, i.e. ions, excited and neutrals. The thesis project presented in this report, executed in collaboration with T4i S.P.A, proposes an updated numerical tool which solves the fully coupled continuity and momentum equations for the neutrals species in the plasma. The new solver is implemented with OpenFOAM, a finite volume library written in C++, and the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) is utilised to resolve the pressure-velocity coupling in the continuity and momentum equations. Four different test cases are considered: a one-dimensional typical discharge, a cylindrical discharge, the Schwabedissen GEC-ICP reactor experiment and the Piglet helicon reactor of Lafleur. The obtained results have been compared against the original drift-diffusion solver, and when available, with experimental data. The new tool produced similar results to the older one, even though the neutrals density computed with the former generally presented stronger gradients. Additionally, in the case of the GEC-ICP and Piglet reactors, the agreement in terms of electrons density computed with the new solver was satisfactory compared to the empirical data. Nevertheless, all the analysis performed during the thesis project revealed that the keys to obtain physically realistic results are the boundary conditions for the neutrals' pressure and velocity, which greatly affects the outcome of the simulations. Overall, the new solver has shown to provide accurate results with reasonable computational time.