Allen Jackson (Committee Member), James Menart (Advisor), Scott Thomas (Committee Member)
Master of Science in Engineering (MSEgr)
The design and development of ion engines is a difficult and expensive process. In order to alleviate these costs and speed ion engine development, it is proposed to further develop a particle-in-cell (PIC), Monte-Carlo collision (MCC) model of an ion engine discharge chamber, which has previously been worked on by the Wright State Ion Engine Modeling Group. Performing detailed and accurate simulations of ion engines can lead to millions of dollars in savings in development costs.
In order to recognize these savings more work must be done on the present day models used to simulate ion engine performance. The work presented in this thesis is an effort to do this with a computer model of the plasma in the discharge chamber of an ion engine. In particular, this thesis presents a few steps in the process of moving a Wright State developed PIC-MCC computer code, developed specifically for the plasma in the discharge chamber, to include detailed electric field calculations. This is a rather difficult process in that the electric fields present in the discharge chamber are strongly dependent on the location of the charged particles in the plasma. This means there is a strong and unstable connection between the particle position calculation and the electric field calculation. Other difficulties are the relatively large computational domain and the relatively large plasma density present. Because of the computational times involved,PIC-MCC techniques are generally not applied to large computational domains with high particle number densities, but this is the precise physical model that is required to obtain accurate results for the plasma in the discharge chamber of an ion engine.
This thesis presents a few steps taken to get such a program to converge and to run in a stable fashion. Not only is getting the program to converge an issue, but getting convergence times that are less than one week is difficult. By no means is the work in this thesis a complete solution to these problems; the work done here is just a few steps in this process. There are many problems and issues that still need to be addressed.
In addition to discussing the work done to move detailed PIC-MCC calculations with a fully coupled electric field and particle position calculation forward, a good deal of discussion about the physics of ion engines and the computational tools used in this work will be presented. This is done to familiarize the reader with ion engines and so they will understand how difficult it is to develop a model that will accurately predict the performance of an ion engine.
The baseline computer code used in this research is reviewed. The baseline code is called VORPAL, which the Tech-X Corporation developed. VORPAL itself is an outgrowth of a computer program called OOPIC PRO. This project started using OOPIC PRO, but switched to VORPAL, an object orientated, relativistic, plasma simulation code, because of the many benefits it provides.
Following the discussion of VORPAL, techniques used to decrease run time that were undertaken by the ion engine group at Wright State and the Tech-X Corporation are given. These include particle fragmenting and merging, scaling of the discharge chamber, and two-dimensional domain decomposition. Programming issues that were discovered in VORPAL and in an earlier version of VORPAL called OOPIC PRO are discussed.
Due to the sensitivity that PIC-MCC codes have to the time step used and the desire to implement a time throttling technique to reduce computational times, a time step survey is conducted. PIC-MCC codes are extremely sensitive to time step size. It is found that a time step size of 10-12 seconds is the largest time step that can be used.
Department or Program
Department of Mechanical and Materials Engineering
Year Degree Awarded
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