Date of Award

12-2025

Degree Name

Master of Science in Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Kristina Lemmer, Ph.D.

Second Advisor

Nicholas Taylor, Ph.D.

Third Advisor

Muralidhar Ghantasala, Ph.D.

Keywords

COMSOL, impedance matching network, inductively coupled plasma, molecular propellant, nitrogen plasma, radio frequency plasma

Access Setting

Masters Thesis-Open Access

Abstract

The electric propulsion industry is witnessing a strategic interest toward the utilization of molecular propellants as alternatives to noble gases due to increasing cost constraints and logistical requirements. This transition is hindered by a fundamental deficiency in the understanding of molecular plasma and radio frequency physics, including the excitation, dissociation, and ionization cross-sections, which mandates a reliance on computationally inefficient, empirically modified atomic thruster designs. Furthermore, current state-of-the-art electric propulsion devices, such as gridded ion thrusters and Hall effect thrusters, utilize an electrode (cathode) for ionization and are thus susceptible to chemical poisoning and material erosion, which is prevalent while using certain molecular propellants. To overcome the inherent chemical poisoning risk, there is an interest in the employment of electrodeless radio frequency ionization technology to ensure propellant compatibility and operational lifetime.

To explore these molecular ionization mechanisms, an inductively coupled plasma test cell was developed to focus on nitrogen and hydrogen-based plasmas, with the ability to perform laser diagnostics and characterize the radio frequency plasma environment. The work presented here details the design and implementation of the radio frequency electronics system, featuring a nontraditional, test cell mounted impedance matching network and an Arduino-based automatic impedance tuning unit. Analytical characterization and COMSOL modeling of the RF fields and plasma parameters are presented, providing the foundational knowledge necessary to develop high-fidelity computational models. COMSOL simulations revealed the temporal evolution of the test cell plasma, confirming initial capacitive discharge near the walls, which rapidly transitions into a high-density inductive state with peak electron density concentrating at the center due to ambipolar diffusion and the skin effect. This transition is directly correlated with a significant reduction in the magnitude of both the electric and magnetic fields as the highly conductive plasma effectively screens the radio frequency induced fields, confirming the theoretical expressions and characterizations of the transformer model. These COMSOL simulations correlate to electrical feedback and measurements, as well as visual observations, of the test cell and radio frequency plasma during operation. Therefore, this work provides the fundamental expressions, electronics, and global definitions for the test cell to provide initial plasma and RF characterization through analytical and model-based calculations.

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