An Investigation of Plasma Dynamics within Microwave Electrothermal Thruster Cavities
Open Access
- Author:
- Biswas, Saptarshi
- Graduate Program:
- Aerospace Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 30, 2023
- Committee Members:
- Robert Melton, Major Field Member
Amy Pritchett, Program Head/Chair
Michael Lanagan, Outside Unit Member
Sean Knecht, Outside Field Member
Sven Bilén, Chair & Dissertation Advisor
Jesse Kane Mc Ternan, Special Member - Keywords:
- Electric Propulsion
Microwave Electrothermal Thruster
Low Earth Orbit
Satellite Constellation
MET
Microwaves - Abstract:
- Microwave Electrothermal Thrusters (METs) are a type of electric propulsion system that use microwaves to generate a plasma within a resonant cavity, which is then ejected to produce thrust. Microwave power is fed into the resonant cavity in which a transverse magnetic TMz011 mode is established, concentrating the electric field at the antenna and the nozzle ends of the cavity. A plasma generated at the nozzle end of the cavity heats the incoming propellant before it exits the nozzle. METs can operate using different types of propellants, offering system designers many options. Here we focus on hydrogen and ammonia as the propellant choices. Hydrogen is the lightest gas and ammonia is liquid-storable and considered a green propellant by NASA for its low toxicity, and can provide high exhaust velocity because of its low molecular mass. A promising application of METs in space propulsion is for micro- and nanosatellites. This enables these small satellites to have propulsion capabilities, leading to medium specific impulse, yet significant change in velocity (delta-V) for extended missions, as well as mission flexibility. Despite their small size, micro- and nanosatellites are utilized for scientific missions in low Earth orbit, either alone or in constellations, and are now being employed for interplanetary missions as well. Due to their small size, cost-effectiveness, and lower complexity compared to larger satellites, they are becoming increasingly common. However, for ground-based testing of these thrusters, facility effects, mainly buoyancy-induced effects caused due to the local gravitational field on microwave-discharge plasma, must be considered, as they affect overall thruster performance. This dissertation investigates the dynamics of the microwave-discharge plasma, and the impact of buoyancy forces on plasma discharge due to variations in thruster orientation for a 2.45-GHz MET, using nitrogen gas as the propellant. The research found that the location of the plasma discharge within the chamber significantly impacts the performance of the MET, as indicated by a parameter known as the stagnation pressure ratio. Results show that an upward-oriented nozzle performs the best, followed by a sideways-oriented nozzle and a downward-oriented nozzle when the mass flow is more than 200 sccm. For mass flow rates below 200 sccm, a sideways-oriented nozzle performs the best, followed by an upward-oriented nozzle and a downward-oriented nozzle. Additionally, the research found that, for each mass flow rate, an optimum input power yields the maximum stagnation pressure ratio. Increasing the power causes the plasma to move towards the microwave source located at the base of the chamber, whereas decreasing the power below the optimum reduces plasma heating and eventually causes the plasma to die out or oscillate about the axis due to the vortical gas flow near the nozzle and eventually dying out. Numerical analyses were conducted to understand the temporal and spatial history of the plasma location and its impact on impedance and the initial plasma discharge behavior. The simulations provided valuable insights into the impact of plasma ignition on the S11 value of the experimental setup. Specifically, it was observed that the ignition of plasma resulted in the distortion of E-field lines, leading to the loss of their symmetrical structure while still maintaining the TMz011 mode shape, which negatively affected the S11 value, thus causing an impedance mismatch in the resonant circuit. We also investigated a 17.8-GHz MET for CubeSat applications. The 17.8-GHz frequency allows for a small resonant cavity, resulting in a higher power density for the same input power. Experiments were carried out at less than 60-W power levels using only the resonant cavity without a straight nozzle. To support these experiments a novel tuning system was experimentally implemented, increasing the experimental setup’s coupling efficiency to ∼ 99%. The tuning assembly consisted of a variable sliding short and a doorknob protruding inside the waveguide, where an antenna passes through the waveguide into the MET cavity. The height of the antenna inside the MET cavity can be varied, allowing for optimal tuning. The system was also validated using COMSOL Multiphysics simulations. Using gas-dynamic principles, an estimated specific impulse (Isp) of ∼ 1068 s and ∼ 454 s was obtained using hydrogen and ammonia propellant for 60 W at mass flow rates of 196 sccm and 95 sccm, respectively. Additionally, preliminary direct thrust measurements of only the cavity were investigated and are discussed. Overall, this work highlights the potential of METs for CubeSat applications and provides new insights into the underlying plasma physics and the potential of hydrogen and ammonia as propellant choices.