Final PhD Seminar

Computational fluid dynamics and plasma modelling of the Pocket Rocket radiofrequency plasma electrothermal microthruster

Mr Teck Seng Ho

Pocket Rocket is a radiofrequency plasma electrothermal microthruster designed for use aboard miniaturised spacecraft like CubeSats. Using only a few Watts of RF power and under 3 mg/s of Ar propellant, a weakly ionised gamma discharge is generated in a small constricted volume, primarily sustained by high energy secondary electrons emitted from the negatively self-biased walls of the discharge chamber. Ion-neutral charge exchange collisions in the plasma volumetrically heats the background neutral Ar to temperatures up to 1000 K. This raises the local sound speed in the gas, and therefore the velocity at which the propellant exits the thruster, resulting in a higher thrust force than what is attainable from using cold propellant.

Due to Pocket Rocket's small dimensions (4.2 mm in diameter and 18 mm in length), experimental instruments like Langmuir probes cannot accurately provide diagnosis without physically restricting gas flow and electrically perturbing the plasma. Additionally, terrestrial equipment such as a pendulum thrust balance are not sufficiently sensitive for precision measurement of thrust forces on the order of 1 mN.

A computational fluid dynamics (CFD) and plasma simulation technique has been developed to overcome these challenges. Firstly, cold gas flow simulations were performed to study low pressure (~ 1 Torr) boundary layer effects present in Pocket Rocket, and compared with theoretical predictions and experimental results to ensure accuracy. Secondly, plasma simulations are built upon the verified cold gas flow model, and matched with experimental plasma measurements. The end result is a calibrated CFD-plasma model of Pocket Rocket which permits noninvasive full 2D axisymmetric examination, not only spatially but also temporally on the nanosecond scale. The model allows the extraction of fluid parameters like pressure, flow velocity, temperature profiles, and most importantly thrust calculations; as well as plasma parameters like electron energy and density, ion velocity, plasma potential, reaction rates, and heating mechanisms taking place in the discharge. Using this calibrated model as a foundation, modifications such as a converging-diverging nozzle or different electrode geometries are implemented to explore further enhancements to the Pocket Rocket design.

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