title: Plasma-based micro-propulsion system for nanosatellites: design, nanofabrication, and characterisation of plasma-based micro-thruster chip
Creators: name: Guevara Jelid, Hassam Israel
Publisher: name: University of Glasgow
Year: 2024

description: Micro-electromechanical systems have experienced a major expansion in their use as flow control devices and CubeSats' onboard attitude and orbit determination systems (AOCS) in recent years, where plasma thrusters have a potential application in this field. As a result, further understanding of plasma micro-thrusters and plasma processes at micrometer scale sizes is required. Consequently, the propulsion system under development in this project, aims to integrate electrostatic and cold gas thruster in a two-mode operation. Being in specific, a plasma-based micro-thruster chip was fabricated for the micro-propulsion system used in nano and picosatellites, enabling accurate manoeuvre operations in space and producing thrust in the range of nanonewtons (nN) to millinewtons (mN). 

The micro-thruster chip, is conceived as a breakthrough hybrid design chip combining the two-mode operation systems (electrostatic and cold-gas) into a hybrid propulsion system, increasing the flexibility to adapt the thrust control and power consumption according to the mission requirements by switching on or off the current through the electrodes. 
Computational simulations were used to evaluate geometric possibilities in defining the shape and location of the electrode of the micro-thruster chip. A significant problem was presented on the inclusion of plasma in simulations due to the method’s limitations and the high processing power requirements, which are numerically expensive. Therefore, a novel code was developed to simulate micro-discharge interaction with the flow as a slim alternative based on the dsmcFOAM+ solver from the OpenFOAM platform. The code reduces the computational burden by simulating ionisation without introducing new electrons as particles, increasing the velocity of an existing number of equivalent gas particles in the mesh and finding a numerical solution to the Fowler-Nordheim equation. 

As a result, an in-house 5 mm x 7.4 mm x 0.75 mm plasma based micro-thruster chip was designed to connect to the microSD card's electrical connection. A comprehensive method for the nanofabrication of the micro-thruster was established based on fifty cleanroom processes, including photolithography, wet etching, dry etching, metal deposition, and bonding procedures inside the James Watt Nanofabrication Centre (JWNC). The neutraliser designing and manufacture are no cover in this project. 

After the micro-thruster was manufactured, a bespoke experimental setup to characterise the performance of the chip as a cold gas thruster was built based on a prior PIV configuration from the TSI time-resolved particle image velocimetry (PIV) system. The experimental setup was constrained when magnification lenses were used to see the micro-size jet, which reduced the field of view for capturing exhaust particles from the micro-thruster traveling at higher velocities ( 8 m/s), encouraging the design and fabrication of a calibration sheet with micrometre dots for camera alignment to be used for higher magnification lenses. In the attempt to obtain the characterisation of the velocities on the jet, it was able to capture velocities up to 12 m/s, being a significant increase but not sufficient for the full jet velocities profile that allows a further reliable thrust calculation. Therefore, no electrostatic performance features were investigated given by the restrictions on equipment capabilities for the detection and readiness of high voltage (200 V - 1.2 KV) with low current (1 μA – 100 μA). Therefore, the most significant contributions or findings of this current project are: 

• The creation of a micro-discharge algorithm for the dsmcFOAM+ solver on OpenFOAM that can simulate DC discharge at the micro-scale while taking the enhancement factor for micro-gaps into account. 
• Design and nanofabrication of a novel plasma-based micro-thruster chip prototype. 
• Feasibility of using SPR220-7 positive photoresist and physical vapour deposition were used to deposit 500nm-thick Au (gold) through a 100 m trench (PVD). 
• Identification of the particle image velocimetry (PIV) system limitations in the development of an experimental testing rig for micro-thrusters based on cold gas tests.