Supersonic Constricted Plasma Flows

The Pocket Rocket electrothermal microthruster is a miniaturised electric propulsion system designed for nanosatellites operating in space. A weakly ionised capacitively coupled plasma is ignited in the flowing Ar gas propellant within a constricted discharge chamber at 1 Torr using less than 10 W of radiofrequency power. The discharge can operate either continuously or in rapid pulsed mode since plasma breakdown initiates almost instantaneously on a μs time scale. The propellant is heated to temperatures approaching 1000 K and is expanded through a converging-diverging nozzle into vacuum at supersonic velocities. Thrust on the order of 1 mN is generated as a reactionary force to the linear momentum of the expelled neutral gas propellant. This thesis presents a comprehensive model of Pocket Rocket developed with computational fluid dynamics and plasma simulations. Boundary layer effects are significant in the rarefied flow within the constricted discharge chamber. A slip boundary condition with the appropriate tangential momentum and thermal accommodation coefficients must be used to produce results that precisely match experimental measurements. The problem of including vacuum regions within a fluid simulation domain is unconventionally circumvented by taking advantage of the flow velocity choking. The computed sonic surface, thrust force, and specific impulse are in good agreement with theoretical predictions. Volumetric plasma-induced heating of the background neutral gas is primarily due to ion-neutral charge exchange collisions, with very little contribution from electron-neutral elastic collisions. The propellant temperature is described by two local models based on the different ion transport behaviour in the plasma bulk and plasma sheath. The most dominant process is surface bombardment by ions accelerated through the plasma sheath, which heats the discharge chamber wall and is responsible for the creation of secondary electrons that sustain the gamma mode discharge. The geometrical area asymmetry of the grounded and powered electrodes results in a self-bias that manifests as a spatially nonuniform negative charging within the dielectric discharge chamber wall. In the thin sheath regime, the self-biased waveform has a diminished trailing edge at each positive peak, and asymmetrically displaced negative peaks due to the extraneous impedance of the dielectric wall. This leads to a redefinition of the self-bias voltage that uses the maxima envelope of the self-biased waveform instead of the mean, which maintains consistency with different extraneous impedances. The performance of Pocket Rocket is improved by optimising the physical and electrical geometry for thrust and boundary layer effects, and plasma confinement is achieved through the formation of a conical plasma sheath at the nozzle throat. Enhanced recombination in the supersonic expanding plume creates a neutral exhaust, thereby avoiding contamination of externally mounted solar panels and interference with sensitive instruments. Most importantly, the combination of flow velocity choking and plasma confinement results in a convergent plasma simulation that accurately models plasma expansion into vacuum. The computational fluid dynamics and plasma modelling technique and analysis presented in this thesis are not restricted to the Pocket Rocket discharge and may be adapted for other discharges at different pressure regimes and physical scales

Redefinition of the self-bias voltage in a dielectrically shielded thin sheath RF discharge

In a geometrically asymmetric capacitively coupled discharge where the powered electrode is shielded from the plasma by a layer of dielectric material, the self-bias manifests as a nonuniform negative charging in the dielectric rather than on the blocking capacitor. In the thin sheath regime where the ion transit time across the powered sheath is on the order of or less than the Radiofrequency (RF) period, the plasma potential is observed to respond asymmetrically to extraneous impedances in the RF circuit. Consequently, the RF waveform on the plasma-facing surface of the dielectric is unknown, and the behaviour of the powered sheath is not easily predictable. Sheath circuit models become inadequate for describing this class of discharges, and a comprehensive fluid, electrical, and plasma numerical model is employed to accurately quantify this behaviour. The traditional definition of the self-bias voltage as the mean of the RF waveform is shown to be erroneous in this regime. Instead, using the maxima of the RF waveform provides a more rigorous definition given its correlation with the ion dynamics in the powered sheath. This is supported by a RF circuit model derived from the computational fluid dynamics and plasma simulations

Current-Free Electric Double Layer in a Small Collisional Plasma Thruster Nozzle Simulation

A computational fluid dynamics and plasma model of a collisional (~ a few Torr) radiofrequency (at 13.56 MHz) argon plasma capacitively coupled in a converging-diverging nozzle (applied to the optimization of electrothermal plasma thrusters for space use) shows the formation of a strong stationary current-free double layer (CFDL) at the 1.5 mm diameter nozzle throat for a downstream pressure of ~ 0.1 Torr. The cycle average magnitude of the double layer potential is ΔΦDL = 77 V and the electron temperature at the high potential edge of the double layer is kBTe = 2.64 eV, yielding a strength of ΔΦDL/(kBTe) ~ 30. The double layer is 1.2 mm wide which corresponds to ~ 90 Debye lengths. The axial electric field of the double layer accelerates ions along the nozzle to a maximum drift velocity of 17 km s−1, about 3.3 times the ion sound speed, and their kinetic energy is transferred to neutrals by ion-neutral charge exchange collisions. The ion transit time τi through the potential structure spontaneously forming at the nozzle throat is about 5 times the radiofrequency excitation period τRF. These findings are discussed in the broader context of double layer physics and the dynamics of their formation as well as in the context of electrothermal thruster optimization in which neutral propellant heating via ion-neutral charge exchange collisions is the main source of thrust

A Comprehensive Cold Gas Performance Study of the Pocket Rocket Radiofrequency Electrothermal Microthruster

This paper presents computational fluid dynamics simulations of the cold gas operation of Pocket Rocket and Mini Pocket Rocket radiofrequency electrothermal microthrusters, replicating experiments performed in both sub-Torr and vacuum environments. This work takes advantage of flow velocity choking to circumvent the invalidity of modeling vacuum regions within a CFD simulation, while still preserving the accuracy of the desired results in the internal regions of the microthrusters. Simulated results of the plenum stagnation pressure is in precise agreement with experimental measurements when slip boundary conditions with the correct tangential momentum accommodation coefficients for each gas are used. Thrust and specific impulse is calculated by integrating the flow profiles at the exit of the microthrusters, and are in good agreement with experimental pendulum thrust balance measurements and theoretical expectations. For low thrust conditions where experimental instruments are not sufficiently sensitive, these cold gas simulations provide additional data points against which experimental results can be verified and extrapolated. The cold gas simulations presented in this paper will be used as a benchmark to compare with future plasma simulations of the Pocket Rocket microthruster.This research was funded by the Australian Research Council Discovery Project DP140100571, by the Australian Space Research Program [Wombat upgrade as part of the Australian Plasma Thruster (APT) project] of the Department of Industry, Innovation, Science, Research and Tertiary Education (Australian Government), and Lockheed Martin US (Pocket Rocket thruster Research and Development contract)

Performance modelling of plasma microthruster nozzles in vacuum

Computational fluid dynamics and plasma simulations of three geometrical variations of the Pocket Rocket radiofrequency plasma electrothermal microthruster are conducted, comparing pulsed plasma to steady state cold gas operation. While numerical limitations prevent plasma modelling in a vacuum environment, results may be obtained by extrapolating from plasma simulations performed in a pressurised environment, using the performance delta from cold gas simulations performed in both environments. Slip regime boundary layer effects are significant at these operating conditions. The present investigation targets a power budget of ∼10 W for applications on CubeSats. During plasma operation, the thrust force increases by ∼30% with a power efficiency of ∼30 μNW−1. These performance metrics represent instantaneous or pulsed operation and will increase over time as the discharge chamber attains thermal equilibrium with the heated propellant. Additionally, the sculpted nozzle geometry achieves plasma confinement facilitated by the formation of a plasma sheath at the nozzle throat, and fast recombination ensures a neutral exhaust plume that avoids the contamination of solar panels and interference with externally mounted instruments

Improving SAR image classification in tropical region through fusion with SPOT data

This paper investigates various SAR digital filtering techniques to remove speckles for image classification using fused SAR and SPOT XS image. The fused image classification is then compared with the classified SPOT XS image. The result has shown that the use of the Enhanced Frost digital filtering technique for SAR image and the fusion with SPOT XS gives a very similar classification with comparison to the SPOT XS image classification.</p

Spatio-temporal plasma heating mechanisms in a radio-frequency electrothermal microthruster

Low-power micro-propulsion sources are currently being developed for a variety of space missions. Electrothermal plasma thrusters are of specific interest since they enable spatial control of the power deposition to the propellant gas. Understanding the mechanisms whereby electrical power is coupled to the propellant will allow for optimization of the heating and fuel efficiencies of electrothermal sources. Previous studies of radio frequency (RF) plasmas have shown a dependence of the gas and electron heating mechanisms on the local collisionality. This is of particular importance to thrusters due to the large pressure gradients that exist between the inlet and outlet when expanding into vacuum. In this work, phase-resolved optical emission spectroscopy and numerical simulations were employed to study plasma heating in an asymmetric RF (13.56 MHz) electrothermal microthruster operating in argon between 186-226 Pa (1.4-1.7 Torr) plenum pressure, and between 130-450 V (0.2-5 W). Three distinct peaks in the phase-resolved Ar(2p 1) electron impact excitation rate were observed, arising from sheath collapse heating, sheath expansion heating, and heating via secondary electron collisions. These experimental findings were corroborated with the results of two-dimensional fluid/Monte Carlo simulations performed using the Hybrid Plasma Equipment Model (HPEM). The influence of each mechanism with respect to the position within the plasma source during an α-γ mode transition, where plasma heating is driven via bulk and sheath heating, respectively, was investigated. Sheath dynamics were found to dictate the electron heating at the inlet and outlet, this is distinct from the center of the thruster where interactions of secondary electrons were found to be the dominant electron heating mechanism. Optimization of the heating mechanisms that contribute to the effective exhaust temperature will directly benefit electrothermal thrusters used on miniaturized satellite platforms

Neutral gas heating and ion transport in a constricted plasma flow

Ion-neutral charge exchange collisions are demonstrated to be the dominant heating mechanism in a weakly ionised ∼1  Torr Ar capacitively coupled radiofrequency plasma flowing through a cylinder. In this rarefied regime, thermal conduction is ineffective. The neutral gas temperature is significantly higher in the plasma bulk than in the plasma sheath due to different plasma parameters and ion transport behaviours in these regions. This study is achieved in a computational fluid dynamics and plasma simulation, and is applicable to similar plasmas at different pressures and physical scales