Ho, Teck Seng
Description
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...[Show more] 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.
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