Amelia Greig

Direct measurement of neutral gas heating in a radio-frequency electrothermal plasma micro-thruster

Direct measurements and modelling of neutral gas heating in a radio-frequency (13.56 MHz) electrothermal collisional plasma micro-thruster have been performed using rovibrational band matching of the second positive system of molecular nitrogen (N2) for operating pressures of 4.5 Torr down to 0.5 Torr. The temperature measured with decreasing pressure for 10 W power input ranged from 395 K to 530 K in pure N2 and from 834 K to 1090 K in argon with 1% N2. A simple analytical model was developed which describes the difference in temperatures between the argon and nitrogen discharges.

Volume and surface propellant heating in an electrothermal radio-frequency plasma micro-thruster

This research was partially funded by the Australian Space Research Program (APT project) and the Australian Research Council Discovery Project (No. DP140100571).

Simulation of main plasma parameters of a cylindrical asymmetric capacitively coupled plasma micro-thruster using computational fluid dynamics

Computational fluid dynamics (CFD) simulations of a radio-frequency (13.56 MHz) electro-thermal capacitively coupled plasma (CCP) micro-thruster have been performed using the commercial CFD-ACE+ package. Standard operating conditions of a 10 W, 1.5 Torr argon discharge were used to compare with previously obtained experimental results for validation. Results show that the driving force behind plasma production within the thruster is ion-induced secondary electrons ejected from the surface of the discharge tube, accelerated through the sheath to electron temperatures up to 33.5 eV. The secondary electron coefficient was varied to determine the effect on the discharge, with results showing that full breakdown of the discharge did not occur for coefficients coefficients less than or equal to 0.01.

Plasma Catalytic Synthesis of Ammonia Using Functionalized-Carbon Coatings in an Atmospheric-Pressure Non-equilibrium Discharge

We investigate the synthesis of ammonia in a non-equilibrium atmospheric-pressure plasma using functionalized-nanodiamond and diamond-like-carbon coatings on α-Al2O3 spheres as catalysts. Oxygenated nanodiamonds were found to increase the production yield of ammonia, while hydrogenated nanodiamonds decreased the yield. Neither type of nanodiamond affected the plasma properties significantly. Using diffuse-reflectance FT-IR and XPS, the role of different functional groups on the catalyst surface was investigated. Evidence is presented that the carbonyl group is associated with an efficient surface adsorption and desorption of hydrogen in ammonia synthesis on the surface of the nanodiamonds, and an increased production of ammonia. Conformal diamond-like-carbon coatings, deposited by plasma-enhanced chemical vapour deposition, led to a plasma with a higher electron density, and increased the production of ammonia.

Spatiotemporal study of gas heating mechanisms in a radio-frequency electrothermal plasma micro-thruster

A spatiotemporal study of neutral gas temperature during the first 100 s of operation for a radio-frequency electrothermal plasma micro-thruster operating on nitrogen at 60 W and 1.5 Torr is performed to identify the heating mechanisms involved. Neutral gas temperature is estimated from rovibrational band fitting of the nitrogen second positive system. A set of baffles are used to restrict the optical image and separate the heating mechanisms occurring in the central bulk discharge region and near the thruster walls. For each spatial region there are three distinct gas heating mechanisms being fast heating from ion-neutral collisions with timescales of tens of milliseconds, intermediate heating with timescales of 10 s from ion bombardment on the inner thruster tube surface creating wall heating, and slow heating with timescales of 100 s from gradual warming of the entire thruster housing. The results are discussed in relation to optimising the thermal properties of future thruster designs.

Plume Characteristics of an Electrothermal Plasma Microthruster

High-resolution digital images of the expansion plume from an electrothermal capacitively coupled radio frequency (13.56 MHz) plasma microthruster were captured for argon and nitrogen discharges in lowand high-pressure operating regimes. The sheath near the thruster exit is clearly visible in all images, and the observed expansion plume features are consistent with previous experiments on electron density profiles within the discharge.

Atmospheric plasma thruster: Theory and concept

Dielectric barrier discharge plasma actuators generate a net response force that exhibits potential for thrust applications such as propulsive systems. In this work, an angled actuator in which the exposed and encased electrodes are not parallel has been investigated using direct force measurements and particle image velocimetry. It was shown that the induced force was nonlinearly increased by increasing the angle between the electrodes. In addition, the direction of the upstream component of the ionic wind was changed by varying the electrode angle. Modifying the angle between the electrodes changes the electric field strength in the vicinity of the plasma actuator, thereby changing the response force produced. Analytical calculations were used to compare expected results with results obtained experimentally. Then, using the results obtained experimentally, a plasma thruster was designed as proof of concept for dielectric barrier discharge plasma actuators as propulsive devices.

Neutral gas temperature estimates and metastable resonance energy transfer for argon-nitrogen discharges

Rovibrational spectroscopy band fitting of the nitrogen (N2) second positive system is a technique used to estimate the neutral gas temperature of N2discharges, or atomic discharges with trace amounts of a N2 added. For mixtures involving argon and N2, resonant energy transfer between argon metastable atoms (Ar*) and N2 molecules may affect gas temperature estimates made using the second positive system. The effect of Ar* resonance energy transfer is investigated here by analyzing neutral gas temperatures of argon-N2 mixtures, for N2 percentages from 1% to 100%. Neutral gas temperature estimates are higher than expected for mixtures involving greater than 5% N2 addition, but are reasonable for argon with less than 5% N2 addition when compared with an analytic model for ion-neutral charge exchange collisional heating. Additional spatiotemporal investigations into neutral gas temperature estimates with 10% N2 addition demonstrate that although absolute temperature values may be affected by Ar* resonant energy transfer, spatiotemporal trends may still be used to accurately diagnose the discharge.

Investigation into the effect of electrode angle on force production of a dielectric barrier discharge plasma actuator

Dielectric Barrier Discharge (DBD) plasma actuators impart a momentum transfer to the surrounding medium that in turn produces a net response force. The amount of force generated is directly proportional to the electric field strength. The electric field strength of a DBD plasma actuator can be modified by changing the angle between the electrodes. A larger electrode angle produces a higher magnitude electric field and therefore higher magnitude response force. Direct force measurements were made to measure the change in response force produced with changing electrode angle, with higher response forces generated experimentally with larger electrode angles. By analyzing the electrode configuration of a DBD plasma actuator as a capacitor, a theoretical relationship between electrode angle and thrust produced was developed and matched to the experimentally determined results. Results indicate changing the angle between the electrodes increases the electric field strength and therefore response force.

A Radio Frequency Plasma Micro-Thruster: Characterization of Various Discharge Gases Through Optical Diagnostics

Images of the discharge cross section of an electro-thermal radio frequency plasma microthruster were taken using a digital camera in conjunction with bandpass filters for argon, nitrogen (N2) and carbon dioxide (CO2) discharges. The bandpass filters were chosen to isolate a particular emission line (transition), such that the light intensity imaged was related to electron density using the coronal model of the transition. From this, the profile of electron density across the discharge diameter was inferred for powers between 5 and 50 W and pressures between 0.35 and 6.0 Torr. For all three discharge gases a mode change occured from a maximum intensity peak on the central axis at lower pressure to an annulus of maximum intensity located approximately mid-radius at higher pressure. The pressure at which this occured differed for all three gases.