Development of Cubesat Thrusters

The popularity of the cubesat form factor has increased dramatically in the last few years, resulting in unprecedented access to space by smaller or medium sized organisations, universities and even smaller countries that could not afford such access in the past. However, the vast majority of the cubesats currently in orbit do not have a propulsion system. Such a system is invaluable as it can extend the satellite's time in space, assist with space debris avoidance manoeuvres, coordinate constellation orbits and facilitate a timely and controlled re-entry at the end of the mission. Various satellite propulsion methods have been developed and tested successfully in space over the past 50~years but it is not always possible to scale down a thruster to match the strict requirements of cost, power, size and so on, set by the cubesat platform. The objective of this thesis is to explore a number of options that, directly or indirectly, can help with the development of cubesat thrusters. The presented work includes a cost efficient method of measuring the performance of a cubesat thruster in the lab, using a load cell on a hanging pendulum thrust balance. The method shows good performance in comparison with the most expensive displacement measurement method, normally achieved by a laser interferometer. The thesis continues with the enhancement of the Pocket Rocket, an existing cubesat electrothermal RF thruster. The changes introduced are centred around a change from a capacitively coupled plasma system to an inductive one. These changes result in a more compact and efficient RF matching system which improves the overall efficiency of the thruster system, which is an important improvement for power-constrained cubesats. The new Inductive Pocket Rocket is placed in the Wombat space simulation chamber and a set of direct thrust measurements are made, showing the increase in thrust of up to 50%, then the plasma is ignited. Continuing on that theme, the thesis presents a very efficient RF source produced in collaboration with the Stanford Power Lab. It is based on the Class-E amplifier topology, which operates the power semiconductor at the two extremes - fully conducting to fully cut off, spending little time in the linear, lossy region of the transfer curve. This RF source, matched with a suitable DC power supply, exhibits electrical efficiency of over 91% resulting not only in better use of the limited power resources on-board the cubesat, but also less waste heat. The thesis continues with exploring an alternative propellant, naphthalene. Naphthalene sublimates at relatively low temperatures, producing enough vapour pressure to sustain the operation of a cold gas thruster. Cold gas thrusters generally have lower performance compared to other types of thruster systems for satellites but have the engineering advantage of simplicity. To that, naphthalene adds the advantage of having a propellant that is stored in solid state, resulting in negligible propellant storage pressure and higher propellant storage density, compared to the more common gas propellants such as krypton and xenon. Naphthalene also has excellent compatibility with the materials commonly used in the construction of satellites, eliminating the materials compatibility problems (corrosion), that increase the engineering challenge of other, more reactive solid propellants such as iodine. A functional proof of concept thruster is built and its performance is tested in the space simulation chamber, showing the reliable production of up to 0.6 mN of thrust

Naphthalene as a cubesat cold gas thruster propellant

The “cubesat” form factor (multiples of 10 × 10 × 10 cm3 volume and 1.33 kg mass) has been adopted as the defacto standard for a cost effective and modular, nano-satellite platform. Many commercial options exist for nearly all components required to build such satellite; however, there is a limited range of thruster options that suit the power and size restrictions of a cubesat. This work presents the design, implementation and direct thrust measurements of a proof of concept cubesat cold gas thruster system using naphthalene (C10H8) as the propellant. The proposed design is optimized for simplicity to match the requirement of entry level cubesat missions, yet, due to the properties of naphthalene, it can achieve a total impulse in the order of tens of newton-seconds

Tuning the Superconducting Linac at Low Beam Intensities

The ANU Heavy Ion Accelerator Facility (HIAF) is comprised of a 15 MV electrostatic accelerator followed bya superconducting linac booster. Employment of double terminal stripping allows the system to accelerate beams with mass up to 70 amu. The disadvantage of double terminal stripping is low beam intensity of few particle nanoamps delivered to the linac. One of the linac set up procedures developed at ANU utilises a U-bend at the end of the linac. One special wide Beam Profile Monitor (BPM) is installed after the 90 degree magnet. The technique allows correct setting of phase by observing the displacement of beam profile versus phase shift of the last phase locked resonator. In this paper, a simple method has been proposed to improve sensitivity of a commercially available BPM for efficient operation with low beam intensities. Verification of BPM with enhanced sensitivity is accomplished during routine linac operations and it is supplemented by longitudinal phase space simulations.This work has been supported by Australian Federal Government Superscience/EIF funding under the NCRIS mechanism

Measurements of low-level anthropogenic radionuclides from soils around Maralinga

The isotopes 239Pu and 240Pu are present in surface soils as a result of global fallout from nuclear weapons tests carried out in the 1950's and 1960's. These isotopes constitute artificial tracers of recent soil erosion and sediment movement. In practic

High voltage performance degradation of the 14ud tandem accelerator

The 14UD at the Australian National University's Heavy Ion Accelerator Facility (HIAF) operated at a maximum voltage of 15.5 MV after the installation of tubes with a compressed geometry in the 1990s. In recent years, the performance of the accelerator has shown a gradual decline to a maximum operation voltage of ~14.5 MV. There are some fundamental factors that limit the high voltage performance, such as SF6 gas pressure, field enhancement due to triple junctions and total voltage effect. In addition, there are nonfundamental factors causing high voltage degradation. These are: operation with faulty ceramic gaps; operation at inappropriate voltage and SF6 pressure combinations; SF6 leaks into the vacuum space; use of SF6 and O2 as a stripper gases; poor electron suppression in the high energy stripper and frequent use of highly reactive ions such as sulphur and fluorine. In this paper we will discuss factors that limit the high voltage performance. The main outcomes of a preliminary investigation of titanium (Ti) electrodes removed from the accelerator after a few decades of operation will be reported. The investigation confirmed contamination of Ti electrodes with unstable films containing traces of oxides, sulphur and fluorine. The rehabilitation strategies for the accelerator will be discussed.This work is supported by Australian Federal Government Superscience/EIF funding under the NCRIS mechanis

Cache Oblivious Matrix Transposition: Simulation and Experiment

A cache oblivious matrix transposition algorithm is implemented and analyzed using simulation and hardware performance counters. Contrary to its name, the cache oblivious matrix transposition algorithm is found to exhibit a complex cache behavior with a cache miss ratio that is strongly dependent on the associativity of the cache. In some circumstances the cache behavior is found to be worst than that of a naïve transposition algorithm. While the total size is an important factor in determining cache usage efficiency, the sub-block size, associativity, and cache line replacement policy are also shown to be very important

Development of a fast voltage control method for electrostatic accelerators

The concept of a novel fast voltage control loop for tandem electrostatic accelerators is described. This control loop utilises high-frequency components of the ion beam current intercepted by the image slits to generate a correction voltage that is applied to the first few gaps of the low- and high-energy acceleration tubes adjoining the high voltage terminal. New techniques for the direct measurement of the transfer function of an ultra-high impedance structure, such as an electrostatic accelerator, have been developed. For the first time, the transfer function for the fast feedback loop has been measured directly. Slow voltage variations are stabilised with common corona control loop and the relationship between transfer functions for the slow and new fast control loops required for optimum operation is discussed. The main source of terminal voltage instabilities, which are due to variation of the charging current caused by mechanical oscillations of charging chains, has been analysed

Terminal voltage stabilization of pelletron tandem accelerator

An NEC terminal voltage stabiliser TPS-6.0, based on conventional corona control, has been installed and investigated on the ANU 14UD tandem accelerator. The fluctuations in the charge transport of the electrostatic pelletron generator and their correlation with mechanical oscillations of the chains and terminal voltage ripple have been analysed. Emphasis during commissioning is placed on the components of the two-loop feedback system and on the application of this system to production of high energy-resolution beams. The relationship between transfer functions for the two loops required for optimum operation is discussed. This system produces the beam position at the image slit of the 90 energy-analysing magnet with long-term stability equivalent to a 3.9 kV FWHM fluctuation of the terminal voltage. The concept of novel fast control loop utilizing the high-frequency component from the image slits to control the voltage of the last gap of high-energy acceleration tube is described

Ultrahigh impedance method to assess electrostatic accelerator performance

This paper describes an investigation of problem-solving procedures to troubleshoot electrostatic accelerators. A novel technique to diagnose issues with high-voltage components is described. The main application of this technique is noninvasive testing of electrostatic accelerator high-voltage grading systems, measuring insulation resistance, or determining the volume and surface resistivity of insulation materials used in column posts and acceleration tubes. In addition, this technique allows verification of the continuity of the resistive divider assembly as a complete circuit, revealing if an electrical path exists between equipotential rings, resistors, tube electrodes, and column post-to-tube conductors. It is capable of identifying and locating a “microbreak” in a resistor and the experimental validation of the transfer function of the high impedance energy control element. A simple and practical fault-finding procedure has been developed based on fundamental principles. The experimental distributions of relative resistance deviations (ΔR/R) for both accelerating tubes and posts were collected during five scheduled accelerator maintenance tank openings during 2013 and 2014. Components with measured ΔR/R>±2.5% were considered faulty and put through a detailed examination, with faults categorized. In total, thirty four unique fault categories were identified and most would not be identifiable without the new technique described. The most common failure mode was permanent and irreversible insulator current leakage that developed after being exposed to the ambient environment. As a result of efficient in situ troubleshooting and fault-elimination techniques, the maximum values of |ΔR/R| are kept below 2.5% at the conclusion of maintenance procedures. The acceptance margin could be narrowed even further by a factor of 2.5 by increasing the test voltage from 40 V up to 100 V. Based on experience over the last two years, resistor and insulator failures are less common now due to the routine use of the noninvasive ultrahigh impedance technique