A dual Stage ion engine for high impulse missions

In this paper, the applicability of dual stage ion opticsand in particular of the so-called dual stage ion engine to highpower, high specific impulse missions will be evaluated. First, theperformance limits of conventional two gridded ion engines (GIE)will be discussed and the advantages provided by dual stage ionengines reported. The limits of applicability of a dual stage ionengine will be analyzed analytically and the results confirmednumerically. The lifetime and performance of a three griddeddual stage ion engine (DS3G) will be numerically investigated andcompared to those of a conventional GIE assessing for the firsttime in the open literature under what condition dual stage ionoptics provide performance improvements over conventional GIEsand what is its impact on the thruster lifetime. Dual stage ionengines have been found to be capable of providing higher thrustdensity and longer lifetime with respect to conventional griddedion engines

Hollow cathode chemical modelling

In this paper the state of hollow cathode life time modelling at the University of Southampton will be reported. Two models have been developed: one for BaO depletion from the hollow cathode insert and another for low work function compounds deposition and desorption. The model developed to predict BaO depletion from hollow cathode insert will be presented together with some comparison between experimental and numerical data to prove its validity.A model for low work function compounds deposition and desorption will also be presented. This model will be used to simulate the NSTAR cathode showing a very conservative estimate of the cathode life due to conservative character of the hypotheses made in the model development and due to the chosen criteria for the end of life

Solar energetic particle events: phenomenology and prediction

Solar energetic particle events can cause major disruptions to the operation of spacecraft in earth orbit and outside the earth's magnetosphere and have to be considered for EVA and other manned activities. They may also have an effect on radiation doses received by the crew flying in high altitude aircraft over the polar regions. The occurrence of these events has been assumed to be random, but there would appear to be some solar cycle dependency with a higher annual fluence occurring during a 7 year period, 2 years before and 4 years after the year of solar maximum. Little has been done to try to predict these events in real-time with nearly all of the work concentrating on statistical modelling. Currently our understanding of the causes of these events is not good. But what are the prospects for prediction? Can artificial intelligence techniques be used to predict them in the absence of a more complete understanding of the physics involved? The paper examines the phenomenology of the events, briefly reviews the results of neural network prediction techniques and discusses the conjecture that the underlying physical processes might be related to self-organised criticality and turbulent MHD flows

Results of the qualification test campaign of a Pulsed Plasma Thruster for Cubesat Propulsion (PPTCUP)

Pulsed Plasma Thruster for Cubesat Propulsion (PPTCUP) is an ablative pulsed plasma thruster designed with the aim of providing translational and orbital control to Cubesat platforms. The qualification model presented in this paper has been developed by Mars Space Ltd, Clyde Space Ltd and the University of Southampton to produce a versatile “stand-alone” module that can be bolted on the Cubesat structure, allowing the orbital control along the X or Y-axis of the satellite. An extensive and complete test campaign to qualify the unit for space flight, which includes electromagnetic compatibility (EMC) characterization, thermal cycling and mechanical tests, has been performed according to the NASA GEVS procedures. PPTCUP is characterized by an averaged specific impulse of 655±58 s and a deliverable total impulse of 48.2±4.2 Ns. Finally, it has been found that the unit is compliant with the EMC requirements and can successfully withstand the thermal and mechanical loads typical of a Cubesat space mission

Development of an engineering optimization tool for miniature Pulsed Plasma Thrusters

Pulsed Plasma Thrusters (PPT) are an established technology for compact thrust propulsion systems. Although PPT optimization has been performed previously it requires complex numerical codes. A 0D pulsed inductive acceleration model has been developed which links together the dynamics of the current sheet with the plasma dimensions and ionization processes. The model novelty is in a self-consistent estimation of the plasma sheet properties (temperature, density, thickness) driven by the magnetic pinch pressure and propellant ablation together with its simplicity. Parametric studies have been performed in an attempt to arrive at optimized design solutions for small PPTs

Development of an engineering optimization tool for miniature pulsed plasma thrusters

Pulsed Plasma Thrusters (PPT) are an established technology for compact thrust propulsion systems. Although PPT optimization has been performed previously it requires complex numerical codes. Although the scaling laws have been suggested they mainly applicable for large thrusters when edge effects can be neglected. A new 0D pulsed inductive acceleration model has been developed which links together the dynamics of the current sheet with the plasma dimensions and ionization processes. The model novelty is in a self-consistent estimation of the plasma sheet properties (temperature, density, thickness) driven by the magnetic pinch pressure and propellant ablation together with its simplicity. Parametric studies have been performed in an attempt to arrive at modified scaling laws for small PPTs

Investigation of heaterless hollow cathode breakdown

The development of long life high powered (>50A) hollow cathodes is of importance to meet the demand of increasingly powerful Gridded Ion engines and Hall Effect thrusters. High power cathodes typically operate at greater temperature ranges, which poses a significant challenge to maintain heater reliability. The heater component commonly used to raise the insert to emissive temperatures, has inherent reliability issues from thermal fatigue caused by thermal cycling with large temperature variations. A self-heating hollow cathode allows for potentially higher reliability through design simplicity of removing the heater component, and in addition there can be savings in mass, volume, ignition time and power. This study characterizes the initiation of the start-up process for a heaterless hollow cathode. As such the study analyses conditions of the initiation as a function of detailed geometrical and physical parameters. The Paschen curve can be seen to give a qualitative explanation for the breakdown voltage variance. The quantitative variations between the empirical results and Paschen curve are discussed in relation to non-uniform pressure simulations

An investigation into the glow discharge phase of an LaB6 heaterless hollow cathode

Hollow cathodes typically operate through the use of low work function emitters to deliver thermionic current. To achieve high thermionic current the emitters require heating to around 1500 K for barium oxide cathodes and over 1900 K for lanthanum hexaboride cathodes. Conventionally a heater component is utilised to raise the emitter to the required thermionic temperatures for ignition, however this has drawbacks: firstly additional mass and volume for the heater component is required, secondly there are reliability issues due to thermal cycling and high temperature variation, and finally there are long ignition times, up to 10 minutes, due to indirect heating of the insert. Thus replacing the heater component with a simpler and potentially faster ignition system will be highly advantageous. Conventional hollow cathodes can be cold started, though this leads to high voltages combined with unacceptable mass flow rates (order of magnitude higher).We are investigating an alternative approach to ignition by developing dedicated heaterless hollow cathodes (HHC) that meet the internal pressures required at nominal mass flow rates. In which the emitter heating is driven by a discharge between the keeper and the emitter. This method allows for direct heating of the emitter, lowering the overall HHC ignition time to as low as 2 seconds, without requiring additional components. Though to date HHC’s have only demonstrated lifetimes of hundreds of hours. This is primarily due to the absence of thermionic emission during the breakdown stage, such that higher breakdown potentials are used compared with conventional ignition. Hence the sputter erosion yields can be higher due to the higher energy ion bombardment and in addition cathodic spots can form through ignition, due to over powering, thus causing high localised erosion. This study investigates a novel power switching sequence to ignite the heaterless hollow cathode, which can enable repeatable ignition at relatively low voltages (<500V) and flow rates (<20 sccm), thus resulting in low erosion. This is achieved though adapting the voltage and current though through ignition to understand their influence on repeatability and erosion. This is examined through an experimental campaign conducted on the 20A heaterless hollow cathode under development at the University of Southampton. Results have shown that discharge stability can be increased by limiting current though the use of electrical ballasts due to the plasmas negative resistance characteristics observed. Erosion analysis is being conducted though the following diagnostics: scanning electron microscope for erosion detection, spectroscopy for species identification and periodic mass measurements for erosion quantification

Asteroseismology of red giants & galactic archaeology

Red-giant stars are low- to intermediate-mass ($M \lesssim 10$~M$_{\odot}$) stars that have exhausted hydrogen in the core. These extended, cool and hence red stars are key targets for stellar evolution studies as well as galactic studies for several reasons: a) many stars go through a red-giant phase; b) red giants are intrinsically bright; c) large stellar internal structure changes as well as changes in surface chemical abundances take place over relatively short time; d) red-giant stars exhibit global intrinsic oscillations. Due to their large number and intrinsic brightness it is possible to observe many of these stars up to large distances. Furthermore, the global intrinsic oscillations provide a means to discern red-giant stars in the pre-helium core burning from the ones in the helium core burning phase and provide an estimate of stellar ages, a key ingredient for galactic studies. In this lecture I will first discuss some physical phenomena that play a role in red-giant stars and several phases of red-giant evolution. Then, I will provide some details about asteroseismology -- the study of the internal structure of stars through their intrinsic oscillations -- of red-giant stars. I will conclude by discussing galactic archaeology -- the study of the formation and evolution of the Milky Way by reconstructing its past from its current constituents -- and the role red-giant stars can play in that.Comment: Lecture presented at the IVth Azores International Advanced School in Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta, Azores Islands, Portugal in July 201