Optimization and Automatic Control of Radio-Frequency Plasma Thrusters for Space Applications

Helicon Plasma Thrusters represent a recent and innovative type of space thrusters, currently receiving the attention of the research community thanks to (i) a very simple structure, based on a dielectric tube(discharge chamber) in which plasma is generated, an RF antenna for propellant ionization and a magneto-static field which confines and conveys the plasma, (ii) the lack of neutralization cathodes and other electrodes immersed in the plasma, resulting in a potentially long lifetime and (iii) the potential capability of operating with different propellants, both mono-atomic and molecular. This PhD thesis reports the development of an innovative experimental set-up for the characterization and optimization of Helicon Plasma Thrusters (HPT) at the electric propulsion facility of CISAS, composed of a test-bed, including a reconfigurable plasma source and an array of plasma diagnostics, and an innovative, high efficiency RF power generation system, intended to test kW-class thrusters. The activity was carried out both autonomously by the electric propulsion group and within the italian research program SAPERE - STRONG (Space Advance Project Excellence in Research and Enterprise -System, Technologies and Research for Global National Operativity)[1], aiming at the development of a kW-class HPT prototype working on Argon or CO2. The activity was carried out as a development of the technology and know-how deriving from program HPH.Com, carried out at CISAS from 2008 to 2012 with the objective of developing a 50 W HPT employing Argon [2], during which an innovative, high performance RF plasma source was developed. The new set-up constitutes in fact an upgrade of the existing one, which features (i) an high vacuum pumping system, (ii) a re-configurable low power plasma source and (iii) an array of plasma diagnostic systems, including a microwave interferometer for plasma density measurement, optical spectrometers and Faraday probes for plume current measurements. [3] The general configuration of the high power HPT, over which the design of the set-up was based, was determined by means of a plasma source global model, developed within another PhD work at CISAS. The resulting configuration employs roughly 30-80 times the mass flow rate of HPH and roughly 20-30 times the power. During these simulations the size of the thruster was minimized, resulting in a 7-10 times higher power density with respect to HPH.Com. The development of an high power set-up suitable for testing such thrusters presented a series of issues: * the technology of the plasma source, in particular with respect to the RF system, was developed and tested for operation with Argon 50-100 W operation, thus requiring further theoretical and experimental analyses in order to (i) characterize its performance with new propellants (namely CO2), (ii) optimize its design, (iii) investigate its power scalability, with particular attention to the increased power density, (iv) develop analysis and design tools for the RF system; * during the tests the thruster must be re-configured (in terms of geometry and magnetic field) and the input parameters (propellant type and mass ow rate, input power) are varied, resulting in a wide variety of possible operating regimes. The electrical impedance of the antenna will change accordingly, resulting in awidely variable electrical load for the power supply; * the existing plasma diagnostics, developed for HPH.Com, could not be suitable for operation at the desired power level with the chosen propellants; These issues were faced by means of a working program articulated in several phases. As a first step a theoretical and experimental survey was carried out on the RF circuit of the thruster, aiming at its frequency-domain and time-domain characterization both with and without plasma. The collected data were employed as a benchmarck from which to develop dedicated numerical codes, namely (i) a Simulink lumped-parameters model,employed for theoretical study and performance analysis of existing antennae, and (ii) an antenna preliminary design tool, based on a coupled Matlab/FEMM (Finite Elements Method Magnetics) capable of performance prediction from antenna geometry and materials. Along with this theoretical-experimental studys a refinement of theelectrical design of the thruster was performed by means of an experimental/numerical approach, resulting in the maximization of the electric field generated by the antenna at a given input voltage. This process was followed by a low power HPT optimization with the modified antenna. Operation with CO2 was explored with the existing low power set-up, in order to identify eventual issues related to the employment of this propellant. The results have shown that CO2 can be employed in the HPT without significant issues, but requires, for optimal performance, a different plasma source configuration with respect to Argon. In this occasion the employability of the existing plasma diagnostics with CO2 was analysed and verified. The capability of the vacuum pumping system and the plasma diagnostics of operation with the high power HPT was analyzed, concluding that the existing diagnostics are suited for high power operation. Issues related to the higher power density were explored by means of an intermediate power testing at 500 W with the HPH.Com set-up. This activity allowed to (i) identify thermal and electrical issues and their related solutions, (ii) investigate the power scaling of the HPT plasma source performance, in terms of emission spectra and plasma density The experience and the data collected during this work enabled the design and development of an highly reconfigurable plasma source, characterized by a variable-geometry discharge chamber and a magnetic field which can be varied in position, intensity and shape. Along with the work on the plasma source itself, a dedicated RF power generation system was developed in cooperationwith RESIA, a CISAS partner. The system is composed of (i) an innovative RF amplifier, capable of high efficiency operation with a widely variable load and (ii) an automated control system, capable of avoiding power re ections from the load in any operating condition. An high power HPT prototype was also subject to preliminary design, in order to study solutions useful for the prosecution of program SAPERE - STRONG. Chapters 1 and 2 provide a general introduction and an overlook on the existing set-up for HPT testing at CISAS; the theoretical and experimental study on the RF system is illustrated in Chapter 3, while Chapter 4 reports the antenna design refinement and low power thruster optimization activity. Chapter 5 treats all the experimental, theoretical and design activites related to the development of an high power test-bed capable of multi-propellant operation, while the design and deveolpment of the RF power generation system is illustrated in Chapter 6. As an accessory activity, the algorithms for Faraday probe data analysis were upgraded

Beam-forming capabilities of a plasma circular reflector antenna

A gaseous plasma antenna array (PAA) is an aggregate of plasma discharges and possibly conventional metallic radiating elements, and it constitutes a promising alternative to metallic antennas for applications in which fast reconfiguration of radiation pattern, and gain is desired; such properties can be achieved by exploiting the electronic switch on/off condition of plasma discharges, and tuning of the plasma parameters. Here, the authors present a reconfigurable PAA that features a central metallic half-wavelength dipole working around 1.45\u2005GHz, surrounded by a planar circular lattice of cylindrical plasma discharges. Customised plasma discharges have been realised, and filled with argon gas at 2\u2005mbar so as to have a complete control on the plasma discharge properties (e.g. plasma frequency, collisional frequency). The magnitude of the reflection coefficient, and the gain pattern on the H-plane have been investigated numerically and experimentally; numerical and experimental results exhibit a good agreement and show that the central intrinsically omnidirectional antenna can provide simple beamforming capabilities upon turning on a subset of plasma discharges; as these plasma discharges are turned on, the authors have observed a maximum gain of 3c5\u2005dBi, a half-power beam width of 80 18, and an angular steering resolution of 3c15 18

REGULUS CubeSat Propulsion System: In-Orbit Operations

A robust, versatile, and cost-effective propulsion system to provide wide mobility to small satellite platforms and nanosatellite deployers. A Plug&Play propulsion system designed to be easily integrated into different satellite platforms and to match customer\u27s requirements, with minimal customization efforts and costs

REGULUS Iodine Electric Propulsion System Integration in CubeSats’ Platforms and Testing

REGULUS is an electric propulsion (EP) system for CubeSats at TRL8 and now waiting for the IoD flight in late 2020. REGULUS system is provided for integration with all electronics, fluidic line, iodine tank and structures for total mass below 3 kg. Thanks in particular to the Magnetically Enhanced RF Plasma Thruster (MEPT) technology and the use of iodine propellant, the system can provide 3000Ns of total impulse in a 93.8 x 95.0 x 150.0 mm volume performance, fitting in a 1.5U Cubesat. REGULUS includes the whole propulsion package for integration in CubeSats and MicroSats as well as small CubeSat carriers. The system is composed by the thruster, the electronics (PPUs and PCU) the fluidic line and the tank. The main features of REGULUS are the presence of a simple architecture, a thruster with no neutralizer and grids, no high DC-voltage PPU and the use of solid iodine as propellant, that can be substituted with Xenon fluidic line and tank when required. Its first mission will be onboard of Unisat-7 by GAUSS. The flight will take place in late 2020 in a Soyuz flight. During the mission, REGULUS will allow Unisat-7 to perform an orbit descending maneuver, drag compensation in VLEO and decommissioning

Optimization and Automatic Control of Radio-Frequency Plasma Thrusters for Space Applications

Helicon Plasma Thrusters represent a recent and innovative type of space thrusters, currently receiving the attention of the research community thanks to (i) a very simple structure, based on a dielectric tube(discharge chamber) in which plasma is generated, an RF antenna for propellant ionization and a magneto-static field which confines and conveys the plasma, (ii) the lack of neutralization cathodes and other electrodes immersed in the plasma, resulting in a potentially long lifetime and (iii) the potential capability of operating with different propellants, both mono-atomic and molecular. This PhD thesis reports the development of an innovative experimental set-up for the characterization and optimization of Helicon Plasma Thrusters (HPT) at the electric propulsion facility of CISAS, composed of a test-bed, including a reconfigurable plasma source and an array of plasma diagnostics, and an innovative, high efficiency RF power generation system, intended to test kW-class thrusters. The activity was carried out both autonomously by the electric propulsion group and within the italian research program SAPERE - STRONG (Space Advance Project Excellence in Research and Enterprise -System, Technologies and Research for Global National Operativity)[1], aiming at the development of a kW-class HPT prototype working on Argon or CO2. The activity was carried out as a development of the technology and know-how deriving from program HPH.Com, carried out at CISAS from 2008 to 2012 with the objective of developing a 50 W HPT employing Argon [2], during which an innovative, high performance RF plasma source was developed. The new set-up constitutes in fact an upgrade of the existing one, which features (i) an high vacuum pumping system, (ii) a re-configurable low power plasma source and (iii) an array of plasma diagnostic systems, including a microwave interferometer for plasma density measurement, optical spectrometers and Faraday probes for plume current measurements. [3] The general configuration of the high power HPT, over which the design of the set-up was based, was determined by means of a plasma source global model, developed within another PhD work at CISAS. The resulting configuration employs roughly 30-80 times the mass flow rate of HPH and roughly 20-30 times the power. During these simulations the size of the thruster was minimized, resulting in a 7-10 times higher power density with respect to HPH.Com. The development of an high power set-up suitable for testing such thrusters presented a series of issues: * the technology of the plasma source, in particular with respect to the RF system, was developed and tested for operation with Argon 50-100 W operation, thus requiring further theoretical and experimental analyses in order to (i) characterize its performance with new propellants (namely CO2), (ii) optimize its design, (iii) investigate its power scalability, with particular attention to the increased power density, (iv) develop analysis and design tools for the RF system; * during the tests the thruster must be re-configured (in terms of geometry and magnetic field) and the input parameters (propellant type and mass ow rate, input power) are varied, resulting in a wide variety of possible operating regimes. The electrical impedance of the antenna will change accordingly, resulting in awidely variable electrical load for the power supply; * the existing plasma diagnostics, developed for HPH.Com, could not be suitable for operation at the desired power level with the chosen propellants; These issues were faced by means of a working program articulated in several phases. As a first step a theoretical and experimental survey was carried out on the RF circuit of the thruster, aiming at its frequency-domain and time-domain characterization both with and without plasma. The collected data were employed as a benchmarck from which to develop dedicated numerical codes, namely (i) a Simulink lumped-parameters model,employed for theoretical study and performance analysis of existing antennae, and (ii) an antenna preliminary design tool, based on a coupled Matlab/FEMM (Finite Elements Method Magnetics) capable of performance prediction from antenna geometry and materials. Along with this theoretical-experimental studys a refinement of theelectrical design of the thruster was performed by means of an experimental/numerical approach, resulting in the maximization of the electric field generated by the antenna at a given input voltage. This process was followed by a low power HPT optimization with the modified antenna. Operation with CO2 was explored with the existing low power set-up, in order to identify eventual issues related to the employment of this propellant. The results have shown that CO2 can be employed in the HPT without significant issues, but requires, for optimal performance, a different plasma source configuration with respect to Argon. In this occasion the employability of the existing plasma diagnostics with CO2 was analysed and verified. The capability of the vacuum pumping system and the plasma diagnostics of operation with the high power HPT was analyzed, concluding that the existing diagnostics are suited for high power operation. Issues related to the higher power density were explored by means of an intermediate power testing at 500 W with the HPH.Com set-up. This activity allowed to (i) identify thermal and electrical issues and their related solutions, (ii) investigate the power scaling of the HPT plasma source performance, in terms of emission spectra and plasma density The experience and the data collected during this work enabled the design and development of an highly reconfigurable plasma source, characterized by a variable-geometry discharge chamber and a magnetic field which can be varied in position, intensity and shape. Along with the work on the plasma source itself, a dedicated RF power generation system was developed in cooperationwith RESIA, a CISAS partner. The system is composed of (i) an innovative RF amplifier, capable of high efficiency operation with a widely variable load and (ii) an automated control system, capable of avoiding power re ections from the load in any operating condition. An high power HPT prototype was also subject to preliminary design, in order to study solutions useful for the prosecution of program SAPERE - STRONG. Chapters 1 and 2 provide a general introduction and an overlook on the existing set-up for HPT testing at CISAS; the theoretical and experimental study on the RF system is illustrated in Chapter 3, while Chapter 4 reports the antenna design refinement and low power thruster optimization activity. Chapter 5 treats all the experimental, theoretical and design activites related to the development of an high power test-bed capable of multi-propellant operation, while the design and deveolpment of the RF power generation system is illustrated in Chapter 6. As an accessory activity, the algorithms for Faraday probe data analysis were upgraded.I propulsori al plasma di tipo Helicon rappresentano una famiglia recente ed innovativa di propulsori spaziali, che sta attualmente ricevendo l'attenzione delle comunità di ricerca grazie ad (i) una struttura molto semplice, basata su un tubo in materiale dielettrico (la camera di scarica) in cui viene generato il plasma, un'antenna RF per propellente ionizzazione e un magnetostatico settore che limita e trasmette il plasma, (ii) la mancanza di catodi di neutralizzazione e di altri elettrodi immersi nel plasma, il che ne estende potenzialmente la vita operativa e (iii) la potenziale capacità di operare con diversi propellenti, sia mono-atomici che molecolari. Questa tesi di dottorato riporta lo sviluppo di un innovativo set-up sperimentale per la caratterizzazione e l'ottimizzazione di Helicon Plasma Thruster (HPT) presso la facility di propulsione elettrica del CISAS, comprendente un test-bed, composto da una sorgente di plasma riconfigurabile e un complesso di apparati di diagnostica di plasma, e un innovativo sistema di generazione di potenza RF ad alta efficienza, destinato a testare propulsori aventi potenze in ingresso dell'ordine del kW. L'attività è stata svolta sia autonomamente dal gruppo di propulsione elettrica e nell'ambito del programma di ricerca italiano SAPERE - STRONG (Space Advance Project Excellence in Research and Enterprise - System, Technologies and Research for Global National Operativity) [1], mirante allo sviluppo di un prototipo di HPT da 1-1.5 kW funzionante con Argon o CO2. L'attività costituisce uno sviluppo della tecnologia e del know-how derivante dal programma HPH.Com, portato avanti presso il CISAS dal 2008 al 2012 con l'obiettivo di sviluppare un HPT da 50 W impiegante Argon [2], durante il quale è stata sviluppata una innovativa sorgente dil plasma in RF ad alte prestazioni (Figura[1]). Il nuovo set-up costituisce infatti un aggiornamento di quello esistente, che presenta (i) un sistema di pompaggio ad alto vuoto, (ii) una sorgente di plasma riconfigurabile, progettata per bassa potenza (fino a 100 W) e (iii) una serie di dispositivi diagnostici per il plasma, compreso un interferometro a microonde per la misura della densità del plasma, spettrometri ottici e sonde di Faraday per le misure di corrente nel plume emesso dal motore. La configurazione generale dello HPT ad alta potenza, sulla base della quale quale è stata basata la progettazione del nuovo set-up, è stata determinata mediante simulazioni condotte con un modello globale della sorgente di plasma, sviluppato nell'ambito di un altro lavoro di dottorato presso il CISAS. La configurazione risultante impiega circa 30-80 volte la portata massica di HPH e circa 20-30 volte la potenza. Nelle simulazioni è stata imposta la minimizzazione delle dimensioni del thruster, in modo da competere più efficientemente con altre tecnologie propulsive caratterizzate da prestazioni similari (propulsori ad effetto Hall, propulsori a ioni), con il risultato di arrivare ad una densità di potenza per unità di superficie maggiore di 7-10 volte rispetto ad HPH.Com. Lo sviluppo del set-up ad alta potenza destinato a testare simili propulsori ha dovuto fronteggiare una serie di problematiche, che possono essere così riassunte: * la tecnologia alla base della sorgente di plasma, in particoalr modo per quanto riguarda il circuito in RF, è stata sviluppata e testata per impiego con Argon a livelli di potenza compresi fra 50-100 W. Ciò porta alla necessità di una ulteriore analisi teorica e sperimentale per (i) caratterizzarne le prestazioni con nuovi propellenti (CO2 nello specifico), (ii) ottimizarne il design, (iii) indagarne la scalabilità, in particolare tenendo conto della maggiore densità di potenza, (iv) sviluppare strumenti di analisi e design del circuito in RF; * durante le prove il propulsore deve essere ri-congurato (in termini di geometria e campo magnetico) ed i parametri di input (tipo di propellente, portata di massa, potenza di ingresso) vengono variatii, portando ad una grande varietà di possibili regimi di funzionamento. L'impedenza elettrica dell'antenna cambierà di conseguenza, venendo vista come un carico elettrico ampiamente variabile dal sistema di generazione di potenza elettrica; * gli apparati diagnostici per il plasma esistenti sono stati sviluppati durante HPH.Com per essere usati su plasmi di Argon a bassa potenza. Ciò implica che i loro range di funzionamento potevano non essere adatti per il funzionamento ad alta potenza e/o per l'impiego con altri propellenti ; *l'accresciuta densità di potenza rispetto ad HPH comporta maggiori carichi termici e più alte tensioni e correnti, con rischi di danneggiamento potenzialmente accresciuti. Questi problemi sono stati affrontati tramite un programma di lavoro articolato in varie fasi. Come primo passo è stata condotta un'indagine teorica e sperimentale a bassa potenza sul circuito RF del propulsore, volta a caratterizzarne l'impedenza complessa nel dominio della frequenza e nel dominio del tempo, con e senza il plasma. Sulla base dei dati raccolti è stato possible sviluppare (i) un affidabile modello a parametri concentrati, implementato in Matlab e Simulink, impiegato per studi teorici e per l'analisi prestazionale delle antenne esistenti e (ii) uno strumento numerico di progettazione preliminare per le antenne, sulla base di un algoritmo accoppiato Matlab / FEMM (Finite Elements Method Magnetics) capace di prevedere le prestazioni elettriche di un'antenna sulla base della sua struttura e dei materiali. Assieme a questo studio teorico/sperimentale è stato portato avanti anche un processo di raffinazione del design del circuito RF del propulsore, condotto anche in questo caso per mezzo di test sperimentali e analisi numeriche, miranti alla massimizzazione del campo elettrico generato dall'antenna per un dato voltaggo applicato ai suoi morsetti di ingresso. Questo processo è stato seguito da una ottimizzazione bassa potenza dell'HPT con l'antenna modificata, allo scopo di caratterizzarne le prestazioni in configurazione aggiornata. L'incognita circa i funzionamento con CO2 è stata affrontata mediante test a bassa potenza con tale gas, condotti impiegando il set-up di HPH.Com, al fine di individuare eventuali problematiche legate all'impiego di questo propellente. I risultati hanno dimostrato che la CO2 può essere impiegata come propellente senza problemi significativi, ma richiede, per ottenere prestazioni ottimali, una configurazione sorgente di plasma differente rispetto all' Argon. In questa occasione l'impiegabilità delle diagnostiche di plasma esistenti con CO2 è stato analizzata e verificata. La capaciàa del delle diagnostiche di operare ai regimi di potenza e portata massica richiesti dal nuovo propulsore è stata analizzata, concludendo che gli apparati attualmente disponibili sono adatti per il funzionamento ad alta potenza. Gli effetti della superiore densità di potenza sono stati esplorati tramite test alla potenza intermedia di 500 W con il set-up di HPH.Com, sottoponendolo dunque ad una densità di potenza 10 volte superiore a quella già testata. Questa attività ha permesso di (i) individuare i problemi termici/elettrici legati alla maggiore densiàa di potenza ed elaborare relative soluzioni, (ii) studiare il modo in cui le prestazioni della sorgente di plasma sviluppata per gli HPT scalano con la potenza, in termini di spettri di emissione e densità del plasma, (iv) verificare che non insorgano fenomeni di instabilità di plasma. L'esperienza ed i dati raccolti durante questo lavoro hanno reso possiblie la progettazione e lo sviluppo di una sorgente di plasma riconfigurabile, caratterizzata da una camera di scarica a geometria variabile e di un campo magnetico che può essere variato in posizione, intensità e forma. Parallelamente allo sviluppo della sorgente di plasma è stato inoltre portato avanti lo sviluppo di un sistema di generazione di potenza in RF. Tale attività è stata condotta in cooperazione con RESIA, un partner del CISAS durante HPH.Com, specializzato in realizzazioni elettroniche non convenzionali. Il sistema è composto da (i) un innovativo amplificatore RF, in grado di funzionare ad alta efficienza con un carico ampiamente variabile e (ii) un sistema di controllo automatizzato, in grado di evitare le riflessioni di potenza dal carico in ogni condizione operativa. In aggiunta a questa attività si è inoltre proceduto alla progettazione preliminare di un HPT ad alta potenza impiegante anidride carbonica come propellente, allo scopo di identificare soluzioni progettuali utili per la futura prosecuzione del programma SAPERE - STRONG. I capitoli 1 e 2 forniscono un'introduzione generale ed un preliminare sguardo d'insieme sul set-up per il testing e lo sviluppo di propulsori Helicon al CISAS; lo studio teorico e sperimentale sul sistema RF è illustrato nel capitolo 3, mentre il capitolo 4 riporta il processo di raffinazione del design dell'antenna e la successiva ottimizzazione a bassa potenza del propulsore. Il capitolo 5 tratta tutte le attività sperimentali, teoriche e progettuali legate allo sviluppo di un banco di prova ad alta potenza in grado di funzionare con diversi propellenti, mentre il design e deveolpment del sistema di generazione di potenza RF è illustrato nel capitolo 6. Come attività accessoria sono stati aggiornati gli algoritmi per l'analisi dei dati della sonda di Faraday