IEPC Presented at the 35th International Electric Propulsion Conference Georgia Institute of Technology Atlanta, Georgia USA

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1 Highly Efficient Miniaturized Hall-Type Thruster and Rotamak Projects in Plasma Sources and Application Centre at the Nanyang Technological University, Singapore IEPC Presented at the 35th International Electric Propulsion Conference Georgia Institute of Technology Atlanta, Georgia USA Shuyan Xu, 1 Mark Lim, 2 Shiyong Huang 3, Luxiang Xu 4, Igor Levchenko Plasma Sources and Applications Centre / Space Propulsion Centre Singapore, Nanyang Technological University Kateryna Bazaka 5 and Igor Levchenko 6 School of Chemistry, Physics, and Mechanical Engineering Queensland University of Technology, Australia Abstract: This paper presents the status of the development of electric propulsion technology at the Plasma Sources and Applications Centre / Space and Propulsion Center Singapore (PSAC-SPCS), National Institute of Education, Nanyang Technological University, Singapore. Our team at PSAC-SPCS, NIE is focused on the development, commissioning, optimization and operation of two types of highly distinctive space propulsion systems a miniaturized Hall-thruster for cube- and nanosats propulsion, and a radio frequency rotating magnetic field driven Gradually-Expanded-Rotamak (GER) electromagnetic thruster. Conceptualization, physical understanding and modelling, engineering development and performance characterization are under the main focus of the work. The supporting technologies, including Space Environment Simulation Facility, Thruster Performance Measurement System, and in situ Plasma Diagnostics System will also be briefly discussed. P N sp F = Power, W = Specific impulse, s = Thrust, N Nomenclature I. Introduction icro and nanosatellites including Cubesats are in focus of the present day space exploration trends. Due to the Mdrastic miniaturization of electronics and power systems, small satellites are now capable of performing the whole spectrum of functions intrinsic to large orbital system, with the apparently higher affordability of small systems which could be launched in tens and even hundreds at a time, thus significantly reducing the launch price. Satellite constellations featuring stunning possibilities could be imagined, but small satellites need small yet robust, long-operating, highly efficient thrusters 1-4. Newly established Space and Propulsion Center at Nanyang Technological University, Singapore, has started several fascinated project on development of miniaturized space propulsion platforms Professor, National Institute of Education / Plasma Sources and Applications Center, shuyan.xu@nie.edu.sg 2 Research Scientist, Plasma Sources and Applications Center, lim_jian_wei_mark@moe.edu.sg 3 Research Scientist, Plasma Sources and Applications Center, shiyong.huang@nie.edu.sg 4 Research Scientist, Plasma Sources and Applications Center, luxiang.xu@nie.edu.sg 5 Assistant professor, School of Chemistry, Physics, Mechanical Engineering, kateryna.bazaka@qut.edu.au 6 Research Scientist / Adjunct Professor, Plasma Sources and Applications Center, levchenko.igor@nie.edu.sg 1

2 II. Miniaturized Hall Thrusters The miniaturized Hall-type thruster project is one of the major directions of the studies being conducted in the Plasma Sources and Applications Centre / Space and Propulsion Center (PSAC-SPCS), NIE, Nanyang Technological University, Singapore. Currently, we have developed the base design and several modifications of Hall thrusters of mm dia. form-factor, and highly efficient cathode capable of ensuring very high energy efficiency of the developed thruster. Intense fire tests are in progress with the help of custom-designed Space Simulator involving large (about 10 m3) vacuum chamber and powerful pumping and heating/cooling system, as described in more details below. Basically, we have designed and tested the two modifications of the Hall thrusters, namely classically designed platform with the embracing electromagnetic oils, and hybrid system which incorporates Samarium-Cobalt permanent magnets to create the base magnetization in the magnetic circuit, and internal adjustment coil to precisely control the shape and strength of the magnetic field in the channel. Figure 1 below shows some features of the designed system. Figure 1. General design (a), 3D model (b), and the two variants of the magnetic system of miniaturized Hall thruster designed at PSAC/SPCS (c,d). Internal coil has 1000 turns of 0.3 mm wire. A set of 24 to 50 SamariumCobalt permanent magnets ensure the principal magnetization, inner coil allows for a fast, flexible adjustment of the magnetic field configuration. 2

3 Figure 2. Photographs of the operating miniaturized Hall thruster designed at PSAC/SPCS, side (a) and face (b) views. Xenon supply rate is 1 mg s -1, power 100 W, discharge voltage 200 V. No overheating and excessive channel wall wear were detected. Several hundred switch pulses were fired without any fault. Magnetic field at the bottom of channel is only fractions of µt, which can nevertheless help to guide slow electrons towards anode. The Singapore-based Plasma Sources and Application Centre / Space Propulsion Centre, Singapore (PSAC/SPCS) has developed, tested and optimized the two highly efficient versions of the miniaturized Hall-type thrusters: a version with electromagnetic coils, and a hybrid version with external permanent magnets and internal adjustment coil (Figures 3 and 4). Both versions were tested and optimized, and similar results were obtained (see Table I which lists the main parameters and characteristics of these thrusters). Specifically, these thruster feature the folowing advantages: 1. Highly efficient, light (80 g) thermionic cathode capable of working without external heating, consuming only 0.1 mg s -1 of xenon; 2. Optimized anode ensuring very uniform supply of working gas to the accelerating chamber, and proper position relatively to magnetic field; 3. Optimized configuration of magnetic field for the both coil-based and permanent-magnet based design versions, with the inner coil used for a fine adjustment of the filed configuration at the channel face; 4. Very compact light-weight design, not exceeding 450 g for the coil-based version and 350 g for the permanent magnet-based version; 5. Short ignition time (several seconds), including cathode heating up. Table I. The main parameters of Hall thruster. Parameter Basic Hybrid Mass, g Thrust, mn Power, W Operating voltage, V Operational current, A Thrust at 50 W 3 mn 3 mn Xe flow rate, mg s Specific impulse, s Efficiency, % Proven life, h 1000 Under test 3

4 Figure 3. Measured characteristics of the miniaturized thruster. (a) Dependence of thrust on applied power P and propellant flow rate; (b) Dependence of specific impulse Isp on applied power and propellant flow rate. The thruster has demonstrated relatively high efficiency (35 %) with very small integral plasma divergence angle not exceeding 28 (measured in situ during the thruster operation in chamber using rebotised measured system, described below), thus strongly confirming the optimization approach. III. Rotamak-Like Thruster Project Spherical plasma sources and related Rotamak configurations have been studied extensively since the late 1970 s as a potential candidate for realizing nuclear fusion as a source of long-term energy security. A spherical discharge vessel, through manipulation of the configurations of the external sources enables one to operate in a field reversed configuration and other current drive schemes. The far reaching implications of earlier theoretical and experimental studies of the spherical plasma source laid the groundwork for the development of a gradually expanded rotamak (GER) device as an up and coming contender for electric propulsion. The Space and Propulsion Centre (SPC) in Singapore at the National Institute of Education, Nanyang Technological University has concluded preliminary studies on the utilization of a spherical plasma source for propulsion based applications. Figure 4. PSAC s Gradually-Expanded-Rotamak prototype (lab model) operating. Intense stable plasma is ignited and sustained in a wide range of parameters. 4

5 Figure 5. (a) Artistic representation of thruster based on the Gradually-Expanded-Rotamak principle (currently under development in PSAC/SPCS); (b) Measured dependence of electron temperature and electron density on radio frequency power. Preliminary studies done at the SPCS have shown the ability for the generation of dense plasmas in a spherical vessel that can be sustained with a pair of parallel RF coils mounted outside the confinement vessel. The schematic diagram of the experimental set-up of the discharge vessel is shown in Figure 18. The spherical discharge vessel was constructed out of Pyrex and measures 28 cm in its internal diameter. The discharge vessel is also outfitted with connections to a vacuum pump out suite as well as gas inlets for the introduction of discharge gases. The vacuum suite used in this work comprises of a two-stage rotary and turbo-molecular pump system which enables a pumping capacity of 450 l/s and a base pumping pressure of ~ Pa. The working gas used in this work is high-purity argon which is introduced into the vessel through a glass tube connecter through a KYKY mass flow controller which accurately controls the flow rate. IV. Test Facilities at the Plasma Sources and Applications Centre / Space and Propulsion Center, Singapore The test facilities / Space Conditions Simulation system built at the Plasma Sources and Applications Centre / Space and Propulsion Center, Singapore, is based on the large vacuum chamber which features the following main parameters: Dimensions: 4.75 m (L) 2.3 m (D) Base pressure: ~ Pa Pumping: Dry + Turbo + Cryo pumps Pumping rate: 50, 000 L/s Leakage : Pa L/s Workload: < 500 kg Plume diagnostics: thrust, impulse, efficiency, IEDF, EEDF, RGA. The robotic actuation suite comprising digital electronics, microcontrollers, custom-made hardware and accessorial software for diagnostics and characterization of electric propulsion thrusters in a large vacuum space environment simulator was also designed, built and tested. The components of the characterization suite are have been aggregated into modular add-on units for rapid re-arrangement and easy customization to suit various applications. The modules include a force calibration system for accurate thrust measurements in various thrust stands, and spatially resolved measurements in a multi-probe array comprising Langmuir and Faraday probes for plume diagnostics of plasma thrusters. Plume and thrust characteristic of a miniaturized Hall-type thruster were successfully measured insitu, enabling users to vary process controls and efficiently optimize the thruster performance. The system revealed a further potential for real-time flight mission diagnostics and control. The developed system has been deployed in a large space simulation facility, and the systems were tested with an operational Hall thruster developed by the Centre. 5

6 Figure 6. (a) Photograph of thruster in chamber; (b) Chamber before attachment of pumps and other equipment; and (c) equipment installation on the chamber. VI. Conclusion Miniaturized electric propulsion devices are the heart and pivotal sub-system of small satellites and satellite systems. Further exploration of Moon, manned Mars exploration, sending long-living probes to Jupiter and Saturn, comets, asteroids and deep space, and much more intense usage of near-earth space for the benefit of the whole mankind for advanced communication, global internet access, precise weather prediction and many other practical aims. All these tasks require efficient, reliable, robust control systems capable of controlling the spacecraft velocity vector, as well as orientation and location in space with the maximum possible mass and energy efficiency of the propulsion devices (thrusters). They should work in adverse space conditions (low and high temperatures and extremely high rates of temperature change, vacuum, radiation, possible attack of high-speed dust particles) for long time reaching years, with very high system fault tolerance. In this paper we have briefly outlined the works at the Plasma Sources and Applications Centre / Space and Propulsion Center Singapore (PSAC-SPCS), National Institute of Education, Nanyang Technological University, Singapore, on building the electric propulsion capabilities and designing, testing and optimization of several various types of miniaturized electric thrusters. Several efficient systems were successfully designed, built and tested. Acknowledgments The authors acknowledge the kind technical support of K. L. Ong, E. L. Goh, Y. J. C. Chan, J. W. Lim, G. S. Tan and all other members of PSAC-SPCS. This work was supported by the National Research Foundation, OSTin, and Academic Research Fund AcRF RP 6/16 SX. J. W. M. Lim acknowledges the financial support of the NIE PhD scholarship. I. L. acknowledges the support from the School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology. References 1 Mazouffre, S., EP elements and technologies: Electric propulsion for satellites and spacecraft: established technologies and novel approaches, Plasma Sources Sci. Technol. 25, 2016, Lukas, J., Teel, G., Kolbeck, J. & Keidar, M, High thrust-to-power ratio micro-cathode arc thruster, AIP Advances 6, 2016, Levchenko, I., Keidar, M., Ostrikov, K., Electron transport across magnetic field in low-temperature plasmas: An alternative approach for obtaining evidence of Bohm mechanism, Physics Letters A 373, 2009, pp Walker, M., Electric Propulsion, Aerospace America 12, 2005, pp K. Lemmer, Propulsion for CubeSats, Acta Astronautica 134, 2017, Anders, A., Schein, J., Q,i N., Pulsed vacuum-arc ion source operated with a "triggerless" arc initiation method. Rev. Sci. Instrum., vol.71, no.2, pt.1-2, Feb. 2000, pp Charles, C., Boswell, R.W., Bish, A., Khayms, V. and Scholz, E. F., Direct Measurement of Axial Momentum Imparted by an Electrothermal Radiofrequency Plasma Micro-Thruster, Front. Phys. 4, 2016, Levchenko, I., Bazaka, K., Keidar, M., Xu, S., Fang, J., Hierarchical multi-component inorganic metamaterials: intrinsically driven self-assembly at nanoscale, Adv. Mater., in press 2017, DOI: /adma