Publications
A hollow cathode with a modular design has been developed to assist with laboratory testing of plasmabased thrusters for satellite applications. This novel modular design includes interchangeable components for varying the geometry and tailoring the configuration to specific applications, as well as easing the replacement of individual components in the case of damage. The modular hollow cathode also presents unconventional design features aimed at improving the heating efficiency: the heater is in direct contact with the emitter and the keeper is not in physical contact with the cathode base. The modular hollow cathode development has been based on a combination of theoretical modelling and experimental testing. The influence of the novel mechanical assembly has been investigated by characterising the operational envelope at different propellant mass flow rates for xenon and krypton. The modular hollow cathode has demonstrated stable operation by sustaining discharge currents between 0.5 and 4 A at different conditions. Finally, the cathode has been coupled with a Hall-type plasma thruster operating in the 0.3–2.5 A anode current range. This paper outlines development, experimental validation of this peculiar mechanical cathode configuration, covering plasma and thermal modelling, standalone testing, and coupled Hall-type thruster operation.
The possibility of efficiently exploiting Very Low Earth orbits (VLEO) poses significant technological challenges. One of the most demanding constraints is the need to counteract the drag generated by the interaction of the spacecraft with the surrounding atmosphere. Funded by the European Commission under the H2020 programme, the Air-breathing Electric THrustER (AETHER) project aims at developing the first propulsion system able to maintain a spacecraft at very-low altitudes for an extended time. The main objective of the project is to demonstrate, in a relevant environment, the critical functions of an air-breathing electric propulsion system, and its effectiveness in compensating atmospheric drag. This achievement will involve multiple research activities, among which: (i) the characterization of specific application cases through an extensive market analysis in order to define specific requirements and constraints at different design levels, (ii) fulfilment of pertinent testing conditions of flight conditions on-ground, relevant to the specific mission cases, (iii) the development of critical technologies, in particular those relevant to the collection, the ionization and the acceleration of rarefied atmospheric mixtures and (iv) the testing of the RAM-EP thruster to assess the system performance. In this paper, the main activities foreseen in the AETHER project are described, providing the detailed perspective towards an effective exploitation of the project outcomes for a possible future in-orbit demonstration.
Abstract The air-breathing electric propulsion (ABEP) concept refers to a spacecraft in very-low Earth orbit (VLEO) ingesting upper atmospheric air as propellant for an electric thruster. This compensates atmospheric drag and allows the spacecraft to maintain its orbital altitude, removing the need for on-board propellant storage and allowing an extended mission duration which is not limited by propellant exhaustion. There is a need for development of a robust, high current density and long life cathode (or neutralizer) for air-breathing electrostatic thrusters as conventional thermionic hollow cathodes are susceptible to oxygen poisoning. An Air-breathing Microwave Plasma CAThode (AMPCAT) is proposed to overcome this issue through the use of a microwave plasma discharge, producing an extracted current in the order of 1 A with 0.1 mg s-1 of air. In this paper, the effect of varying magnetic-field strength and topology is investigated by using an electromagnet coil, which reveals a significantly different behaviour for air compared to xenon. The extracted current with xenon increases by 3.9 times from the zero-field value up to a peak around 150 mT magnetic-field strength at the antenna, whereas an applied field does not increase the extracted current with air at nominal conditions. A non-zero magnetic-field with air is however beneficial for current extraction at reduced neutral densities. A distinct increase in extracted current is identified at low bias voltages with air for a field strength of around 50 mT at the internal microwave antenna, consistent across varying field topologies. The effect of a lowered magnetic-field strength in the orifice region is investigated through the use of a secondary coil, resulting in an extracted current increase of 25 % for a relaxation from 6 mT to 1 mT, and demonstrating the beneficial impact of a locally reduced field strength on electron extraction.
The work presented in this paper addresses specific issues of plasma cathodes operated on alternative propellants aiming at improving the current extraction of the technology using a novel DC plasma neutraliser developed by a partnership between the Surrey Space Centre and the Institute for Plasma Science and Technology of the Italian Research Council. This neutraliser uses thin diamond films on metallic disks (the cathode). The experimental development of the diamond-based plasma neutraliser (DBC) is described along with the preliminary characterisation and the resulting operating envelope. The highest achieved performance by the DBC was 0.215 A at 125 W and 10 sccm (0.67 mg/s) mass flow rate of krypton. Efficiency metrics (such as electron extraction power efficiency, and gas utilisation factor) for the neutraliser were compared for different diamond cathodes with unique film properties and operating conditions.