Thermionic cathodes are essential for the operation of various electrostatic propulsion devices. They strongly influence the performance and lifetime of the propulsion system. In this study, a 1 A-class LaB6 laboratory model hollow cathode has been tested with krypton in diode and triode configurations in order to assess the cathode discharge mode transition behavior. Measurements have been performed over a range of krypton mass flow rates (0.1, 0.15, and 0.21 mg/s, or 1.6, 2.4, and 3.4 sccm), keeper (0.1, 0.15, and 0.2 A), and anode currents (0.1–1 A) at a fixed cathode-to-anode distance. Seven criteria were used to distinguish between the spot and plume mode operations. The results show that the mode transition in low-current cathodes may be a nonlinear phenomenon, and only some of the existing mode transition criteria can be used to accurately predict the spot/plume discharge regions at low emission currents.

The age of space electric propulsion arrived and found the space exploration endeavors at a paradigm shift in the context of new space. Mega-constellations of small satellites on low-Earth orbit (LEO) are proposed by many emerging commercial actors. Naturally, the boom in the small satellite market drives the necessity of propulsion systems that are both power and fuel efficient and accommodate small form-factors. Most of the existing electric propulsion technologies have reached the maturity level and can be the prime choices to enable mission versatility for small satellite platforms in Earth orbit and beyond. At the Plasma Sources and Applications Centre/Space Propulsion Centre (PSAC/SPC) Singapore, a continuous effort was dedicated to the development of low-power electric propulsion systems that can meet the small satellites market requirements. This review presents the recent progress in the field of electric propulsion at PSAC/SPC Singapore, from Hall thrusters and thermionic cathodes research to more ambitious devices such as the rotamak-like plasma thruster. On top of that, a review of the existing vacuum facilities and plasma diagnostics used for electric propulsion testing and characterization is included in the present research.

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The development of plasma-based propulsion thrusters for spacecraft has seen a rapid growth over the past few decades, with the number of spacecraft including small satellites and Cubesats increasing exponentially. Although traditional chemical propulsion is still widely employed in space flights, it cannot meet the more challenging requirements for deep space travel due to low specific impulse. Electric propulsion thrusters have already helped humans travel further from Earth and have the potential to be developed for interstellar flights due to their advantages such as high velocity increments, long operational lifetimes, high impulse-to-weight ratios and high impulse-to-power ratios. The electrodynamic thrusters have significant potential for applications in the remote regions of space, and several types of electrodynamic plasma thrusters are currently under investigation. In this paper we present conceptual experiments to study a miniaturized Rotamak-type device initially proposed for the controlled thermonuclear fusion, with a view to assess its potential for the application as a small space thruster. An outline of the physical characteristics of the experiments that has been carried out, and measurements were done to try to elucidate the important mechanisms at work in the Rotamak, which will help design next-generation thruster capabilities. A discussion is also presented about the Rotamak systems and the opportunities they present for space applications.

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