A simple theoretical model for ion emission from ionic liquids

Mechanism for thrust generation for an ionic liquid electrospray thruster using porous glass: the next-generation ultra-compact, energy-saving, and highly efficient space propulsion device

Key research points

An easy-to-use theoretical model was developed that can calculate the ion current emitted from a porous glass emitter. The modelled phenomenon is of significant interest as a propulsion system for small satellites.
The model successfully reproduced experimental results for the ion current with high accuracy.
Use of the model is benefitial to predict and design the performance of porous electrosprays, enabling high-ΔV missions by small satellites.

Summary

A simple model that can calculate thrust for a propulsion system that is promising for small satellite design has been developed. The new theoretical model does not require the challenging multi-scale analysis used by previous calculations, and can replicate the physical mechanism of thrust generation by ion emission from a porous emitter impregnated with ionic liquid in a vacuum. The model successfully reproduced the experimental results of the emitted ion current with high accuracy. It is expected that this model will be important for improving the performance of space propulsion using ionic liquids. The results of this research were published in the international scientific journal "Journal of Applied Physics" (date: June 27th, 2024) and were selected by the journal as a featured article for the particularly noteworthy research result.
The commissioned research was performed at the JAXA Institute of Space and Astronautical Science (ISAS) by TAKAGI Koki (second year doctoral student at Yokohama National University, JST Emergent Project Research Assistant, who completed his master's course at the University of Tokyo), with his host professor, Associate Professor TSUKIZAKI Ryudo, his master's course supervisor, Professor NISHIYAMA Kazutaka (both in the Department of Space Flight Engineering), Dr. YAMASHITA Yusuke (currently at Stanford University), and Associate Professor Takao, Yoshinori (JST Emergent Researcher). The porours thrusters were machined at the Advanced Machining Technology Group of Japan Aerospace Exploration Agency and were developed at ISAS, with experiments conducted at ISAS and Yokohama National University, financially supported by JSPS KAKENHI JP21H01530, the Canon Foundation, the Advisory Committee for Space Engineering of Japan Aerospace Exploration Agency, Grant-in-Aid for Exploratory Research in the area of human space technology and space exploration, and JST FOREST Program under Grant No. JPMJFR2129.

Background

The development of small satellites is currently attracting a great deal of attention in space development. The development of a communication infrastructure using constellations that consist of many satellites is rapidly progressing, creating a new industrial base. However, few options exist for good propulsion systems for small satellites, especially Nano (1-10kg) to Micro (11-200kg) satellites . Conventional chemical propulsion systems are greatly limited by the need to use high-pressure gas, while electric propulsion systems such as ion thrusters and Hall thrusters lack power. In this research, a significantly higher efficiency propulsion system is proposed by storing an ionic liquid propellent in porous glass emitters without the use of high-pressure gas, and thrust is generated by directly releasing ions from the propellant without the need to generate a plasma. It is expected that the establishment of this propulsion technology will enable ultra-small spacecraft to freely power and navigate in space.

Research results

When a high electric field is applied to the tip of the electrospray thruster made of porous glass that is permeated with an ionic liquid (a "salt" in liquid state that is composed only of ions), ions can be efficiently released. Research and development is being conducted worldwide with the aim of using this mechanism as a space propulsion device. However, the physical phenomenon by which ions are extracted from the ionic liquid interface on the porous surface is a complex process occurring on multiple scales, requiring time-consuming numerical simulations to predict how many ions would be released. Drawing on previous research that has suggested that the flow of ions inside the emitter is the determining factor for ion release, the transport of ions suppressed by the porous structure was expressed using a simple model based on Ohm's law, and ions are released from a region where the threshold electric field can be exceeded. Despite this simple model not taking into account the complex surface structure, the current values predicted by the model are in excellent agreement with experimental values.

Experimental and analytical methods

The shape of a porous emitter with minute protrusions of about 1mm was precisely measured, and a numerical calculation was performed to calculate the surface electric field from that shape. The current value from ions emitted by applying a high electric field to the emitter surface impregnated with an ionic liquid contained in a vacuum device was then directly measured. The relationship between the electric field and the current density was formulated, and the resulting theoretical model was used to investigate the effect of differences in ion transport on ion emission due to changes in the porous resistance using different microscale and nanoscale porosity.

Future developments

The use of the theoretical model developed in this paper can dramatically reduce computational costs, and predict the ions released for different designs of thruster shape. It is therefore expected that the model will be used in future thruster development to improve the efficiency of designs to enable higher performance. If the class of propulsion mechanisms modelled by this theoretical model is put into practical use, it is expected that satellite constellations, deep space exploration, and formation flights using small satellites can be realized.

Featured Article

Koki Takagi, Yusuke Yamashita, Ryudo Tsukizaki, Kazutaka Nishiyama, and Yoshinori Takao, "Simple model of multi-scale and multi-site emissions for porous ionic liquid electrospray thrusters", Journal of Applied Physics 135, (2024): doi: 10.1063/5.0195699

Acknowledgements

This research was supported by the JST Emergent Research Support Program JPMJFR2129, and the JAXA Strategic Development Research Grant (Engineering). The porous emitter used in this experiment was made of a porous material provided by Nippon Electric Glass Co., Ltd., and was processed by the Advanced Engineering Group at JAXA ISAS.

Figure: Overview of the theoretical model, showing how the flow from the inside to the surface of the porous emitter determines ion emission. (Source: published paper. Licensed under Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International (CC BY-NC-ND) license.)