There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This contribution presents a short review of electric propulsion (EP) technologies for satellites and spacecraft. Electric thrusters, also termed ion or plasma thrusters, deliver a low thrust level compared to their chemical counterparts, but they offer significant advantages for in-space propulsion as energy is uncoupled to the propellant, therefore allowing for large energy densities. Although the development of EP goes back to the 1960s, the technology potential has just begun to be fully exploited because of the increase in the available power aboard spacecraft, as demonstrated by the very recent appearance of all-electric communication satellites. This article first describes the fundamentals of EP: momentum conservation and the ideal rocket equation, specific impulse and thrust, figures of merit and a comparison with chemical propulsion. Subsequently, the influence of the power source type and characteristics on the mission profile is discussed. Plasma thrusters are classically grouped into three categories according to the thrust generation process: electrothermal, electrostatic and electromagnetic devices. The three groups, along with the associated plasma discharge and energy transfer mechanisms, are presented via a discussion of long-standing technologies like arcjet thrusters, magnetoplasmadynamic thrusters, pulsed plasma thrusters and ion engines, as well as Hall thrusters and variants. More advanced concepts and new approaches for performance improvement are discussed afterwards: magnetic shielding and wall-less configurations, negative ion thrusters and plasma acceleration with a magnetic nozzle. Finally, various alternative propellant options are analyzed and possible research paths for the near future are examined.

http://iopscience.iop.org/article/10.1088/0963-0252/25/3/033002/pdf

Electric propulsion for satellites and spacecraft: established technologies and novel approaches

This review presents the basics of plasma discharges applied to electric spacecraft propulsion. It briefly reports on the mature and flown technologies of gridded ion thrusters and Hall thrusters before exploring the recent yet immature technology of plasma thrusters based on expansion from low pressure high density inductively coupled and wave-excited plasma sources, e.g. the radiofrequency helicon source. Prototype development of plasma engines for future space travel is discussed using the example of the helicon double layer thruster. A summary of highlights in electric propulsion based space missions gives some insight into the challenges of future high power missions in more remote regions of space.

/pdf/plasmas-for-spacecraft-propulsion-4uknp98tfc.pdf

Plasmas for spacecraft propulsion

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

Space micropropulsion systems for Cubesats and small satellites: from proximate targets to furthermost frontiers

Drastic miniaturization of electronics and ingression of next-generation nanomaterials into space technology have provoked a renaissance in interplanetary flights and near-Earth space exploration using small unmanned satellites and systems. As the next stage, the NASA’s 2015 Nanotechnology Roadmap initiative called for new design paradigms that integrate nanotechnology and conceptually new materials to build advanced, deep-space-capable, adaptive spacecraft. This review examines the cutting edge and discusses the opportunities for integration of nanomaterials into the most advanced types of electric propulsion devices that take advantage of their unique features and boost their efficiency and service life. Finally, we propose a concept of an adaptive thruster.

/pdf/recent-progress-and-perspectives-of-space-electric-4xllnmkzvs.pdf

Recent progress and perspectives of space electric propulsion systems based on smart nanomaterials.

The axial force imparted from a magnetically expanding current-free plasma is directly measured for three different experimental configurations and compared with a two-dimensional fluid theory. The force component solely resulting from the expanding field is directly measured and identified as an axial force produced by the azimuthal current due to an electron diamagnetic drift and the radial component of the magnetic field. The experimentally measured forces are well described by the theory.

/pdf/electron-diamagnetic-effect-on-axial-force-in-an-expanding-1hdyewdbni.pdf

Electron diamagnetic effect on axial force in an expanding plasma: experiments and theory.

Development of electrodeless radiofrequency plasma thrusters, e.g., a helicon thruster, has been one the of challenging topics for future high-power and long-lived electric propulsion systems. The concept simply has a radiofrequency plasma production/heating source and a magnetic nozzle, while it seems to include many aspects of physics and engineering issues. The plasma produced inside the source is transported along the magnetic field lines and expands in the magnetic nozzle, where the plasma is spontaneously accelerated into the axial direction along the magnetic nozzle, yielding a generation of the thrust force. Hence, the plasma transport and spontaneous acceleration phenomena in the magnetic nozzle are key issues to improve the performance of the thrusters. Since the thrust is equal in magnitude and opposite in direction to momentum flux exhausted from the system, the direct measurement of the thrust can reveal not only the thruster performance but also fundamental physical quantity of plasma momentum flux. Here studies on fundamental physics relating to the thruster development and the technology for the compact and efficient system are reviewed; the current status of the thruster performance is shown. Finally, a recently proposed future new application of the thruster is also discussed.

/pdf/helicon-type-radiofrequency-plasma-thrusters-and-magnetic-y0d7ht0qlj.pdf

Helicon-type radiofrequency plasma thrusters and magnetic plasma nozzles

Direct thrust measurements of a permanent magnet helicon double layer thruster have been made using a pendulum thrust balance and a high sensitivity laser displacement sensor. At the low pressures used (0.08 Pa) an ion beam is detected downstream of the thruster exit, and a maximum thrust force of about 3 mN is measured for argon with an rf input power of about 700 W. The measured thrust is proportional to the upstream plasma density and is in good agreement with the theoretical thrust based on the maximum upstream electron pressure.

/pdf/direct-thrust-measurement-of-a-permanent-magnet-helicon-3g4dqw71w8.pdf

Direct thrust measurement of a permanent magnet helicon double layer thruster

In the high potential plasma, upstream of the double layer, the measured electron energy distribution function EEDF shows a very clear change in slope at energies break corresponding to the double layer potential drop. Electrons with lower energy are Maxwellian with a temperature of 8 eV, whereas those with higher energy have a temperature of 5 eV. The EEDF in the downstream plasma has a temperature of 5 eV. Over the range of pressures wherein the double layer and accelerated ion beam are detected by analysis of a retarding field energy analyzer, the strength of the double layer corresponds to the energy where the slope changes in the EEDF break. We deduce that the downstream electrons come from upstream electrons that have sufficient energy to overcome the potential of the double layer, and that only a single upstream plasma source is required to maintain this phenomenon. © 2007 American Institute of Physics. DOI: 10.1063/1.2803763 High energy charged particles in space are thought to be accelerated by mechanisms such as proposed by Fermi, by waves, and more recently, by electric double layers DLs. 1‐3 Although some experimental data obtained from probes on satellites are available, they are rarely sufficient to fully develop self-consistent models of such space DLs and assumptions on the form of the accelerated and trapped particle distribution functions have to be made. Perkins and Sun predicted the existence of current-free double layer solutions, 4 and soon after their prediction, current-driven laboratory double layers were set up in a current-free “mode,” 5‐7 confirming the prediction. More recently, a new class of currentfree double layers were experimentally found in expanding

/pdf/measurement-of-the-energy-distribution-of-trapped-and-free-3y1c0dl6c7.pdf

Measurement of the energy distribution of trapped and free electrons in a current-free double layer

Cross-field diffusion and plasma expansion in a rapidly diverging magnetic nozzle are controlled while maintaining constant plasma production in a contiguously attached radio frequency plasma source. It is demonstrated that the measured electron-diamagnetic-induced axial momentum increases with increasing magnetic field strength to approach the theoretical limit derived using an ideal nozzle approximation. The measured axial momentum exerted onto the axial and radial plasma source boundaries validate the prediction from a maximum electron pressure model on the back wall and from a zero net axial momentum model on the radial wall.

/pdf/approaching-the-theoretical-limit-of-diamagnetic-induced-538m84hptg.pdf

Kazunori Takahashi

Papers

Electron diamagnetic effect on axial force in an expanding plasma: experiments and theory.

Helicon-type radiofrequency plasma thrusters and magnetic plasma nozzles

Direct thrust measurement of a permanent magnet helicon double layer thruster

Measurement of the energy distribution of trapped and free electrons in a current-free double layer

Approaching the theoretical limit of diamagnetic-induced momentum in a rapidly diverging magnetic nozzle.