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

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.

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Helicon-type radiofrequency plasma thrusters and magnetic plasma nozzles

This article presents a review of work performed over the past ten years in France, centered on the utilization of laser-induced fluorescence (LIF) spectroscopy to diagnose the low-pressure magnetized dc discharge of a Hall thruster (HT). The latter is a gridless electric propulsion device in a crossed electric and magnetic field configuration, which is used onboard satellites and space probes for various types of maneuvers. Although the design of a HT is relatively simple, the physical mechanisms that govern thrust generation and efficiency are not yet fully understood. Characterization of the ion and atom velocity distribution function (VDF) appears to be a powerful way to obtain insights into the underlying physics. The VDF of xenon and krypton—the most common propellants—is therefore locally interrogated by means of LIF on excited levels. In this review emphasis is placed on time-averaged and time-resolved continuous-wave LIF measurements, associated quantities and recent outcomes. Results will be presented concerning a variety of phenomena: velocity vector field structuring, ion population interaction, electric field generation, ion magnetic drift, apparent atom acceleration, interaction of the plasma plume with background gas and low-frequency electric field oscillations, to name only a few.

https://iopscience.iop.org/article/10.1088/0963-0252/22/1/013001/pdf

Laser-induced fluorescence diagnostics of the cross-field discharge of Hall thrusters

: During the early development stages of Hall thruster technology, plasma research and propulsion advancements centered primarily on 300 V, 1600 s specific impulse operation. Since the first thruster firing on a Soviet satellite in 1972, extensive investigations of the plasmadynamic discharge phenomena and operating characteristics progressed the propulsion concept to a high-level of performance suitable for a wide range of near-earth maneuvers and interplanetary missions. The expanded performance envelope is primarily a function of improvements in thruster lifetime, thermal margin, discharge stability, and power system capability. Advancements in the Hall thruster propulsion system have enabled a wider range of input parameters to the thruster, including the applied anode potential. Operation in the low discharge voltage regime is associated with a decline in total thruster efficiency. This dissertation is intended to investigate low voltage Hall thruster physics, identify dominant performance loss mechanisms, and determine the discharge characteristics that drive efficiency.

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Investigation of low discharge voltage Hall thruster characteristics and evaluation of loss mechanisms

This paper reviews and discusses recent experimental, theoretical, and numerical studies of plasma-wall interaction in a weakly collisional magnetized plasma bounded with channel walls made from different materials. A low-pressure E ×B plasma discharge of the Hall thruster was used to characterize the electron current across the magnetic field and its dependence on the applied voltage and the electron-induced secondary electron emission (SEE) from the channel wall. The presence of a depleted anisotropic electron energy distribution function with beams of secondary electrons was predicted to explain the enhancement of the electron cross-field current observed in experiments. Without the SEE, the electron cross-field transport can be reduced from anomalously high to nearly classical collisional level. The suppression of the SEE was achieved using an engineered carbon-velvet material for the channel walls. Both theoretically and experimentally, it is shown that the electron emission from the walls can limit the maximum achievable electric field in the magnetized plasma. With nonemitting walls, the maximum electric field in the thruster can approach a fundamental limit for a quasi-neutral plasma.

Effect of Secondary Electron Emission on Electron Cross-Field Current in $E \times B$ Discharges

A flight-model SPT-100 Hall thruster from the Fakel Experimental and Design Bureau was characterized with respect to performance and plume properties at The Aerospace Corporation and at L-3 Electron Technologies, Inc. over vacuum facility background pressures ranging from 2.3×10 -4 to 9.5×10 -3 Pa (1.7×10 -6 to 7.1×10 -5 Torr). Thrust and xenon mass flow rate measured at both facilities agreed to within the measurement uncertainties. Thrust decayed exponentially, while flow rate increased linearly with decreasing pressure. Using simple curve-fits for extrapolation, thrust, specific impulse, efficiency, and flow rate were projected to change by -4, -10, -14, and 7% respectively at zero pressure relative to 6.7×10 -3 Pa (5.0×10 -5 Torr). A decrease in net ion accelerating voltage and an increase in plume divergence were identified as contributors to decreased thrust with decreasing pressure. The amplitude of discharge current oscillations decreased linearly with decreasing background pressure, while the center frequency of the oscillations also decreased, from 26 to 16.5 kHz, but in non-linear fashion. Neutral density near the exit plane of Hall thrusters can be quite low, O(10 18 ) m -3 , such that background neutrals from the residual gas in the vacuum chamber may significantly influence ionization and conduction there.

The Effect of Background Pressure on SPT-100 Hall Thruster Performance

Ion flux, energy, and charge-state measurements for the BPT-4000 Hall thruster

It is shown experimentally that the cylindrical Hall thruster (CHT) produces a highly ionized plasma flow with mass utilization efficiencies greater than 100% due to generation of multicharged ions. For the CHT employing geometries with and without a short annular section, used to enhance ionization, plume divergence reductions of approximately 25% were demonstrated by running a cathode-keeper discharge along with the main cathode-anode discharge. Thruster anode efficiencies varied from approximately 15% to 35% over input powers from 70 to 220 W. A 2-A keeper discharge resulted in an approximately 20% increase in anode specific impulse for both geometries, and the specific impulse for the case of no annular section was, on average, 13% higher than that with the annular section. The multicharged ion fraction increased with keeper current, and the quantity of channel erosion products in the plume was correlated with that of multicharged ions.

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Ionization, Plume Properties, and Performance of Cylindrical Hall Thrusters

Plume characterization tests with the 36-cm NEXT ion engine are being performed at The Aerospace Corporation using engineering-model and prototype-model thrusters. We have examined the beam current density and xenon charge-state distribution as functions of position on the accel grid. To measure the current density ratio j++/j+, a collimated Eprobe was rotated through the plume with the probe oriented normal to the accel electrode surface at a distance of 82 cm. The beam current density jb versus radial position was measured with a miniature planar probe at 3 cm from the accel. Combining the j++/j+ and jb data yielded the ratio of total Xe+2 current to total Xe+1 current (J++/J+) at forty operating points in the standard throttle table. The production of Xe+2 and Xe+3 was measured as a function of propellant utilization to support performance and lifetime predictions for an extended throttle table. The angular dependence of jb was measured at intermediate and far-field distances to assist with plume modeling and to evaluate the thrust loss due to beam divergence. Thrust correction factors were derived from the total doubles-to-singles current ratio and from the far-field divergence data

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Spatially-Resolved Beam Current and Charge-State Distributions for the NEXT Ion Engine

An investigation of a fully cylindrical Hall thruster was performed using laser induced fluorescence (LIF) to measure ion velocity profiles in the plume. The measurements confirm a previously reported 9% increase in the exhaust energy when the cathode keeper draws an excess current (overrun mode). Furthermore, the velocity directions in the plume remain relatively unchanged for the cusped and direct magnetic field configuration in both overrun and nonoverrun modes. Previously reported plume narrowing in the overrun mode was confirmed and found to be due to the shift of the acceleration and ionization regions toward the anode. The electric field inferred from the LIF measurements allowed calculation of the electron E×B drift. Close to the centerline of the thruster, electrons drift azimuthally with velocity decreasing away from the centerline, thus creating shear. This shear can be a source of plasma instabilities and influence electron transport. Further away from the centerline, electrons drift in the op...

Kevin D. Diamant

Papers

The Effect of Background Pressure on SPT-100 Hall Thruster Performance

Ion flux, energy, and charge-state measurements for the BPT-4000 Hall thruster

Ionization, Plume Properties, and Performance of Cylindrical Hall Thrusters

Spatially-Resolved Beam Current and Charge-State Distributions for the NEXT Ion Engine

Laser induced fluorescence measurements of the cylindrical Hall thruster plume