Ion Thruster Research Articles

Analysis of Sputtering Yield Measurements for Ion Thruster Grid Materials

Grid assembly is one of the key components of an ion thruster and directly affects the performance and life of the thruster. The measurement of the sputtering yield of the grid assembly under ion beam bombardment is highly significant for predicting the lifetime of the grid assembly and ion thruster. This study systematically summarizes the main methods currently used for sputtering yield measurement of grid materials, analyzes the advantages and disadvantages of different measurement methods, and provides suggestions for sputtering yield measurements in the low-energy () range. In addition, this study compares the sputtering resistance properties of metal- and carbon-based grid materials and summarizes the influence of key core parameters, such as the surface roughness, surface morphology, binding energy, and incident angle, on the sputtering yield. The results can be used to guide the correction of the sputter yield theoretical formula and the numerical simulation of sputtering erosion.

Geometry Design Optimization of High-Frequency Ion Thrusters and Ion Sources

Geometry Design Optimization of High-Frequency Ion Thrusters and Ion Sources

Numerical study of the radio-frequency biased accelerating system in ion thrusters

A 2D-3V implicit immersed-finite-element particle-in-cell (IFE-PIC) model is introduced to investigate the radio-frequency (RF) self-bias accelerating system applied in the RF ion thruster. A set of holes in a two-grid system with slit apertures is simulated in Cartesian coordinates. The characteristics of the plasma plume, such as the ion density, the neutralization rate and the ion and electron current density were investigated for different RF voltage amplitudes (600−1200 V) and frequencies (6−30 MHz). Furthermore, the performance of the thruster was also carefully studied. The simulation results show that a well-focused plasma beam can be formed when the voltage amplitude is larger than 900 V and the frequency exceeds the reciprocal of ion transit time (≥12 MHz) in our simulation cases. The performance of the system can be evidently improved by increasing the voltage amplitude and the frequency, and the losses of the particle and thrust are reduced correspondingly. The bulk region of the plasma beam downstream shows good quasi-neutrality, and the ions are dominant in the peripheral region when a well-focused state is achieved. The high ion density beamlet in the periphery of the ion beam is closer to the axis when the voltage amplitude is increasing, while it is expanded radially when increasing the frequency. Backstream electrons have been observed upstream, and this mainly occurs in the phase in which the electron cannot escape.

A Case Study about Innovative Electromagnetic Engineering Education based on Capacitor Charging and Discharging

Capacitor charging and discharging is basic knowledge in electromagnetic education. This work proposes an innovative course teaching method of “self-learning as the mainstay, supplemented by circuit examples”, which can improve the student’s knowledge absorption and enthusiasm in the class. The simulation tool Multisim is used as an auxiliary software to strengthen the connection between theory and practice, which reviews the knowledge they have learned and helps students to develop the habit of using simulation software to assist their learning. This innovative course teaching method improves the role of students in the classroom and cultivates the ability of students to learn independently and practice automatically, lays a foundation for the design of electromagnetic theories in the future.

Modeling grid erosion in the NEXT ion thruster using the CEX2D and CEX3D codes

NASA’s Evolutionary Xenon Thruster (NEXT) is a candidate for future deep space missions that offers high efficiency and specific impulse over a large power throttling range. One of the key life-limiting components is the ion accelerator system, which is subject to sputter erosion by low energy discharge plasma ions incident on the upstream screen grid and higher energy charge exchange ions that impact the downstream accelerator grid. The grid erosion codes CEX2D and CEX3D were validated with data from tests of NEXT as well as the NSTAR ion thruster and then used to assess the time to failure in space due to screen grid erosion and electron backstreaming caused by accelerator grid aperture erosion. Screen grid erosion was found to be important only at the lowest throttle levels, and was conservatively estimated to lead to failure after processing over 900 kg of xenon. The first failure mode at high power levels was found to be electron backstreaming due to accelerator grid hole wall erosion, which would occur after processing over 700 kg of propellant.

Twist-tunable polaritonic nanoresonators in a van der Waals crystal

Optical nanoresonators are key building blocks in various nanotechnological applications (e.g., spectroscopy) due to their ability to effectively confine light at the nanoscale. Recently, nanoresonators based on phonon polaritons (PhPs)—light coupled to lattice vibrations—in polar crystals (e.g., SiC, or h-BN) have attracted much attention due to their strong field confinement, high quality factors, and their potential to enhance the photonic density of states at mid-infrared (mid-IR) frequencies, where numerous molecular vibrations reside. Here, we introduce a new class of mid-IR nanoresonators that not only exhibit the extraordinary properties previously reported, but also incorporate a new degree of freedom: twist tuning, i.e., the possibility of controlling their spectral response by simply rotating the constituent material. To achieve this result, we place a pristine slab of the van der Waals (vdW) α-MoO3 crystal, which supports in-plane hyperbolic PhPs, on an array of metallic ribbons. This sample design based on electromagnetic engineering, not only allows the definition of α-MoO3 nanoresonators with low losses (quality factors, Q, up to 200), but also enables a broad spectral tuning of the polaritonic resonances (up to 32 cm−1, i.e., up to ~6 times their full width at half maximum, FWHM ~5 cm−1) by a simple in-plane rotation of the same slab (from 0 to 45°). These results open the door to the development of tunable and low-loss IR nanotechnologies, fundamental requirements for their implementation in molecular sensing, emission or photodetection applications.

Prediction and optimization of thrust performance from plasma diagnostics in the inductively coupled plasma of an RF ion thruster

Ion thrusters have acquired extensive applications in the space industry with their advantages of high efficiency, high specific impulse, and long lifetime. To operate a radio-frequency (RF) ion thruster successfully, a high-frequency current must be generated to facilitate the discharge of inductively coupled plasma. In this study, with support of trial and error, it is discovered that the thrust performance of an RF ion thruster depends on not only the RF power but also on other critical parameters, such as the propellant flow rate, ion grids voltages, and RF input power. A surrogate model, namely – the Kriging model, is employed to simplify the estimation and optimization of thrust performance. The thrust and specific impulse are selected to be the crucial metrics for used Kriging models for categorizing all types of propulsions. On the basis of Kriging model simulations, the designed RF ion thruster can achieve a thrust of 1.9 mN, a specific impulse of 1649.5 s, and a thrust efficiency of 48% under a propellant flow rate of 4 sccm, grids’ voltage difference of 2500 V, and an RF input power of 40 W.

Three-dimensional analysis of RF-biased ion optics with misalignments of apertures

Accelerator grid hole shift is a critical reason for erosion failure of optic systems in ion thrusters, which may also cause an unexpected roll torque about the ion beam axis. A three-dimensional (3D) model of ion optics is developed based on a particle-in-cell Monte Carlo collision method to investigate the plasma dynamics and performance of radio frequency (RF) grid systems with misalignments of apertures, which are compared with those in the direct current (DC) grid system. For the benchmark case, the 3D model gives a better agreement with the experiments in the ion energy distribution function (IEDF) compared with the two-dimensional model from a previous publication, with 36.4% and 47.9% relative error reduction of the peak position and full width at half maximum (FWHM), respectively, indicating the effectiveness of the developed 3D model. Simulations show that in the RF grid system the ion beamlet is deflected in the direction opposite to the shift of the accelerator grid hole, while electrons move first in the hole shift direction, and then deflect in the opposite direction. The average ion beamlet deflection angle calculated is consistent with the predictions from linear optical theory in both the RF and DC grid system. The amplitude of beamlet deflection angle fluctuation with time decreases with the increase of RF frequency. When the grid holes shift, the ion beamlet will deflect with the divergence angle almost unchanged in the DC grid system, while the beamlet divergence angle increases in the RF grid system. When RF frequency is low, the big vortex-like structure in the electron velocity phase diagram breaks into small vortices, showing a reduced oscillation intensity. The hole shift also causes high-frequency oscillation in the shift direction. In terms of performance, the RF grid system is more sensitive to grid hole shift than the DC grid system.

Computational and experimental research on the performance of ECRIT ion source with nitrogen propellant

Electron cyclotron resonance ion thruster (ECRIT) with a diameter of 2 cm has the characteristics of no hot cathode and high specific impulse, which is suitable for the air-breathing electric propulsion system. In order to adapt to the atmospheric composition characteristics of nitrogen and oxygen in low orbit, the computational and experimental research on the performance of the ECRIT ion sourse with nitrogen propellant is an important basis for analyzing the feasibility of applying ECRIT to the air-breathing electric propulsion system. In this paper, the global model of the nitrogen ECRIT ion source with a diameter of 2 cm is established to calculate its performance. Then, the computational results are compared with the experimental results to analyze the difference. The research results show that when the input power of the ion source is 8 W and the gas flow rate is 2 ml/min, the computational and experimental results of the extracted ion beam current and thrust reach the maximum with the extracted beam current of 16.2 and 12.5 mA and the thrust of 476.6 and 368 μN, respectively. When the input power is 8 W and the gas flow rate is 0.6 ml/min, the computational and experimental results of the specific impulse are 2 095.8 and 1 855.6 s, both reaching the maximum value. The relative errors between the computational and experimental results of the extracted ion beam current, thrust and specific impulse all range from 2% to 32%. When the input power and gas flow rate used are 8 W and 1 ml/min in calculation, and 8 W and 0.8 ml/min in experiment, the ion source is on the optimal operating state. At this situation, the computational and experimental propellant utilization efficiencies with 17.8% and 16.2% respectively are high, and the ion energy loss with 443.9 and 596.2 W/A respectively is low.

On the space-charge effects in the beam extraction process of ion thrusters: the roles of compensating electrons and changing beam radius

Space-charge effects limit the beam-extraction capability of the ion optics and thus hinder the miniaturization and other performance improvements of ion thrusters. This paper presents numerical studies of the space-charge effects in ion optics using hybrid and full particle-in-cell (PIC) simulations, and proposes a modified Child–Langmuir (CL) law. As the injected current increases, the parallel-plane electrode system which corresponds to the classical CL law will reach an unstable and oscillatory state, while the ion optics system remains stable because the electrons from the bulk plasma compensate for the space-charge effects. Furthermore, the radial expansion of the ion beam and the loss of ions on the grids can counteract the space-charge effects when the injected current increases. In general, the space-charge effects in ion optics are self-consistently adjusted by the compensating electrons and the variation of the beam radius. Accordingly, we identify a region in ion optics where, generally, no electrons exist to exclude the influence of electron compensation, and then we modify the CL law of this region by taking into account the effect of the change in the beam radius. We validate the modified CL law and demonstrate its effectiveness in predicting the operating points of the ion optics, such as the perveance-limit point.

A novel and efficient dual-antenna micro plasma thruster

A novel dual-antenna micro plasma thruster (DMPT) driven by radio-frequency (rf) power supply is proposed and characterized experimentally. The Langmuir probe (LP) and the retarding potential analyzer (RPA) are applied to measure the plasma parameters. The results show that compared to the single-antenna, the dual-antenna yields higher electron density, electron temperature, and ion energy. The measurements from the rf I–V probe indicate that the power coupling efficiency for the dual-antenna is about 2.6 times higher than that for the single-antenna. The plasma potential drop in the plume of the DMPT is the reason for the formation of ion beam. The neutral gas temperature is measured by rovibrational ban matching of the first negative band system of ionic nitrogen (N2+) for operating powers of 20 W up to 100 W. The total thrust of the DMPT has been calculated by the sum of the ion thrust and the neutral gas thrust. The total thrust at 100 sccm argon flow rate and 100 W rf power is ∼2.23 mN, which indicates an up to 168% thrust gain compared to the cold gas thrust, and a significant 11% total thrust is attributed to the ions’ electrostatic acceleration. The specific impulse and thruster efficiency at the above experimental conditions are ∼76 s and ∼0.8%, respectively. These initial results show that the dual-antenna scheme has better performance and is more promising than the traditional single-antenna design for electrodeless micro-electric propulsion. This novel and efficient micro plasma thruster is particularly useful for cubic satellites and scientific experiments in space (such as gravitational wave detection) which require gesture and orbit control of great precision.

Ion thruster accelerator grid erosion mechanism under extreme conditions of cylindrical erosion and chamfer erosion

In this paper, the abnormal experimental phenomenon on barrel erosion under extreme working conditions in the ultra-long life experiment (>10000 h) of ion thruster ion optics is studied by the Immersed-Finite-Element Particle-In-Cell Monte-Carlo-Collision (IFE-PIC-MCC) method and the grid erosion evaluation model. The transport process of beam ions and Charge Exchange (CEX) ions in the grid system, and the characteristics and mechanisms of the aperture barrel erosion under extreme erosion conditions (i.e. the cylindrical erosion and chamfer erosion) were systematically studied. Thanks to the advantage of the IFE method for dealing with complex boundaries in structured mesh, the aperture barrel erosion morphology of the accelerator grid is reconstructed accurately based on the experimental results. The results show that, with the evolution of working conditions, the mechanism of the aperture barrel erosion changes significantly, which relies heavily on the accelerator grid morphology. The change of the accelerator grid aperture barrel morphology has a significant effect on the behavior of CEX ions, and only affects the local electric field distribution, but has no effect on the upstream plasma sheath. As the erosion progresses, the erosion position moves downstream along the grid aperture axis direction, and the erosion range becomes narrower. Regardless of the erosion phase, the erosion rate of the CEX ions located downstream of the decelerator grid is the largest. The erosion rate is related to the mean incident energy and angle, and their variation is closely related to the position and trajectory of CEX ions.

Static aeroelasticity of the propulsion system of ion propulsion unmanned aerial vehicles

“Ionic wind” generators are used as the main propulsion system in ion propulsion unmanned aerial vehicles (UAVs). Owing to the large size and poor stiffness of the electrode array in the propulsion system, the electrode array is prone to deformation under the flight load. In this work, the thrust characteristics and static aeroelastic properties of “ionic wind” propulsion systems were analyzed in detail. The simulation model for an “ionic wind” propulsion system was established by coupling a two-dimensional gas discharge model with a gas dynamics model. The influences of electrode voltage, spacing, size, and shape on the performance of the propulsion system were investigated. The fluid-solid interaction method was used to solve static aeroelastic characteristics under deformation. The aerodynamic and thrust performances of the elastic state and the rigid state were compared. It was found that the operating voltage, the distance between two electrodes, and the emitter radius had greater impacts on the thrust of the propulsion system. The propulsion system had a small contribution to the lift but a large contribution to the drag. In the elastic state, the lift coefficient accounted for 12.2%, and the drag coefficient accounted for 25.8%. Under the action of the downwash airflow from the wing, the propulsion system formed an upward moment around the center of mass, which contributed greatly to the pitching moment derivative of the whole aircraft. In the elastic state, the pitching moment derivative accounted for 29.7%. After elastic deformation, the thrust action point moved upward by 28.7 mm. Hence, the no lift pitching moment is reduced by 0.104 N·m, and the pitching moment coefficient is reduced by 0.014, causing a great impact on the longitudinal trimming of the whole aircraft.

Optimal Interplanetary Transfer of Solar Wind Ion Focusing Thruster-Based Spacecraft

The Solar Wind Ion Focusing Thruster (SWIFT) is a highly-innovative propellantless propulsion concept, recently proposed by Gemmer and Mazzoleni. In its nominal configuration, a SWIFT consists of a conically-shaped mesh of positively-charged conducting tethers, with its vertex linked to the spacecraft and its axis oriented towards the Sun. The SWIFT collects and filters the solar wind plasma and suitably directs the positive ions, which are then accelerated by an ion thruster. Such a device is theoretically able to generate a deep-space propulsive acceleration that comes, in part, from the solar wind dynamic pressure impinging on the conical grid and, in part, from the positive ion beam. In particular, the orientation of the ion beam may be chosen in such a way as to set the resultant propulsive acceleration and steer the spacecraft. The aim of this paper is to analyze the performance of a SWIFT-propelled spacecraft in an orbit-to-orbit two-dimensional interplanetary transfer. To that end, some mission scenarios are studied, in an optimal framework, by minimizing the total flight time necessary for the spacecraft to complete the transfer as a function of the propulsion system performance parameters. Numerical simulations are used to compare the optimal flight times calculated in simplified Earth–Venus and Earth–Mars transfers with those obtained by considering other propellantless propulsion systems.

Plasma transport simulation under different conditions and optimization analysis of dual-stage grid ion thruster

In order to study the extraction and acceleration mechanism of the dual-stage grid, a three-dimensional model based on the Particle-In-Cell/ Monte Carlo Collision (PIC/MCC) method is performed. Dual-stage grid ion thruster is a new type of electrostatic ion thruster, which can break through the limitations of traditional gridded ion thrusters, and greatly improve the specific impulse. The high performance also makes the grid sensitive to operating parameters. In this paper, the influence of grid parameters on xenon ion thruster’s performance in a wide range is systematically simulated, and the optimal operating condition is given. Both the over-focusing of the plume, and the transparency of the screen grid are improved, and the grid corrosion is reduced through simulation optimization. The specific impulse under the given working conditions is 9877.24 s and the thrust is 7.28 mN. Based on the simulation optimization, the limitation of the dual-stage grid is discussed. The grid performs well under high voltage conditions (>3000 V) but not well under low voltage conditions (<2000 V). Finally, since argon is cheaper and more advantageous in future engineering applications, the plasma distribution and grid extraction ability under xenon and argon are analyzed and compared to study the flexibility of the dual-stage grid ion thruster. The simulation results show that a set of optimal parameters is only applicable to the corresponding propellant, which needs to be optimized for different propellant types.

Simplified Optimization of the Magnetic Configuration of HEMP-Thrusters

One type of ion thruster that has gained attention in recent years is the High Efficiency Multistage Plasma (HEMP) thruster. Optimizing the performance of these thrusters can be challenging due to the complexity of the underlying physics. Since the construction of new designs is expensive, cheaper methods for optimization, e.g., numerical optimization, are being sought. This paper presents a fast, analytical approach to finding realistic starting points for the magnetic geometry design of HEMP thrusters. First, a ratio of length to radius is presented, where the magnetic field is especially parallel at the center of the magnetic ring. This result is confirmed with the open-source library magpylib. Its speed and accuracy qualify this tool for further optimization processes. Here, we present some simple performance indicators, which may be beneficial to characterize the magnetic field structure for further optimization.

Influence of coil structure and position on the performance of miniature radio frequency ion thrusters

In this article, the influence of coil structure and position on the energy coupling efficiency of the miniature radio frequency (RF) ion thruster (μRIT) HRIT-4 with a diameter of 4 cm was studied. The experimental results show that the performance of the thruster can be improved by increasing the number of coil turns or decreasing the turn spacing, but if the distance between the coil and the screen grid is too small, the performance of the thruster will be significantly reduced. It is found that increasing the number of coil turns or decreasing the turn spacing will increase the resistance of the coil itself, but it will also obviously increase the inductance. Increasing the number of coil turns will also increase the resistance of the plasma source, and finally improve the performance of the thruster. On the other hand, due to the mutual inductance between the coil and the screen grid, the mutual inductance will be enhanced when the distance between the coil and the screen grid is reduced, and the mutual inductance will reduce the inductance of the thruster and increase the resistance of the thruster, eventually leading to the performance degradation of the thruster.

Measurement and diagnosis of miniaturized ion thruster plume

A planar Langmuir probe, which can be used for both ion electric propulsion beam characteristics measurement and plasma diagnosis, was developed and applied to plume measurement and diagnosis experiments of an 8 cm Kaufman ion thruster to address the growing demand for miniaturized ion thruster plume measurement and diagnosis. The experimental results show that the plume area exhibits good symmetry along the central axis, and the beam dispersion angle of the 8 cm ion thruster is approximately 22.7°. The radial distributions of the floating potential, plasma space potential and ion density are similar, showing a high distribution in the middle and a relatively low distribution at the two ends. The miniaturized ion thruster plume measurement and diagnosis system is not only easy to manipulate, but also fast and efficient for plasma plume measurement and diagnosis, which provides more complete data support for analyzing the working state of the miniaturized ion thrusters and optimizing the design of the thruster structure.

Optimized design of the ion optics based on the over-perveance characteristics of the beam current

To illustrate the erosion mechanism of the edge aperture by high-energy ions, a three-dimension simulation model was established, and a particle-in-cell method was applied in tracking the hitting processes as well as a Monte Carlo collision method was used to deal with the particles’ collision. Numerical results showed that the potentials interacted among the sheaths of adjacent holes and the beam ions were over-focused in the accelerator gap, causing some ions to be intercepted on the downstream wall of the decelerator grid. This was a typical under-perveance characteristic of the beam current, operating at significantly less than the optimal perveance and corresponding to lower beam currents than optimal combination, which pushed the sheath to the left farther into the plasma. The farther the grid was, the more serious erosion patterns developed. In addition, the charge exchange ions were attracted and bombarded on the decelerator and the accelerator grids, respectively. The ion optics of the 30 cm ion thruster was optimized from four-zone to three-zone, where the plasma density upstream of the screen grid was increased by 1.5% and the perveance characteristics of the beam ions were improved such that many ions struck the edge hole. The background pressure was reduced from 3.0 × 10−3 to 8.0 × 10−4 Pa to decrease the erosion depths of the grids caused by the charge exchange ions, about 9.09% and two orders of magnitude, respectively, for the accelerator and decelerator grids.

A Coupled 0-D Plasma Thermal Model for Hollow Cathodes

Hollow cathodes are essential components of dc electric thrusters for space applications, such as ion and Hall thrusters. Since the late 1970’s, several mathematical models have been proposed with the goal of characterizing the working principle and the plasma properties of such devices. Currently the more advanced 2-D models are used to effectively describe the onset of instabilities inside the plasma plume, but the increased complexity and the required calibrations make them not particularly suitable for cathode prototyping. Zero-dimensional models, on the other hand, can be used quickly and effectively to compute the main plasma parameters. In spite of their good qualities, reduced models usually do not perform well when used for very different cathodes with respect to the ones they were created for, especially due to the strong assumptions and free parameters which must be tuned to the specific case. By coupling a newly developed 0-D plasma model with a robust finite element analysis thermal simulation, the proposed model aims at filling this lack of flexibility, enabling the fast simulation of cathodes in a wide range of discharge currents and geometries. The model is able to compute self consistently all the physical quantities of interest, except the effective emission length, being the only free parameter. The model includes the approximate computation of the plasma sheath density on the emitter surface and the charge exchange heating (CEX) for the heavy species temperature. The thermal simulation is fully parametrized to quickly test different geometries and materials, while the prototyping tool allows great flexibility in terms of propellants and emissive insert materials.