Gridded Ion Thrusters Research Articles

Deposition characteristics and influence regularity of sputtered products in a low-power RF gridded ion thruster

RF ion thruster has a broad prospect of development via its simple structure, high specific impulse and electrode-less discharge. During the RF operating process, an important but ignored generally issue is that the high-energy metal particles generated by sputtering grid system can diffuse towards the ionization chamber and then deposit on the wall to form a conductive film. As a result, the RF power feed can be significantly inhibited which affects the thruster perform and reliability. In this work, the axial distribution of film thickness on the ionization chamber has been measured which has the characteristic of nonlinear distribution. And the film roughness can be significantly affected due to the gradient of film thickness and deposition temperature. Another corresponding measurement of plasma parameter evolutions in the ionization chamber has been carried out within 6000 min of thruster operation. Results shows that the presence of the deposited film can result in a ∼30 % decrease in the plasma density, which weakens the grid sputtering erosion and subsequently leads to a decrease in film growth rate. The combination of contributed to the degradation of the thruster performance, with the effect of deposited film accounting for about 41 %.

A review of the impact of ground test-related facility effects on gridded ion thruster operation and performance

A key consideration in the interpretation of ground test data of electric propulsion devices purposed for spaceflight is understanding how facility-effects influence thruster operation. This understanding is critical to the prediction of actual thruster performance in space. The necessity of science-based predictions gleaned from ground tests are particularly critical at higher thruster power levels. Operation of engines at higher power levels in vacuum chambers leads to considerable elevation in background pressure, background plasma density, and backsputter rates. This review examines the influence of ground test facility effects on gridded ion thruster operation. Ground test operation is compared with flight data, where available, to obtain a clear picture of operational differences. Mitigation strategies to alleviate facility effects are also commented upon.

Modelling an Ion Thruster for a Small Spacecraft in Very Low Earth Orbit

A gridded ion thruster running on two different propellants (Xenon and Iodine) and sited within a 3U CubeSat satellite, is modelled in a Low-Earth-Orbit (altitude 400 km), using a Particle-in-Cell approach. Charged exchange collisions cause a plume backflow, which results in erosion or contamination of external spacecraft surfaces. In particular, this research is motivated by evaluating the risks to solar panel arrays from plume backflow. At low altitudes there is a wide range of ambient species able to interact electrostatically with the ion plume. In particular one must consider the plasma potential of the incoming particle flux at the solid boundaries as well as a sheath comparison and the temperature of the solar panel surfaces. This enables evaluation of the effect of temperature on the charged exchange region. A Particle-in-Cell is used, with a hybrid approach where electrons are treated as a fluid and the propellant as kinetic particles. Our results compare surface ablation for two propellants, focusing on solar panel arrays which are considered to be especially vulnerable to the backflow of ions from the plume expansion. It is found that the flux of incoming particles increases for lower satellite surface temperatures. Furthermore, Xenon results in having a lower overall effect on the sensitivity of the particle flux in comparison to Iodine.

Kinetic simulations of low-pressure inductively coupled plasma: an implicit electromagnetic PIC/MCC model with the ADI-FDTD method

Low-pressure inductively coupled plasma (ICP) is promising for space electric propulsion. For the first time, an implicit electromagnetic particle-in-cell Monte Carlo collision model based on the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method is developed to investigate low-pressure xenon plasma characteristics of a miniature ICP source. The induced simulated electric field is well consistent with that calculated by the finite element method, indicating that this method can provide an accurate estimation of the electromagnetic field. The simulation time step used in the ADI-FDTD method is no longer restricted by the Courant–Friedrichs–Lewy constraints. Compared with the FDTD method, the ADI-FDTD method increases the size of the time step and significantly improves computational efficiency. The method is validated by comparing the simulated and measured electron density and plasma potential profile and reasonable agreement is reached. Therefore, the model is used to investigate the temporal and spatial distribution of plasma properties and the influence of the current amplitude of radio frequency (RF) coil, applied frequency of RF coil and neutral gas pressure on the plasma dynamics in the ionization chamber of a miniature gridded RF ion thruster. To explain the influence of the operating parameters, a concept called ‘the energy relaxation characteristics of electrons in response to the change of electric field’ is proposed and verified. The simulations also find that the oscillation frequency of plasma properties is twice the applied frequency of RF coil. The oscillation characteristics reveal the dynamic energy balance in the ICP. The experiment on the gridded RF ion thruster BHRIT-4 confirms the oscillation by measuring the plasma sheath potential.

Kinetic simulation of ion thruster plume neutralization in a vacuum chamber

The electrical environment of a ground vacuum testing chamber creates facility effects for gridded ion thrusters. For example, it is well known that the plume from the thruster generates current paths that are very different from what occurs in space, and the neutralization of this plume is also different. For reasons such as this, it is important to clarify how the experimental testing environment affects plasma flows, but understanding this effect solely through ground experiments is difficult. To that end, this study utilizes particle-in-cell and direct simulation Monte Carlo methods to simulate xenon beam ions and electrons emitted from a neutralizer. First, we compare simulations conducted within the chamber to those conducted in space, demonstrating that grounded chamber walls increase the electric potential and electron temperature. Next, we investigate the impact of the neutralizer’s position and the background pressure on the plume in the vacuum chamber. We find that as the neutralizer position moves closer to the location of maximum potential, more electrons are extracted, resulting in increased neutralization of the plume. We also observe that high background pressure generates slow charge-exchange ions, creating ion sheaths on the side walls that alter ion current paths. Finally, we discuss how the potential at the thruster and neutralizer exits affects the plume. The relative potential of the neutralizer to the vacuum chamber wall is observed to significantly influence the behavior of the electrons, thereby altering the degree of plume neutralization. These findings are shown to be consistent with experimental results in the literature and demonstrate the promise of high-performance simulation.

Three-Dimensional Kinetic Simulations of Carbon Backsputtering in Vacuum Chambers from Ion Thruster Plumes

Gridded ion thrusters are tested in ground vacuum chambers to verify their performance when deployed in space. However, the presence of high background pressure and conductive walls in the chamber leads to facility effects that increase uncertainty in the performance of the thruster in space. To address this issue, this study utilizes a fully kinetic simulation to investigate the facility effects on the thruster plume. The in-chamber condition shows a downstream neutral particle density 100 times larger than the in-space case due to ion neutralization at the wall and limited vacuum pump capability, resulting in a significant difference in the density and distribution of charge-exchange ions. The flux, energy, and angle of charge-exchange ions incident on the chamber wall are found to be altered by the electron sheath, which can only be simulated by the fully kinetic approach, as opposed to the conventionally used quasi-neutral Boltzmann approach. We also examine the effect of backsputtering, another important facility effect, and find that it does not necessarily require a fully kinetic simulation as the incident flux and energy of the sampled charge-exchange ion are negligibly small. Finally, we demonstrate that the carbon deposition rate on the thruster is significantly influenced by the angular dependence of the sputtered carbon, with a nearly 50% effect.

Magnetic confinement less microwave discharge gridded ion thruster

A watt-level microwave discharge is induced without magnetic confinement for a high-precision gridded ion thruster, making use of a coaxial transmission line resonator. The thruster is characterized by performance measurements and plasma diagnosis, and a description of the operating mechanism of this thruster is given in this paper. A Faraday probe and a retarding potential analyzer are employed for the diagnosis. The results show that the plume divergence increases due to the electric field formed between the plume and the ion-induced electron emission cathode. A discharge mode transition is found during the increase of the microwave power, in which the bulk heating mode is converted to the surface heating mode. The magnetic-less microwave discharge gridded ion thruster performs a continuously adjustable thrust range of 5–115 μN and a highest total efficiency of 17.2%. Compared to the common ECR ion thruster, this thruster is free from magneto-static interference on the instruments and the additional magnetic momentum on the spacecraft.

Iodine Electric Propulsion System Thrust Validation: From Numerical Modeling to In-Space Testing

In this paper, we complete a full-thrust audit of an iodine-based gridded ion thruster. Prior results have demonstrated excellent agreement between indirect and direct laboratory thrust estimates. Here, thrust estimates from numerical modeling, indirect laboratory testing from diagnostic probes and propulsion system telemetry, indirect in-space testing from onboard propulsion system telemetry, and direct in-space testing by analyzing orbital maneuvers are compared to demonstrate consistency between the four methods and complete the thrust audit. Results from recent in-space testing of the iodine-based thruster demonstrate that thrust estimates from all four methods agree to within three standard deviations of uncertainty for the 11 maneuvers studied. This thrust audit represents a critical step toward improving the understanding and technological maturity of iodine-based gridded ion thrusters for future mission applications, and it demonstrates the utility of recently developed in-space thrust inference techniques for analyzing low-thrust maneuvers.

Effect of spacecraft surface conductivity on voltage of microwave neutralizer in gridded ion thrusters

The μ10 gridded ion thruster, designed and developed at the Institute of Space and Astronautical Science, Japan, is used in the asteroid explorer Hayabusa2. In this propulsion system, the cathode is negatively biased with a power supply. The potential difference between the cathode and the system ground is regarded as an essential indicator of the cathode condition. In the operation of μ10 during the Hayabusa2 mission, it was found that the potential difference increased faster than that measured in a durability test on the ground. In this study, we performed a fault tree analysis of the on-orbit increase in the potential difference between the cathode and the system ground considering the total current balance (including electrons that flow back to the spacecraft). In addition, to understand the effect of the conductive surface around the thruster on the potential difference, we experimentally simulated the change in conductive area. It was found that the potential difference increased under a certain conductive surface condition. The results suggest that the on-orbit increase in the potential difference between the cathode and the system ground is caused by a loss of surface conductivity.

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.

Experimental study of a neutralizer-free gridded ion thruster using radio-frequency self-bias effect

An experimental study on the quasi-neutral beam extracted by a neutralizer-free gridded ion thruster prototype was presented. The prototype was designed using an inductively coupled plasma source terminated by a double-grid accelerator. The beam characteristics were compared when the accelerator was radio-frequency (RF) biased and direct-current (DC) biased. An RF power supply was applied to the screen grid via a blocking capacitor for the RF acceleration, and a DC power supply was directly connected to the screen grid for the DC acceleration. Argon was used as the propellant gas. Furthermore, the characteristics of the plasma beam, such as the floating potential, the spatial distribution of ion flux, and the ion energy distribution function (IEDF) were measured by a four-grid retarding field energy analyzer. The floating potential results showed that the beam space charge is compensated in the case of RF acceleration without a neutralizer, which is similar to the case of classical DC acceleration with a neutralizer. The ion flux of RF acceleration is 1.17 times higher than that of DC acceleration under the same DC component voltage between the double-grid. Moreover, there are significant differences in the beam IEDFs for RF and DC acceleration. The IEDF of RF acceleration has a widened and multi-peaked profile, and the main peak moves toward the high-energy region with increasing the DC self-bias voltage. In addition, by comparing the IEDFs with RF acceleration frequencies of 3.9 and 7.8 MHz, it is found that the IEDF has a more centered main peak and a narrower energy spread at a higher frequency.

Ground-Based Experiment for Electric Propulsion Thruster Plume—Magnetic Field Interaction

Electric space propulsion is a technology which is employed on a continuously increasing number of spacecrafts. While the current focus of their application area is on telecommunication satellites and on space exploration missions, several new ideas are now discussed that go even further and apply the thruster plume particle flow for transferring momentum to targets such as space debris objects or even asteroids. In these potential scenarios, the thruster beam impacts on distant objects and subsequently generates changes in their flight path. One aspect which so far has not been systematically investigated is the interaction of the charged particles in the propulsion beam with magnetic fields which are present in space. This interaction may result in a deflection of the particle flow and consequently affect the aiming strategy. In the present article, basic considerations related to the interaction between electric propulsion thruster plumes and magnetic fields are presented. Experiments with respect to these questions were conducted in the high-vacuum plume test facility for electric thrusters (STG-ET) of the German Aerospace Center in Göttingen utilizing a gridded ion thruster, an RIT10/37, and a Helmholtz coil to generate magnetic fields of varying field strength. It was possible to detect a beam deflection on the RIT ion beam caused by a magnetic field with an Earth-like magnetic field strength.

Measurement and Prediction of Micronewton Class Thrust of Electric Propulsion Based on the Torsional Pendulum and Machine Learning Technique

Accurate and direct measurement of thrust is the most fundamental requirement in the evaluation of electric thruster performance. This article performed the measurement and prediction of the micronewton class thrust of micro gridded ion thrusters (μGITs) based on a novel design of torsional pendulum and high precision neural network models. The developed torsional pendulum adopts specially designed liquid metal connectors to eliminate the interference caused by the power supply cables and propellant feeding lines, realizing an accurate thrust measurement of radio frequency (RF)- and dc-μGIT from micronewton level to millinewton level with a measuring range of 0–10 mN and a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.4~\mu \text{N}$ </tex-math></inline-formula> . Results show that the RF-μGIT generates a thrust of 0.46–2.43 mN for the RF power from 100 to 120 W and the xenon flow rate from 0.75 to 1.50 sccm. The dc-μGIT generates a thrust of 0.016–0.310 mN for the acceleration voltage from 700 to 1050 V and the flow rate from 0.2 to 2.0 sccm. Both the artificial neural network (ANN) and the radial basis function neural network (RBF-NN) are adopted to predict the relationship between the thrust and input parameters of RF-μGIT. The predicted thrusts by ANN deviate about 10% from the measured data, and the maximum relative deviation using RBF-NN model is about 2%. Furthermore, the RBF-NN model is used to obtain the distribution maps of thrust and specific impulse under 600 different working conditions and provides a solution for intelligent regulation of thruster performance. For the first time, the combination of thrust measurement and machine learning (ML) provides a new approach for the fast performance evaluation, prediction and regulation of electric thrusters.

Development and test of a cost-efficient gridded ion thruster propulsion system for small satellites – IonJet

IonJet is an engineering model of a low-cost electric propulsion system for small satellites. Whereas the flow control subsystem is based on the MICROJET 2000 system launched 2016 on the BIROS satellite, the thruster subsystem is based on a novel gridded ion thruster where the neutralizer is integrated in the ion thruster allowing for simultaneous ion and electron extraction. This design allows reducing the number of components and consequently costs and system complexity. The whole system was successfully tested under vacuum conditions regarding ignition and stable operation of the propulsion system. In the performance tests, thrusts of 1 mN could be achieved using Xe gas as propellant at a total power consumption of less than 50 W as well as specific impulses larger than 1000 s, including the gas consumption of the neutralizer. In addition, first tests with Kr gas were performed showing that for the same total power the achievable thrust is reduced to 55% – 68% of that for Xe.

Development and validation of an iodine plasma model for gridded ion thrusters

Iodine is emerging as an attractive alternative propellant to xenon for several electric propulsion technologies due to its significantly lower cost and its ability to be stored unpressurized as a solid. Because of the more complex reaction processes and energy-loss channels in iodine plasmas however, as well as the historical lack of reliable collision cross-section data, the development of accurate theoretical and numerical models has been hindered. Using recently calculated theoretical cross-sections, we present an iodine plasma model and perform a comparison with experimental data obtained from an iodine-fuelled gridded ion thruster. The model is in reasonable agreement with experimental measurements of the ion beam current, propellant mass utilization efficiency, and ion beam composition, and is able to quantitatively and qualitatively reproduce system behaviour as the input mass flow rate and RF power are varied. In addition, both the model and experiment show that the use of iodine can lead to a performance enhancement when compared with xenon. This occurs because of the combination of different iodine reaction processes, collision cross-section values, and inelastic energy thresholds which result in lower collisional energy losses, as well as an increased antenna-plasma power transfer efficiency for thrusters using a radio-frequency inductive coil.

A Global Model Study of Plasma Chemistry and Propulsion Parameters of a Gridded Ion Thruster Using Argon as Propellant

This work reports on the (zero-dimensional) global model study of argon plasma chemistry for a cylindrical thruster based on inductively coupled plasma (ICP) whose output has a system of two grids polarized with each other with direct current potential. The global model developed is based on particle and energy balance equations, where the latter considers both charged and neutral species. Thus, the model allows the determination of the neutral gas temperature. Finally, this study also investigated the role of excited species in plasma chemistry especially in the ions production and its implications for propulsion parameters, such as thrust. For this, the study was carried out in two different scenarios: (1) one taking into account the metastable species Arr and Arp (multi-step ionization), and (2) the other without these species (single-step ionization). Results indicates a distinct behavior of electron temperature with radiofrequency (RF) power for the investigated cases. On the other hand, the gas temperature is almost the same for investigated power range of up to 900 W. Concern propulsion analysis, a thrust of 40 mN at 450 W was verified for case (1), which represents a remarkable thrust value for electric thrusters.

Comparison and Evaluation of Numerical Techniques and Physico-Chemical Algorithms for the Simulation of an Iodine and Xenon Powered Ion Thrusters

An electrostatic ion thruster, modelled on Busek’s BIT-3 [5], is simulated numerically using the open-source software “Starfish”. It is assumed that the thruster is in vacuum conditions propelling a CubeSat in low Earth orbit. Iodine is chosen here as the propellant under test. The results from modelling this relatively recent new fuel are compared to those of the standard propellant of xenon. The plasma in the ion thruster and the associated electric fields are simulated using a particle-based kinetic code in which the hybrid approach of Particle in Cell and Direct Simulation Monte Carlo methods has been used. In modelling these flows, elastic and inelastic collisions can occur involving charge and momentum exchanges. Such collision models use a number of assumptions, e.g., concerning the collisional cross-section area, and in this paper, we present results where the physico-chemical modelling is improved reducing the level of assumptions used. Results are also presented concerning the numerical methods used for the iterative convergence scheme, stochastic sampling, and the importance of the constraints for the mesh size and timestep. It is found that the most appropriate timestep is one which enables both the CFL condition and the highest frequency to be captured. The mesh size affects the choice of the solver being used; the largest the cell sizes the greater the assumption of quasi-neutral flow and thus areas of non-neutrality (such as in the surrounding shealth may be treated inadequately. The subsequent effects on the plume and the thrust produced of using different approaches in modelling the electron temperature distribution are also evaluated to produce a more rigorous modelling methodology.

Integrative simulation of a 2 cm electron cyclotron resonance ion source with full particle-in-cell method

A two-dimensional full particle-in-cell model is developed to simulate the operation process of a 2 cm electron cyclotron resonance ion thruster. The simulation regions include the discharge chamber, grid system, and plume, and the processes of neutral gas injection, plasma discharge, and ion beam extraction are simulated continuously. Neutrals are treated as particles and managed through an adaptive particle management algorithm. The collisions between charged and neutral particles are approached by the method of combining the Monte Carlo and directly simulation Monte Carlo collisions. The integrative simulation provides a global perspective to learn the gridded ion thruster, such as the Child-Langmuir sheath, acceleration grid erosion, plasma bridge, and so on. In this paper, the response of the plasma system to the accelerating voltage is analyzed, which is related to the bulk plasma, antenna floating voltage, and Child-Langmuir sheath. Besides, an ion density wave is observed in plume, which will increase the current of acceleration grid and cause ion backflow.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

Effect of ion beam extraction on neutral density distribution inside a gridded microwave discharge ion thruster

In gridded ion thrusters, it is generally known that the ion extraction from the grid significantly decreases the neutral density. However, because it is difficult to measure the density inside thruster, it has not been revealed how the local density decreases including the inhomogeneous effects (e.g., gas injection, ionization, and ion recombination). To investigate these inhomogeneous effects, two-photon laser-induced fluorescence (TALIF) and Direct Simulation Monte Carlo (DSMC) applied to a gridded microwave discharge ion thruster. First, the TALIF directly measured the density distribution. Second, the DSMC reproduced the measured density distribution. From these results, the main effect of the ion extraction is the decrease of ion recombination at the grid. This decrease makes the density distribution more sensitive to the ionization distribution and increases the inhomogeneity. Based on this knowledge, we propose improvement against ‘simulated experiment without ion extraction’ and ‘decoupled numerical simulation’ which reproduce the density distribution of ion extraction under the condition without ion extraction by decreasing flow rate. Specifically, we suggest that the grid transparency is expanded and the rest discrepancy is adjusted by the flow rate because the difference of the inhomogeneous effect due to gas injection can be reduced.