Miniature Microwave Discharge Ion Thruster Research Articles

Electron extraction mechanism of magnet array microwave discharge neutralizer

Microwave discharge neutralizer is an important part of microwave discharge ion thruster system, which plays a vital role in maintaining potential balance between spacecraft and neutralizing ion beam. Its electron extraction property directly affects the operation condition of ion thruster system. In order to break through the power limit of miniature microwave discharge ion thruster, a magnet array microwave discharge ion thruster system is designed and tested. In the experiment on finalizing magnetic field structure of magnet array microwave discharge neutralizer, an interesting phenomenon is found that the <i>I</i>-<i>V</i> curves of electron current, after rotating the magnetic array orientation, are very different. Defining forward direction of magnet array can normally extract electrons, then backward direction of magnet array can hardly extract electrons. Because the diameter of discharge chamber is only 10 mm, it is too small to perform Langmuir probe diagnosis. And thus, an integrative particle-in-cell method is used to simulate the neutralizer operation processes of two different magnetic field structures, and for the sake of accuracy, real vacuum permittivity is used. The simulation results show good consistence with experimental phenomenon. In the initial discharge process, it is found that the magnetic field gradient leads to different plasma distributions; in electron extraction process, it is found that the potential distribution near the orifice determines the electron extraction property of the neutralizer. Through comparing the plasma parameter distributions under different magnetic field structures and operating voltages, an assumption that the ion is an important factor in electron extraction process is proposed. Then, a simulation that ions disappear artificially outside the orifice is conducted, and the simulation results show that electrons cannot be effectively extracted without ions near the orifice. According to the simulation and experiment results, two necessary conditions are summarized for electron extraction of the neutralizer. The first condition is magnetic field structure: the magnetic field gradient should point towards the orifice to guide plasma migration towards the orifice, the second one is potential distribution: there should be enough ions to lift the potential near the orifice for reducing or breaking the potential well. These two conditions can help understand the electron extraction mechanism of microwave discharge neutralizer and provide theoretical reference for optimizing the performance of neutralizer in future.

Effects of negative ions on discharge characteristics of water plasma source for a miniature microwave discharge ion thruster

This work analyzes the discharge characteristics of water plasma for a miniature microwave discharge ion thruster via three-dimensional particle-in-cell simulations with Monte Carlo collisions. It incorporates the negative ions (H−, O−, and OH−) into the simulation code, aside from the three major positive ion species (H2O+, OH+, and H+), and investigates their effects on the discharge characteristics. On the one hand, the simulation results indicate that H2O+ is the dominant species and the negative ions have little effect on the positive ion density and transport. On the other hand, this study confirms the axial oscillation of the OH− negative ion density, in addition to azimuthal rotation, causing plasma instability with a periodic low-frequency anomalous diffusion. The azimuthal rotation is similar to the phenomena observed in other E × B devices, where the difference in spatial distributions of magnetized electrons and unmagnetized ions causes the local potential hump and the resultant instability. Moreover, the axial oscillation, not observed in the previous study considering only positive ions, is due to the generation of OH− by the charge transfer collision reaction between H2O and H−, and this oscillation frequency is double that of the azimuthal rotation. Both the azimuthal rotation and the axial oscillation result in the periodic double peaks of the potential time evolution. These fluctuations have an influence on electron transport across the magnetic field.This work analyzes the discharge characteristics of water plasma for a miniature microwave discharge ion thruster via three-dimensional particle-in-cell simulations with Monte Carlo collisions. It incorporates the negative ions (H−, O−, and OH−) into the simulation code, aside from the three major positive ion species (H2O+, OH+, and H+), and investigates their effects on the discharge characteristics. On the one hand, the simulation results indicate that H2O+ is the dominant species and the negative ions have little effect on the positive ion density and transport. On the other hand, this study confirms the axial oscillation of the OH− negative ion density, in addition to azimuthal rotation, causing plasma instability with a periodic low-frequency anomalous diffusion. The azimuthal rotation is similar to the phenomena observed in other E × B devices, where the difference in spatial distributions of magnetized electrons and unmagnetized ions causes the local potential hump and the resultant instability. M...

Performance characterization of a miniature microwave discharge ion thruster operated with water

Fundamental characteristics of a microwave-discharge miniature ion thruster using water as a propellant, instead of the conventional xenon, are presented in this paper intended for future applications for CubeSats. Water has advantages in terms of safety, handling ability, and availability compared to any other propellant, and these features are especially important for CubeSats. The ion thruster consists of an ion source and a neutralizer, and both use an electron cyclotron resonance heating to generate the plasma. Water was fed to the ion thruster or the neutralizer as a vapor and the direct thrust measurement was conducted. As a result, the thrust correction factor of the ion source was 0.92 ± 0.074 with a beam acceleration voltage of over 0.80 kV, which was similar to the typical values for xenon ion thruster. The ion beam was extracted at the lower mass flow rate, the highest thrust was 164 ± 13.4 μN, and the highest specific impulse was 665 ± 59 s while neglecting the neutralizer. The electron emission current from the neutralizer was 9.5 ± 0.08 mA, which is sufficient to neutralize the ion beam. These experiments demonstrated the basic features of the miniature ion thruster using water and a future possibility of water propellant utilization for CubeSats.

Three-dimensional particle-in-cell simulation of a miniature plasma source for a microwave discharge ion thruster

We have developed a three-dimensional particle model for a miniature microwave discharge ion thruster to elucidate the mechanism of ECR discharges confined in a small space. The model consists of a particle-in-cell simulation with a Monte Carlo collision algorithm (PIC-MCC) for the kinetics of charged particles, a finite-difference time-domain method for the electromagnetic fields of 4.2 GHz microwaves, and a finite element analysis for the magnetostatic fields of permanent magnets. The PIC-MCC results have shown that the electrons are well confined owing to the mirror magnetic fields and can be effectively heated in the ECR layer downstream of a ring-shaped antenna. The confinement results in the ring-shaped profiles of the plasma density along the antenna. The visual appearance of the plasma discharge of the thruster in operation was also ring-shaped. Moreover, the ions are expected to be accelerated effectively through the grid electrode without a large loss of ions toward side walls, that is, the plasma source developed here would be desirable in ion thrusters.

Measurement of Microwave Absorbance by Plasma of a Miniature Microwave Discharge Ion Thruster

Measurement of Microwave Absorbance by Plasma of a Miniature Microwave Discharge Ion Thruster

Measurements of electron density and temperature in a miniature microwave discharge ion thruster using laser Thomson scattering technique

In order to improve the thrust performance of a miniature microwave discharge ion thruster, the relationship between electron number density/temperature and operational conditions, mass flow rate, incident microwave power and magnetic field strength were measured by means of laser Thomson scattering. A photon counting method and a triple grating spectrometer were used against a small Thomson scattering signal and a strong stray laser light. Electron number density increased with incident microwave power and was saturated at critical incident microwave power; it was about 1.2 × 1018 m−3 at incident microwave power >8 W. In addition, electron number density increased with mass flow rate and became saturated; it was about 1.7 × 1018 m−3 at mass flow rate > 0.04 mg s−1. The electron number density gradually increased with an increase in the number of magnets, i.e. magnetic field strength. There was a sudden jump at thirteen magnets, although the thruster failed to ignite at fourteen magnets. This is because there is an optimum distance between the antenna and the electron cyclotron resonance layer. These results suggest that future improvement in thrust efficiency in miniature microwave discharge ion thrusters may come from the fine adjustment of the magnetic field configuration inside the discharge chamber.

Miniature Microwave Discharge Ion Thruster Driven by 1 Watt Microwave Power

T HE age of space exploration using self-propelled small spacecraft is approaching. In the past decade, research and development of small spacecraft have advanced extensively throughout the world. Numerous small spacecraft have been successfully launched and operated [1–8]. Small-spacecraft missions are increasingly used with widely varying applications. In recent years, planned small-spacecraft missions are increasingly in need of propulsive capability. Propulsion devices supply the spacecraft with attitude control, station-keeping, and orbit transfer functions. Furthermore, propulsion can support future spacecraft missions such as drag-free control from atmospheric or solar pressure, precise constellation flight for interferometer missions, and deorbiting maneuvers of end-of-life spacecraft into the atmosphere. The arrival of propulsion devices suitable for small spacecraft, namely, micropropulsion, is eagerly anticipated [9]. Ion thrusters [10] are promising propulsion devices intended for use not only with standard-sized spacecraft but also with small spacecraft. Their characteristics of high specific impulse (3000 s), high thrust efficiency (50%), usage of an inert propellant (xenon), and continuously controllable thrustmeet the requirements for smallspacecraft missions. Several studies of miniature ion thrusters have been reported for plasma generators of several different types, namely, direct-current electron discharge [11,12], radio frequency discharge [13], and microwave discharge [14–17]. Among these types, microwave discharge ion thrusters have the following unique characteristics suitable for miniaturization. First, the discharge chamber generating the plasma has a simple structure because of its lack of an electron cathode. In this type, electrons are heated using electron cyclotron resonance (ECR); neither a discharge cathode generating primary electrons nor corresponding power supplies are needed. Second, the thruster performance is degraded only slightly by scaling down. In general, discharge loss is affected by the surfaceto-volume ratio of plasma. In microwave discharge ion thrusters, however, the ECR heating zone is confined to the near-surface magnetic field, meaning that the surface-to-volume ratio is not sensitive to the system’s scale. Using these advantages, Takao et al. [14], Yamamoto et al. [15,17], and Nakayama et al. [16] developed miniature ion thrusters driven by microwave discharges and showed good performance. In spite of these benefits, however,miniature ion thrusters have not been used for small spacecraft yet. An important reason is the severe limitation of electrical power available on small spacecraft. In general, the electrical power generated in small spacecraft is about 1 W=kg. For example, the maximum usable power in a 10–50 kg small spacecraft was estimated at only 10–50 W. The miniature ion thrusters developed to date require a total power of at least 30W.Low power limitations have made it difficult to use those ion thrusters on small spacecraft. On the other hand, mass limitations are not so severe problem as that of the electrical power. Nakayama et al. [16] estimated the total dry weight as approximately 4 kg for their miniature microwave discharge ion thruster. Additionally, this weight can be reduced father by decreasing themicrowave power and discharge loss because the microwave power supply has a large fraction of its total weight attributable to the low energy conversion efficiency. Our objective is a miniature ion thruster with a total power consumption of less than 10 W and a total dry weight of less than 2.0 kg. One such miniature ion thruster can propel a 10–20 kg small spacecraft, although several thrusters could propel larger spacecraft of 20–100 kg. Moreover, devices that produce minute and controllable thrust are useful for spacecraft that require precise control of both attitude and position. The major challenge to achieving this goal is to decrease the total required electrical power while maintaining high thruster performance. That is to say that power for generating plasma should be limited to only 1–2Wand the discharge loss should be suppressed to less than 500 W=A. To date, no ion thruster has been reported to operate at such lowpower applied to the plasma and at such a low discharge loss. As a first step toward this goal, we have developed a miniature ion thruster driven by 1 W of microwave power with a 250 W=A ion production cost and 37% mass utilization efficiency. In this paper, we present the characteristics and performance of aminiaturemicrowave dischargebased ion thruster.

Development of Miniature Microwave Discharge Neutralizer for Miniature Ion Thruster

A miniature microwave discharge neutralizer has been developed in order to develop the miniature ion engine system. Extracted electron currents from the microwave discharge neutralizer are measured for two neutralizers, cylindrical discharge chamber with monopole antenna and the same discharge chamber as miniature microwave discharge ion thruster developed at Kyushu University for two orifice diameters. The former shows that the extracted electron current is 14 mA at incident microwave power of 4 W, 6 mA at incident microwave power of 2 W at mass flow rate of 0.005 mg/s. This small extracted current would be due to the large loss on the discharge chamber wall. On the other hand, the best extracted electron current of the latter, is 11 mA at mass flow rate of 0.005 mg/s and incident microwave power of 4 W, by optimizing the orifice diameter or position of the orifice plate. From the poor performance at incident microwave power of 2 W, the plasma- microwave coupling is worse than that of the former.

小型イオンスラスタの推進性能における形状依存性

The effects of antenna shape and magnetic field configuration on thrust performance in a 30W class miniature microwave discharge ion thruster were investigated with the objective of improving its thrust performance. The ion beam currents were measured for three different antenna configurations: disc, star, and cross. The results showed that the optimum antenna shape depends on the desired mass flow level because of the tradeoff between microwave-plasma coupling efficiency and surface area recombination. The ion beam current with various magnitude of magnetic field was measured, since magnetic confinement and microwave-plasma coupling could be affected by the magnetic field configuration. The ion beam current was increased with increasing the number of magnets, though under too much magnitude of magnetic field, the thruster cannot be ignited. So, there is an optimum magnitude of magnetic field. The thrust performance of the miniature microwave discharge ion thruster, i.e., propellant utilization, and ion beam production cost were 0.97 and 600W/A, respectively, at mass flow rate of 0.018mg/s, and incident microwave power of 8W.

Effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster

The effects of magnetic field configuration on thrust performance in a miniature microwave discharge ion thruster were investigated in order to improve thrust performance. First, the extracted ion beam current was measured for various levels of strength of the magnetic field. It was found that there is an optimum magnitude of the magnetic field. That this is due to the tradeoff between magnetic mirror confinement and microwave-plasma coupling was confirmed by measurement of the ion saturation current into the antenna of the ion thruster. The ion saturation current was found to decrease with an increase in magnetic field strength, due to the improvement in magnetic mirror confinement. The estimated electron temperature also decreases with an increase in magnetic field strength. This result shows that the increase in magnetic field strength leads to a decrease in microwave-plasma coupling. Next, the ion beam current for three magnetic field shapes was measured by changing the length of the central yoke. The results show that the optimum magnetic field shape depends on the mass flow rate because of the tradeoff between magnetic confinement and ionization probability. For the configurations tested, the 3 mm length central yoke is optimal for low mass flow, whereas 7 mm is the best for high mass flow. Overall, the extracted ion beam current is 21.4 mA, at a xenon mass flow rate of 0.036 mg/s, beam voltage of 1500 V, and incident microwave power of 16 W.