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.
•Measurement of neutral density distribution inside gridded ion thruster by TALIF.•Comparison of neutral density distribution between without and with ion extraction.•DSMC simulation was performed to reproduce TALIF measurement.•Inhomogeneity of gas injection, ionization, and ion recombination on neutral density.•Expansion of grid transparency is better solution to reproduce sate of ion extraction.
An understanding of the degradation mechanism of a microwave discharge cathode is the key to extending the lifetime of microwave ion thruster systems. This study investigates the effect of nozzle ...contamination by sputtered Ag-polytetrafluoroethylene (PTFE) on microwave discharge cathode performance. The current–voltage characteristics were measured for nominal and contaminated (by PTFE spray with 0.2 µm thick or tape with 0.15 mm thick) cathodes. The contamination thickness and area on the nozzle were varied to investigate the characteristic differences. It was confirmed that the anode voltage increased by 20 V or more in the case of the contaminated cathode. The anode voltage was measured for the sputter-contaminated cathode to evaluate the effect of contamination under more realistic conditions. After 630 h of sputter-contamination operation, it is estimated that sputtered particles were deposited to a thickness of 77 µm at most, and the anode voltage increased by 8 V. The results show that the downstream surface of the nozzle is critical for maintaining cathode performance. The insulating coating formed by the sputtered PTFE may interfere with ion absorption and degrade electron emission capability. A theoretical model based on the extended Brophy model supports these results. This study provides important information for the use of PTFE-based materials around ion thrusters.
This paper reports measurements of the xenon ground state and excited state densities inside a μ10 microwave ion thruster. This thruster exhibits a 40% thrust enhancement upon changing from the ...waveguide to the discharge chamber propellant injection mode. In the present work, the associated mechanism was quantitatively evaluated using two-photon laser induced fluorescence (TALIF) spectroscopy to monitor the thruster waveguide. The 834.7 nm emission from excited state xenon was investigated with a 224.3 nm dye laser to excite the Xe I 5p61 S0 6pʹ 3/22 state, compared with the emission without the laser. The resulting data confirm that the neutral density exhibits a linear relationship with the propellant flow rate in the cold gas and ionized state, while the ion acceleration decreases the neutral density by the same order of magnitude as the propellant utilization efficiency is changed. As the propellant flow rate increases, the collisions of neutrals that generate excited states occur in the waveguide and, when this process plateaus, the ground state emission suddenly increases. Propellant injection from the discharge chamber is evidently effective at suppressing collisions with electrons in the waveguide that generate excited states and that potentially interfere with microwave propagation.
•Xenon neutrals measurements inside a gridded ion thruster in operation.•Two-photon absorption laser-induced fluorescence spectroscopy with error of ±10%.•Comparisons in three modes: cold gas, ionized state, and beam extraction.•Consistent results with predictions on mass utilization efficiencies.
The authors redesigned the discharge chamber for the μ10 microwave discharge ion thruster to improve its thrust performance and succeeded in enhancing the maximum beam current and thrust efficiency. ...However, it was found that the ion current ratio extracted from the discharge chamber with the redesigned configuration was lower than that obtained with the original configuration. To investigate the relationship between ion extraction and the magnetic field geometry, the ion loss current distribution in these two types of discharge chamber were measured by electrostatic probes. Using planar probes with a guard ring, the ion current that flowed into the wall was measured without disturbing the ion beam current. The results show that ionization occurs mainly near the upstream magnet. In addition, the ion flux on the sidewall in the redesigned discharge chamber is about 1.5-2 times larger than that in the original discharge chamber. This suggests that the distance between the edge of the plasmaproduction region and the chamber wall with consideration of the Larmor radius of ions is an important parameter in discharge chamber design. In addition, although the ion beam current showed a tendency to saturate at high microwave power, the ion loss to each part in the discharge chamber increased in proportion to input microwave power. The decrease in the extracted ion ratio in the redesigned discharge chamber is considered to be caused by a decrease in the electrostatic ion transparency of the screen grid. Therefore, in a well-tuned microwave discharge ion thruster, it is difficult to improve the thrust efficiency by increasing the discharge power. A design that suppresses the wall loss of ions is thus important.
•Ion loss in two types of ECR ion thrusters was measured in each parameter.•Ion beam is saturated as increasing input microwave power.•Ion loss distribution depends on magnetic lines geometry near the wall.•Ions are weakly magnetized and lost on the order of their Larmor radius.•Reducing the loss region by strengthening the magnetic field near the wall is important.
In space operation of the microwave discharge ion thruster μ10 on the asteroid explorers Hayabusa and Hayabusa2, the propellant utilization efficiency deteriorated much more than in the ground ...endurance test. In this study, a fault tree analysis and experimental simulations of space operation were performed, focusing on the grid-derived internal carbon contamination. It was found that the performance deterioration due to the waveguide contamination matched that in Hayabusa2 qualitatively and quantitatively. Based on the experimental verification, the future performance is experimentally predicted.
•Performance deterioration investigation for microwave ion thruster μ10 in space.•Fault tree analysis showed possible cause: grid-derived carbon contamination.•Experiments revealed carbon on waveguide decrease propellant utilization efficiency.•Successful thruster performance reproduction in “Hayabusa2” operation by experiment.•Prediction of future thruter performance based on experimental verification.
The microwave power absorption efficiency of the 10 ECR ion thruster, utilized in the Japanese asteroid explorers Hayabusa and Hayabusa2, is investigated in order to allow performance measurement and ...provide information for its improvement. A model detailing the local electron behavior in a real ECR plasma discharge, based the magnetic field characteristics, is presented. Three methods to evaluate the microwave power absorption efficiency are proposed: an estimation based on the chamber geometry and magnetic field characteristics, a measuremen based on performance parameters and a measurement performed with Langmuir probes. The equations used for each method are analytically derived. The local electron behavior model is confirmed with a Langmuir probe experiment. Measurement of the microwave power absorption efficiency is performed with the two independent methods proposed. Results from the two experiments show good agreement with each other and with the theory. Finally, a diffusion model explaining the different electron temperature distributions observed in the chamber is proposed. The model and experiments clarify the physics behind previously observed performance variations and give valuable hints for future chamber improvement.
In electron cyclotron resonance (ECR) thrusters, the plasma mode transition is a critical phenomenon because it determines the maximum thrust performance. In ECR ion thrusters, ionization generally ...occurs in the magnetic confinement region, where electrons are continuously heated by ECR and confined by magnetic mirrors. However, as the flow rate increases, ionization is also observed outside the magnetic confinement region, and this induces the plasma mode transition. In our previous work, two-photon absorption laser-induced fluorescence (TALIF) analysis revealed that the stepwise ionization from the metastable state plays an important role in the ionization process. However, the distribution of the stepwise ionization has not yet been revealed because of the long lifetime of the metastable state. In this study, this distribution was investigated using one experimental and two numerical approaches. First, TALIF was applied to two types of gas injection with clear differences in thrust performance and ground-state neutral density distribution. In the first simulation, the metastable state particle simulation was used to estimate the excitation rate distribution. In the second study, simulations of the electric field of microwaves were used to estimate the contribution of the stepwise ionization to the plasma density. The experimental and numerical results revealed that the stepwise ionization spreads outside the magnetic confinement region because of the diffusion of metastable particles, and this spread induces the plasma mode transition, explaining the difference between the two types of gas injection.
To investigate the neutral xenon density distribution of electric thrusters such as ion and Hall thrusters, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy was applied to a ...microwave cathode. First, the background pressure of the vacuum chamber was measured by TALIF. In the present measurements, the ground state was excited by a 224.29 nm laser, and 834.68 nm fluorescence was detected. The first measurement confirmed that the fluorescence intensity linearly increases with respect to the ground state number density. Based on this result, the density of neutral ground-state xenon was measured at the exit of the nozzle of the microwave cathode. The variation in the density with the microwave power was successfully measured at xenon flow rates of 0.029 and 0.098 mg/s. The measured densities varied from 2.3 × 1019 to 8.4 × 1019 m3 with a maximum error of ±20% due to the plasma fluorescence.
This paper reports the first study to measure xenon neutral ground state particle density of microwave cathode by two-photon laser induced fluorescence spectroscopy (TALIF). Xenon is commonly used as ...a propellant in electric propulsion like Hall thrusters, ion thrusters, and their cathodes. For electric propulsion, information about neutral particles is important such as the ionization degree and the charged exchange collisions (CEX). The measurement target is XeI 5p61S06p3/22, which absorbs at a wavelength of 224.29 nm and emits fluorescence of 834.7 nm. The measurement system was demonstrated for three cases: cold gas, without electron extraction, with electron extraction. From three cases, the measurement system can detect a neutral ground state particle density of 1019m−3 order without and with a plasma. In a cold gas, the neutral ground state particle density is (8.4±0.4)×1019m−3 at 0.098 mg/s. Without electron extraction, the neutral ground density decreases by ionization and excitation With electron extraction, the density varied from 0.6 to 2.3 times compared to without electron extraction depending on anode voltage.
•Order of 1019 m-3 neutral density at ground state can be measured by TALIF.•Detect neutral density decrease by ionization and excitation.•Transition of neutral density is observed depending on anode voltage.
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