Microplasma Thruster Research Articles

Microplasma thruster powered by X-band microwaves

A microplasma thruster of the electrothermal type has been developed with X-band (f = 11 GHz) microwaves and Ar as a propellant. The emphasis was placed on an understanding of distinguished features of the microplasma generation and thrust performance by X-band microwaves, compared with those by S-band (f = 4 GHz) ones. The thruster consisted of a microplasma source 2 mm in the inner diameter and 3–12 mm long with a rod antenna on the axis, followed by a converging-diverging micronozzle. Azimuthally symmetric surface wave-excited plasmas were established by microwaves at powers of ≤6 W, with the source pressure in the range 2–50 kPa at flow rates of 10–70 sccm. The plasma generation, nozzle flow, and thrust performance were numerically analyzed using a two-dimensional fluid model, coupled with an electromagnetic model for microwaves interacting with plasmas in the source region. Simulations indicated that higher frequency f = 11 GHz with the source chamber length Ls ≈ 1/4 of the driving wavelength λ is preferred for the microplasma thruster in terms of efficient plasma generation, gas heating, and thus thrust performance as well as system compactness; moreover, in f = 11 GHz discharges with longer Ls ≈ 3λ/4, standing-wave striation-like plasma structures occur in the axial direction. Experiments were made for f = 11 and 4 GHz microwaves with the respective Ls ≈ λ/4, where the plasma electron density and gas temperature in the microplasma source were measured by optical emission spectroscopy with a small amount of H2 and N2 added. The electron density and gas temperature were in the range of (4–12) × 1019 m−3 and 800–1000 K for f = 11 GHz, being ∼10%–50% higher than those for f = 4 GHz. The thrust performance was also measured by a target-type microthrust stand, giving a thrust, specific impulse, and thrust efficiency in the range 0.2–1.8 mN, 65–90 s, and 2%–14% for f = 11 GHz, which were ∼10%–15% higher than those for f = 4 GHz. These experimental results were consistent with those of simulations, depending on microwave frequency, power, chamber size, and gas flow rate.

Numerical analysis of direct-current microdischarge for space propulsion applications using the particle-in-cell/Monte Carlo collision (PIC/MCC) method

Microdischarges are an important type of plasma discharge that possess several unique characteristics, such as the presence of a stable glow discharge, high plasma density and intense excimer radiation, leading to several potential applications. The intense and controllable gas heating within the extremely small dimensions of microdischarges has been exploited in micro-thruster technologies by incorporating a micro-nozzle to generate the thrust. This kind of micro-thruster has a significantly improved specific impulse performance compared to conventional cold gas thrusters, and can meet the requirements arising from the emerging development and application of micro-spacecraft. In this paper, we performed a self-consistent 2D particle-in-cell simulation, with a Monte Carlo collision model, of a microdischarge operating in a prototype micro-plasma thruster with a hollow cylinder geometry and a divergent micro-nozzle. The model takes into account the thermionic electron emission including the Schottky effect, the secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform argon background gas density in the cathode–anode gap. Results in the high-pressure (several hundreds of Torr), high-current (mA) operating regime showing the behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and anode are presented and discussed. In addition, the results of simulations showing the effect of different argon gas pressures, cathode material work function and discharge voltage on the operation of the microdischarge thruster are presented. Our calculated properties are compared with experimental data under similar conditions and qualitative and quantitative agreements are reached.

Microwave-excited microplasma thruster with helium and hydrogen propellants

Microplasma thruster of electrothermal type has been investigated with feed or propellant gases of He and H2. The thruster consisted of an azimuthally symmetric microwave-excited microplasma source 1.5 mm in diameter and 10 mm long with a rod antenna on axis, and a converging-diverging micronozzle 1 mm long with a throat 0.2 mm in diameter. Surface wave-excited plasmas were established by 4.0-GHz microwaves at powers of ≤ 6 W, with the source pressure in the range 0.5–12 kPa at flow rates of 2–70 sccm. The microplasma generation, micronozzle flow, and thrust performance with He were numerically analyzed by using a two-dimensional fluid model, coupled with an electromagnetic model for microwaves interacting with plasmas in the source region. In experiments, the plasma electron density and gas temperature in the microplasma source were measured at around the top of the microwave antenna, or just upstream of the micronozzle inlet, by optical emission spectroscopy with a small amount of additive gases of H2 and N2. In the case of He propellant, the Stark broadening of H Balmer-β line and the vibronic spectrum of N2 2nd positive (0, 2) band indicated that the electron density was in the range (2-5)×1019m-3 and the gas or rotational temperature was in the range 600–700 K. The thrust performance was also measured by using a target-type microthrust stand, giving a thrust in the range 0.04–0.51 mN, a specific impulse in the range 150–270 s, and a thrust efficiency in the range 2%–12%. These experimental results were consistent with those of numerical analysis, depending on microwave power and gas flow rate. Similar plasma characteristics and thrust performance were obtained with H2 propellant, where the specific impulse of ≤ 450 s was more than 1.5 times higher than that with He, owing to a difference in mass between He and H2. A comparison with previous studies with Ar propellant [T. Takahashi et al., Phys. Plasmas 16, 083505 (2009)] indicated that in the presence as well as absence of plasma discharge, the specific impulse was enhanced by more than 3–5 times with light-mass propellants He and H2 as has been known for large-scale propulsion systems. Thus, it follows that in the microplasma thruster of electrothermal type, the high diffusivity and thermal conductivity of He and H2 in the microplasma source of high surface-to-volume ratios do not lead to a deterioration of the thrust performance, primarily owing to a more significant thermal energy gain due to elastic collisions between electrons and heavy particles in He and H2.

Experimental Investigation of Microhollow Cathode Discharge for the Application to Microplasma Thrusters

As to microhollow cathode discharge (MHCD) in argon, the discharge characteristic was analyzed, the gas temperature was determined from the emission spectroscopy with traces of nitrogen added, and the electron density was also evaluated through the Stark broadening of the Balmer H β line from hydrogen as trace impurity in the gas. The electron density is in the order of 10 14 cm −3 and increases with the increasing gas pressure and discharge current. The gas temperature also increases with the increasing pressure and current, and can reach 1000 K in the stable glow regime. The results show that the extremely small dimensions of the MHCD combining with relative intense and controllable gas heating can be applied as a new micro electro-thermal thruster for micro-satellite propulsion. Due to the heating of the propellant gas in the microdischarge plasma and its expansion through the micro-nozzle, the performance of specific impulse and thrust of the microplasma thruster using the MHCD will be higher than the conventional cold gas microthrusters.

Computational simulation of coupled nonequilibrium discharge and compressible flow phenomena in a microplasma thruster

The microplasma thruster (MPT) concept is a simple extension of a cold gas micronozzle propulsion device, where a direct-current microdischarge is used to preheat the gas stream to improve the specific impulse of the device. Here we study a prototypical MPT device using a detailed, self-consistently coupled plasma and flow computational model. The model describes the microdischarge power deposition, plasma dynamics, gas-phase chemical kinetics, coupling of the plasma phenomena with high-speed flow, and overall propulsion system performance. Compared to a cold gas micronozzle, a significant increase in specific impulse is obtained from the power deposition in the diverging section of the MPT nozzle. For a discharge voltage of 750 V, a power input of 650 mW, and an argon mass flow rate of 5 SCCM (SCCM denotes cubic centimeter per minute at STP), the specific impulse of the device is increased by a factor of ∼1.5 to about 74 s. The microdischarge remains mostly confined inside the micronozzle and operates in an abnormal glow discharge regime. Gas heating, primarily due to ion Joule heating, is found to have a strong influence on the overall discharge behavior. The study provides a validation of the MPT concept as a simple and effective approach to improve the performance of micronozzle cold gas propulsion devices.

Numerical and experimental study of microwave-excited microplasma and micronozzle flow for a microplasma thruster

Plasma and aerodynamic features have been investigated for a microplasma thruster of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The thruster developed consisted of a microplasma source 1.5 mm in diameter, 10 mm long with a rod antenna on axis, and a converging-diverging micronozzle 1 mm long with a throat 0.2 mm in diameter. The feed or propellant gas employed was Ar at pressures of 10–50 kPa with flow rates of 10–70 SCCM (SCCM denotes standard cubic centimeter per minute at STP) and the surface wave-excited plasmas were established by 4.0 GHz microwaves at powers of ≤6 W. Numerical analysis was made for the plasma and flow properties by developing a self-consistent, two-dimensional model, where a two-temperature fluid model was applied to the entire region through the microplasma source to the micronozzle (or through subsonic to supersonic); in the former, an electromagnetic model based on the finite difference time-domain approximation was also employed for analysis of microwaves interacting with plasmas. In experiments, optical emission spectroscopy was employed with a small amount of additive gases of H2 and N2, to measure the plasma electron density and gas temperature in the microplasma source around the top of the microwave antenna, just upstream of the micronozzle inlet; in practice, the numerical analysis exhibited a maximum thereabout for the microwave power density absorbed, plasma density, and gas temperature. The Stark broadening of H Balmer line and the vibronic spectrum of N2 second positive band indicated that the electron density was in the range of (3–12)×1019 m−3 and the gas or rotational temperature was in the range of 700–1000 K. The thrust performance was also measured by using a microthrust stand with a combination of target and pendulum methods, giving a thrust in the range of 0.2–1.4 mN, a specific impulse in the range of 50–80 s, and a thrust efficiency in the range of 2%–12%. These experimental results were consistent with those of numerical analysis, depending on microwave power and gas flow rate.

Microwave-excited microplasma thruster: a numerical and experimental study of the plasma generation and micronozzle flow

A microplasma thruster of the electrothermal type has been developed using azimuthally symmetric microwave-excited microplasmas. The plasma source was ∼2 mm in diameter and ∼10 mm long, and operated at around atmospheric pressures; the micronozzle was a converging–diverging type, having a throat ∼0.2 mm in diameter and ∼1 mm long. Numerical and experimental results with Ar demonstrated that this miniature electrothermal thruster gives a thrust of >1 mN, a specific impulse of ∼100 s and a thrust efficiency of ∼10% at a microwave power of <10 W.

Microplasma thruster for ultra-small satellites: Plasma chemical and aerodynamical aspects

AbstractA microplasma thruster has been developed of electrothermal type using azimuthally symmetric microwave-excited microplasmas. The microplasma source was ~2 mm in diameter and ~10 mm long, being operated at around atmospheric pressures; the micronozzle was a converging-diverging type, having a throat ~0.2 mm in diameter and ~1 mm long. Numerical and experimental results with Ar as a working gas demonstrated that this miniature electrothermal thruster gives a thrust of &gt;1 mN, a specific impulse of ~100 s, and a thrust efficiency of ~10 % at a microwave power of &lt;10 W, making it applicable to attitude-control and station-keeping maneuver for a microspacecraft of &lt;10 kg.

A miniature electrothermal thruster using microwave-excited microplasmas: Thrust measurement and its comparison with numerical analysis

A microplasma thruster has been developed, consisting of a cylindrical microplasma source 10mm long and 1.5mm in inner diameter and a conical micronozzle 1.0–1.4mm long with a throat of 0.12–0.2mm in diameter. The feed or propellant gas employed is Ar at pressures of 10–100kPa, and the surface-wave-excited plasma is established by 4.0GHz microwaves at powers of &amp;lt;10W. The thrust has been measured by a combination of target and pendulum methods, exhibiting the performance improved by discharging the plasma. The thrust obtained is 1.4mN at an Ar gas flow rate of 60SCCM (1.8mg∕s) and a microwave power of 6W, giving a specific impulse of 79s and a thrust efficiency of 8.7%. The thrust and specific impulse are 0.9mN and 51s, respectively, in cold-gas operation. A comparison with numerical analysis indicates that the pressure thrust contributes significantly to the total thrust at low gas flow rates, and that the micronozzle tends to have an isothermal wall rather than an adiabatic.