Optimization of Magnetic Nozzle Configuration and Hybrid Propellant for Radio-Frequency Plasma Micro-Thrusters in Very Low Earth Orbit Applications

Abstract

Very low Earth orbit (VLEO) satellites are confronted with the challenge of orbital decay caused by thin atmospheres, and the volume and power limitations of micro satellites further restrict the application of traditional electric propulsion systems. In response to the above requirements, this study proposes an innovative scheme of radio frequency plasma micro-thrusters based on magnetic nozzle acceleration technology. By optimizing the magnetic nozzle configuration through the system, the plasma confinement efficiency was significantly enhanced. Combined with the mixed working medium (5 sccm Xe + 10 sccm air), the thrust reached 1.7 mN at a power of 130 W. Experiments show that the configuration of the magnetic nozzle directly affects the plasma beam morphology and ionization efficiency, and a multi-magnet layout can form a stable trumpet-shaped plume. The air in the mixed working medium has a linear relationship with the thrust gain (60 μN/sccm), but xenon gas is required as a “seed” to maintain the discharge stability. The optimized magnetic nozzle enables the thruster to achieve both high thrust density (13.1 μN/W) and working medium adaptability at a power level of hundreds of watts. This research provides a low-cost and miniaturized propulsion solution for very low Earth orbit satellites. Its magnetic nozzle-hybrid propellant collaborative mechanism holds significant engineering significance for the development of air-aspirating electric propulsion technology.

1. Introduction

In recent years, with the rapid innovation of aerospace technology, very low Earth orbit (VLEO, usually referring to an altitude of 100–300 km) satellites have gradually become a research hotspot in the field of space exploration and application. Space orbit resources play a crucial role in the national economy and national defense construction, and are widely applied in multiple fields such as global positioning, network interconnection, ground observation, communication and exchange, scientific experiments, and military reconnaissance, demonstrating irreplaceable value [1]. Compared with traditional low-orbit satellites (LEO, 500–2000 km), VLEO satellites have demonstrated irreplaceable application value in fields such as military reconnaissance, high-precision Earth observation, meteorological monitoring, and disaster early warning, thanks to their significant spatial advantages, such as higher remote sensing resolution, lower communication latency, and more flexible mission adaptability. However, this orbital environment also poses severe challenges to the long-term stable operation of the satellite. Due to the significant reduction in orbital altitude, the residual gas resistance of the Earth’s dense atmosphere on satellites has increased significantly, resulting in an accelerated orbital decay rate and a substantial shortening of their lifespan [2]. Although there are a large number of satellites currently in orbit at higher orbits, no satellite has been able to operate stably for a long time within the VLEO altitude range of 100 to 250 km. Although traditional chemical propulsion systems can extend the mission cycle by maintaining frequent orbits, their low specific impulse and rapid fuel consumption severely restrict the payload ratio and mission economy of satellites. Against this backdrop, high-efficiency and long-life electric propulsion systems have emerged as the core solution to break through the technical bottlenecks of VLEO satellites.

Currently, global space agencies are intensifying research and development efforts to enhance the efficiency, environmental adaptability, and reliability of electric propulsion systems for very low Earth orbit (VLEO) applications. Traditional electric propulsion (EP) systems, such as ion and Hall effect thrusters, rely on storing heavy propellant gases like xenon or krypton in high-pressure vessels. For long-durations and low-thrust missions in VLEO, the mass of this stored propellant becomes a major limitation. In contrast, Air-Breathing Electric Propulsion (ABEP) systems offer a promising solution by eliminating the need to carry most, or even all, of the propellant mass. This enables spacecraft equipped with ABEP to carry more scientific instruments or other payloads and operate for significantly longer durations within the usable atmosphere layer. Crucially, their operational lifetime is limited primarily by component durability and orbital decay management, rather than propellant depletion.

An ABEP system functions similarly to a hybrid between a ramjet and an ion thruster. As the spacecraft orbits at high velocity (around 7.8 km/s), atmospheric molecules (primarily N₂ and O₂) collide with it at hypersonic speeds. An aerodynamically designed intake scoop collects these incoming molecules. However, the collected atmospheric gas is extremely diffuse and must be compressed before it can be efficiently utilized for propulsion.

This compression is achieved through mechanical means (pumps or compressors) and/or aerodynamic compression within the intake duct itself. Compressing the gas increases its density and pressure, making the subsequent ionization step more effective. It is important to note that this compression stage requires significant power.

Following compression, the densified gas enters the ionization chamber. There, it is ionized and the resulting ions are accelerated electrostatically to generate thrust.

With the breakthroughs in miniaturized and modular electric propulsion technology, the large-scale application prospects of ultra-low orbit satellites in constellation networking, real-time remote sensing, and other fields are gradually becoming clear. In commercial space missions, the most commonly used micro-thrusters are Hall thrusters, such as the Space X Low Earth Orbit communication constellation, followed by ion thrusters, which have been widely applied in geosynchronous orbit, deep space exploration and other tasks [3,4,5,6,7,8,9,10,11]. Although Hall thrusters and ion thrusters have advantages, such as high specific impulse and large thrust, their system complexity and design cost significantly restrict their application in micro satellite platforms and commercial aerospace. These two types of electric propulsion systems not only require the installation of dedicated neutralizers to eliminate the charge accumulation of ion plumes, but also rely on complex electronic systems. This multi-subsystem integration architecture leads to a relatively large thruster size, low reliability and high single-unit cost. For commercial space missions that emphasize low cost and high reliability (such as the deployment of constellation satellites), these technical and economic characteristics have seriously affected their market applicability. Among them, radio frequency plasma has attracted increasing attention due to its advantages such as high density, no electrodes, fewer design structures and more operation modes [12,13,14,15]. The magnetic nozzle, as the core structure of the radio frequency plasma thruster, has attracted many research groups to join the research one after another since 2020. Sekine et al. from the University of Tokyo began to study the magnetic nozzles of thrusters in the inductive coupling discharge mode [16,17]. Kim et al. from Seoul National University, South Korea [18], Vinci et al. from the ICARE Laboratory of the French Academy of Sciences, Boni et al. from the French National Aerospace Laboratory [19,20] studied the thermodynamic processes of electrons in magnetic nozzles.

In the field of magnetic field research, the influence of magnetic fields on the plasma ionization process, spatial distribution characteristics and thrust performance of traditional electric thrusters such as Hall thrusters and ion thrusters has been systematically studied. However, research on radio frequency plasma micro-thrusters is relatively limited, especially as the magnetic field serves as an effective means to regulate the movement and acceleration of plasma. Study into radio frequency plasma control is still insufficient [19]. The configuration of magnetic nozzles has a very significant impact on RF plasma micro-thrusters. Studying the configuration of magnetic nozzles is of great significance for understanding the plasma acceleration mechanism and performance optimization of RF plasma micro-thrusters.

This paper designs an aspirating radio frequency plasma micro-thruster with an inner diameter of 10 mm in the discharge cavity. By using multiple annular permanent magnets of different sizes to generate a magnetic field around it, the verification of the magnetic nozzle constraint and acceleration mechanism as well as the feasibility study of the mixed working medium are carried out.

2. Operating Principle and Experimental Scheme

2.1. Operating Principle

The RF plasma micro-thruster mainly consists of an RF coil (5 turns), a cylindrical quartz glass discharge chamber with a length of 30 mm and a diameter of 10 mm and a magnetic nozzle composed of samarium cobalt permanent magnets, as shown in Figure 1. Among them, the dotted box represents “groups of permanent magnets with different configurations”, which generate stable plasma based on the principle of inductive coupling self-sustaining discharge, and then accelerate the ejection of the plasma through the magnetic nozzle system.

The inductive coupling plasma ignition mode of the thruster is self-excitation ignition. During the RF energy feeding stage, the RF power supply couples high-frequency electromagnetic energy to the working medium gas in the discharge chamber through an impedance matching network. When the radio frequency electric field intensity exceeds the gas breakdown threshold, free electrons gain sufficient energy in the alternating electric field to collide and ionize with neutral atoms, forming a self-sustaining discharge of radio frequency plasma.

The core of a magnetic nozzle lies in its specific magnetic field design. The magnetic field lines of a magnetic nozzle are usually divergent (similar to a horn shape), generated by electromagnetic coils or permanent magnets, and the magnetic field intensity gradually weakens along the direction of the thruster outlet.

Due to the absence of a steady-state arc in the discharge chamber of the radio frequency discharge type magnetic nozzle thruster and the usually low magnetization degree of heavy particles, the plasma usually lacks rotational kinetic energy. In this case, the energy conversion in the magnetic nozzle mainly occurs through acceleration mechanisms such as bipolar electric fields and antimagnetic mirror acceleration, converting the internal energy of the plasma into axial kinetic energy [21].

Electrons, due to their small mass, are easily confined by magnetic fields and move along magnetic field lines. In the divergent region of the magnetic field, the lateral movement of electrons is restricted, leading to charge separation and the formation of an electrostatic field (electric potential gradient). The electrostatic field exerts Coulomb force on positively charged ions, accelerating them along the axial direction (thrust direction). Ions have a relatively large inertia and are less directly affected by magnetic fields. They mainly rely on electric fields to accelerate to high speeds. Meanwhile, due to the high temperature of the plasma heating the surrounding neutral gas, it acquires a large amount of heat energy, causing the gas to expand and be ejected, generating aerodynamic force, which is one of the main sources of thrust.

Mathematically speaking, the generation of plasma is achieved by the energy fed into the radio frequency antenna. Radio frequency ion thrusters generally adopt inductive coupling methods. Radio frequency coils are wound around the outer surface of the dielectric container and connected to the radio frequency power supply. When a radio frequency current (I_RF) flows through a coil, an axial magnetic field that varies with time will be induced according to Ampere’s law, and it is proportional to the I_RF. The axial magnetic induction intensity is

$$B_z={\mu }_0{\displaystyle \frac{N}{l}}{I}_{\mathrm{RF}}{e}^{i\omega t}$$

(1)

In the formula, μ₀ represents the vacuum permeability, N/l represents the number of coil turns per unit length, I_RF represents the radio frequency current, and $\omega =2\pi f$ represents the radio frequency angular frequency. According to Maxwell’s equations, the time-varying magnetic field B generates a time-varying azimuth electric field E around the axis of the helical coil.

$$\nabla \times \overrightarrow{E}=-{\displaystyle \frac{\partial \overrightarrow{B}}{\partial t}}$$

(2)

The induced angular radio frequency electric field intensity is

$$E_{\theta }=-{\displaystyle \frac{i\omega r}{2}}{B}_{z_0}{e}^{i\omega t}$$

(3)

In the formula, r represents the distance from the axis, and B_z0 is the maximum value of the axial magnetic field. Electrons travel at a very high speed in the discharge chamber. Generally, the time they take to pass through the affected area of the coil is much shorter than the frequency of the electric field. Therefore, they are not affected by the oscillation of the electric field. They are only accelerated and collide with neutral particles to ionize them, forming an inductively coupled plasma (ICP).

As for the plasma extraction process, it mainly relies on the effect of the magnetic nozzle, which can be explained as the extraction of ions by the bipolar electric field, thereby generating a micro-thrust. Here, the principle of the magnetic nozzle is analyzed in detail.

In a magnetic nozzle, the acceleration of charged particles depends on the principle of magnetic mirrors, the conservation of magnetic moment, and the conservation of momentum.

Suppose there is a magnetic field pointing in the z direction, and its magnitude varies in the z direction, making the magnetic field axisymmetric. $\partial B/\partial \theta =0$ and due to the convergence and divergence of magnetic field lines, there must exist a component B_r, and charged particles will inevitably be constrained by the component B_r and thus captured by the magnetic field.

From $\nabla ·B=0$, we can get B_r:

$${\displaystyle \frac{1}{r}}{\displaystyle \frac{\partial }{\partial r}}(r{B}_r)+{\displaystyle \frac{\partial B_z}{\partial z}}=0$$

(4)

Integrate radially over the above equation, if the value of $\partial B_z/\partial z=0$ at z = 0 is given, and since it does not change much with r, it has, approximately,

$$r{B}_r=-{\displaystyle {\int }_0^{r}r\frac{\partial B_z}{\partial z}}dr=-{\displaystyle \frac{1}{2}}{r}^2{[{\displaystyle \frac{\partial B_z}{\partial z}}]}_{r=0}$$

(5)

B_r here is

$$B_r=-{\displaystyle \frac{1}{2}}r[{\displaystyle \frac{\partial B_z}{\partial z}}{]}_{r=0}$$

(6)

The three components of the Lorentz force acting on a particle in a cylindrical coordinate system are, respectively:

$$\begin{array}{l}{F}_r=q(v_{\theta }{B}_z-v_z{B}_{\theta })\\ F_{\theta }=q(-v_r{B}_z-v_z{B}_r)\\ F_z=q(v_{\theta }{B}_{\theta }-v_{\theta }{B}_r)\end{array}$$

(7)

The B_θ here is 0 so there are still four Lorentz force components remaining on the right side. Substituting Equation (5), we can obtain

$$F_z={\displaystyle \frac{1}{2}}q{v}_{\theta }r({\displaystyle \frac{\partial B_z}{\partial z}})$$

(8)

To provide a more in-depth explanation of the working principle of the radio frequency plasma thruster, it is necessary to analyze the plasma dynamic phenomena in the discharge chamber and the magnetic nozzle, respectively. The generation of dense plasma (10¹⁹ m⁻³) is controlled by the propagation of whistle sound waves in the discharge chamber.

Specifically, the power deposition phenomenon stems from the collision process of electromagnetic waves such as dumping. Furthermore, due to the movement of charged particles and the diffusion process that helps achieve stable discharge, the generation of density is non-uniform. The resulting density gradient greatly alters the structure of the wave, so radial non-uniformity can generate local waves. Therefore, the key physical phenomena that control the plasma dynamics in the discharge chamber are the propagation of electromagnetic waves, the transport of plasma and their mutual coupling.

The magnetic nozzle area extends downstream of the plasma source. Here, the plasma is accelerated and eventually separates from the static magnetic field lines. The characteristic of the magnetic nozzle region is the formation of a plume, in which the plasma is rarer than the source (with a density ranging from 10¹⁶ to 10¹⁸ m⁻³). According to the phenomena that control plasma dynamics, two regions can be distinguished in the plume, which are respectively called the near region and the far region. In the near region, particle collisions and the geometry of the applied static magnetic field drive the behavior of the plasma. On the contrary, in the far region, the expansion of plasma is mainly controlled by thermal pressure and bipolar diffusion.

2.2. Design and Experimental Scheme of Micro-Thruster

To explore the influence of the magnetic nozzle configuration of the radio frequency plasma micro-thruster, this paper first attempted multiple magnetic nozzle configurations. By observing the size, divergence degree and color of the plasma plume, the influence of the magnetic nozzle on the plasma density and acceleration was determined. The working medium gas was xenon (Xe).

The experimental system of this project is shown in Figure 2.

2.3. Experimental Facility

In the RPT-1 experiment conducted in this paper, the main experimental devices used include the ground simulation experiment platform for space vacuum environment (Figure 3) and the high-precision micro-thrust measurement equipment (Figure 4). The ground simulation experiment platform for the space vacuum environment can ensure that the vacuum degree reaches the level of 10⁻⁴ Pa before work and remains at the level of 10⁻³ Pa when the thruster is in operation. Its diameter is 1.8 m and the pumping speed is 32,000 L/s.

Thrust measurement is carried out using a high-precision micro-thrust measuring instrument, as shown in Figure 4. It features a large load capacity, high precision (±1%), and easy installation and adjustment. It meets the calibration requirements of technical indicators such as thrust range, thrust resolution, and response time for various micro-thrusters, as shown in

Table 1. Main parameters of the high-precision micro-thrust measuring instrument.

Thrust Measurement Range	Measuring Error	Thrust Resolution
0~2000 μN	≤1%	1 μN

. The relevant principles can be referred to in reference [22].

Considering that the thrust measurement needs to ensure high precision, here follows an explanation of how this thruster filters out external vibrations. By adjusting the center of mass position of the thrust frame to the axis of rotation through weight distribution, the influence of ground vibrations can be significantly reduced. Because, ideally, the vibrations transmitted from the ground to the thrust frame are equivalent to the force acting on the axis of rotation, with a force arm of zero, the thrust frame will not deflect and will have little impact on the measurement. Moreover, most vibrations are of high frequency, and the thrust frame itself is a second-order system with a relatively low natural frequency. After installing the thrusters, the natural frequency may be less than 0.1 Hz, making it insensitive to high-frequency vibrations. Additionally, through low-pass filtering in the software, high-frequency noise is further eliminated.

3. Results

3.1. The Qualitative Influence of Magnetic Nozzle Configuration on Plasma

This subsection systematically studies the influence laws of different magnetic nozzle configurations (including non-magnetic nozzles and magnetic nozzles of different configurations) on the plasma density distribution and plume characteristics. The power of the fixed RF coil in the experiment was 60 W (5.3 MHz), and the flow rate of the Xe working medium was 1 sccm. The magnetic induction intensity at the center of the permanent magnet is 0.1 T.

Due to the large opening of the discharge chamber in the working condition, the internal pressure is too low to be maintained for a long time. To improve the ionization efficiency in the discharge chamber, this study optimized the design of the outlet structure. Theoretical analysis indicates that a fully open outlet structure will lead to an excessively short residence time of neutral gas and an excessively low pressure in the discharge chamber. By installing mica plates at the outlet, the effective conduction area is reduced to 81% of the original discharge chamber. The plasma plume can be stably maintained for a long time, as shown in Figure 5a.

To further optimize the plasma confinement effect, a permanent two-magnet assembly was added on the basis of the improved mica plate outlet, as shown in Figure 5b. This permanent magnet is made of samarium cobalt permanent magnet material and features a ring-shaped structure (inner diameter 30 mm/outer diameter 50 mm/thickness 5 mm). The axial center magnetic induction intensity was measured to be 1000 G by a gaussmeter. When installing the magnet, a polarity configuration with the N pole facing the inner part of the discharge chamber is adopted, aiming to form a converging magnetic field configuration. By comparing the plume morphology in Figure 5a (without a magnetic field) with that in Figure 5b (with a magnetic field applied), a significant confinement effect can be clearly observed: under the influence of the magnetic field, the plasma plume transforms from a dispersed state to a regular trumpet-shaped structure, and the radial expansion is significantly weakened. This phenomenon directly proves that the magnetic field has a significant coring effect on plasma.

To study the influence of magnetic field polarity on plasma, the polarity of the permanent magnet was reversed (N–S poles interchanged) based on the structure in Figure 5b. Experimental observations show (Figure 5c) that neither the plasma ignition characteristics nor the plume morphology have undergone significant changes. Based on theoretical analysis, it can be known that the core mechanism of magnetic nozzle acceleration is bipolar electric field acceleration, and its essence is to constrain the movement of electrons through magnetic field lines. Although the polarity of the magnetic field determines the direction of electron helical motion (left-rotating/right-rotating), this directional difference does not affect the macroscopic behavior of electrons being ejected along the axial direction. Therefore, there is no significant correlation between the plasma state and the polarity of the magnetic field.

To explore the influence of an external magnet on the characteristics of plasma, a third permanent magnet was added on the basis of the structure in Figure 5b, as shown in Figure 5d. Experimental observations show that the plasma density has slightly increased, but the effect is limited. Moreover, the plume constraint pattern did not show significant improvement. This phenomenon stems from the diffusion characteristics of plasma under conditions without solid wall constraints. When the plasma leaves the discharge chamber, its density continuously decreases with the axial distance. Therefore, although increasing the number of magnets can extend the constraint area, due to the free expansion effect, its benefits decrease as the number of magnets increases.

The two permanent magnets in Figure 5b,c are joined together in the same polarity direction, that is, N–S–N–S (S–N–S–N). Experiments further demonstrated that when the alternating polarity arrangement of N–S–S–N or S–N–N–S was adopted, the combination of permanent magnets significantly suppressed the plasma extraction efficiency, and no positive gain effect was observed. This phenomenon contrasts sharply with the aforementioned unipolar arrangement (N–S–N–S or S–N–S–N), indicating that the magnetic pole arrangement has a decisive influence on the plasma transport process.

It can be seen that a mushroom-shaped plasma halo is generated at the outlet of the magnetic nozzle as Figure 5d. From the magnetic flux density model and magnetic field lines numerically simulated, it can be seen that this mushroom-shaped halo is due to some edge-charged particles being guided back to the outlet by the magnetic field lines, as shown in Figure 6a. To verify this view, we further increased the size of the peripheral permanent magnet, and a much larger mushroom-shaped plasma halo can be clearly observed, as shown in Figure 6b.

3.2. Experimental Results and Analysis

Ultimately, we chose the magnetic field configuration shown in Figure 5d for experimental testing. Each of the tests lasts approximately half an hour in total, and for a certain working conditions, the test lasts about one minute. During this period, the thrust measurement value remains relatively stable.

3.2.1. Results and Analysis of Pure Aerodynamic Thrust

In the unignited state, when 1 sccm xenon gas (Xe) is introduced, the change in thrust with the addition of air flow is shown in Figure 7. It can be observed from the figure that when the air flow increases from 0 to 3 sccm, the thrust shows a linear change, and its order of magnitude is approximately tens of micro newtons. According to the data in the illustration, the aerodynamic force of 1 sccm xenon gas is approximately 16 μN, while that of 1 sccm air is approximately 13 μN.

3.2.2. Results and Analysis of Pure Xenon Thrust

Figure 8 and Figure 9, respectively, show the changes in thrust under pure xenon working medium when the radio frequency power is changed with a fixed flow rate and when the flow rate is changed with a fixed radio frequency power.

As can be seen from Figure 8, when the xenon gas flow rate is fixed at 3 sccm, the thrust increases with the increase of radio frequency power. At 40 W, the thrust can reach 200 μN, the specific impulse is 70 s and when the RF power reaches 120 W, the thrust reaches 360 μN, and the specific impulse is 125 s. When the RF power is less than 70 W, the growth slope of the thrust is relatively large, while when the RF power is greater than 70 W, and the growth slope of the thrust decreases. The reason for this phenomenon lies in that as the radio frequency power increases, the ionization process of the plasma approaches saturation, and the thrust gain mainly comes from the aerodynamic force generated by electric heating, while the contribution of electromagnetic acceleration relatively decreases.

Figure 9 shows the relationship between thrust and xenon flow rate under the condition that the working medium is pure xenon (Xe) and the fixed RF power is 60 W. It can be clearly seen from the figure that the thrust shows a linear growth trend as the xenon gas flow rate increases. When the xenon gas flow rate is 1 sccm, the thrust is approximately 160 μN, and the specific impulse is 167 s. When the xenon gas flow rate increases to 12 sccm, the thrust increases to 1000 μN, and the specific impulse is 87 s. Compared with pure aerodynamic power before ignition, the plasma accelerated by magnetic nozzles can significantly enhance the thrust performance of the thruster while effectively focusing the plume.

3.2.3. Results and Analysis of Mixed Propellant Thrust

Under the condition of a fixed RF power of 60 W, air working medium was gradually added to explore the changes in thrust. The results are shown in Figure 10, where the xenon (Xe) flow rate is fixed at 1 to 5 sccm and air is added at a step of 0.5 sccm. To more intuitively demonstrate the influence of changes in air flow rate and xenon gas flow rate on thrust, the air flow rate in the figure is continuously increasing rather than starting from 0 sccm. It can be observed that, under the condition of a fixed xenon gas flow rate, the thrust increases linearly with the increase of air flow rate. For every 0.5 sccm increase in air, the thrust approximately increases by 30 μN. When the xenon gas flow rate is increased, the increase in thrust is even greater. For every 1 sccm increase in xenon gas flow rate, the thrust increases by approximately 100 μN. When the Xe flow rate is 5 sccm and the air flow rate is 10 sccm, the thrust reaches 1123 μN, the specific impulse is 145 s.

Subsequently, the thrust variation was studied when the fixed RF power was 60 W, the fixed air flow rate was 3 sccm, and the xenon (Xe) flow rate was gradually increased. The results are shown in Figure 11. It can be seen from the figure that under the condition of a fixed air flow rate, the thrust shows a linear growth trend as the xenon gas flow rate increases. For every 1 sccm increase in xenon gas flow rate, the thrust increases by approximately 60 μN. This increment value is slightly smaller than the situation shown in Figure 10, indicating that the mixing ratio of air flow and xenon flow has a complex coupling relationship with ionization efficiency and degree, rather than a simple monotonic change. This indicates that the interaction between the two has affected the growth rate of the thrust. In addition, when the air flow rate is changed, the radio frequency point will shift, indicating that the internal matching has changed, that is, the impedance has changed. This indicates that the air has been ionized rather than simply heated.

When the Xe flow rate is fixed at 5 sccm and the air flow rate is 10 sccm, the influence of RF power on thrust is shown in Figure 12. It can be observed that power has a significant impact on thrust. For every 10 W increase, approximately 90 μN can be raised. The thrust reached 1700 μN at 130 W and the specific impulse is 247 s.

Finally, the ignition experiment with pure air working medium was carried out. The results show that within the RF power range of 100 W to 150 W, when the air flow rate is fixed at 1 sccm, adding 0.2 sccm of xenon (Xe) can stably maintain ignition. However, when 0.1 sccm of xenon is added, ignition can only be maintained briefly and cannot be stably maintained under pure air working medium. Though pure air stable maintenance of a working substance was unrealized, this section of the experiment is still a good way to verify the feasibility of the mixed working medium, given that the thrust under the mixed working medium received a significant boost in performance.

4. Conclusions

In this paper, a preliminary study on the air-aspirating radio frequency plasma micro-thruster was carried out using the RPT-1 principle prototype. The most significant breakthrough of this study lies in the fact that through a hybrid propellant strategy, the consumption of the expensive inert gas hybrid propellant, xenon, has been significantly reduced, and the feasibility of using the main components in the air (nitrogen/oxygen) as propellants has been verified, and the optimization of the magnetic nozzle configuration has enhanced the efficiency of the RF micro-thruster. This opens up a new path for the development of extremely low-cost and long-life VLEO satellite propulsion systems. Combining the atmospheric resources of the VLEO environment, this scheme has the potential to achieve continuous air-aspirating operation and is expected to revolutionarily solve the problem of long-term orbit maintenance of microsatellites in VLEO. The optimized magnetic nozzle configuration provides effective design guidance for enhancing the efficiency of RF plasma confinement and acceleration.

The main research conclusions of this article are as follows:

(1)

Through research on pure xenon working medium, it has been verified that magnetic nozzles do indeed have significant confinement and acceleration effects, can effectively confine plasma, and form a distinct waistband effect.

(2)

It is feasible to implement an aspirating radio frequency plasma micro-thruster using Xe and air as the working medium, which can achieve stable plasma self-sustaining stable discharge.

(3)

Under an appropriate mixture ratio of xenon and air, the thrust can be significantly increased. Meanwhile, under higher radio frequency power conditions, the mixed working medium of xenon and air can provide a stronger thrust effect.

(4)

The achieved thrust-to-power ratio in mN/kW which appears to range from ≈18 at 60 W to ≈13 at 130 W.

At present, pure air propellant is difficult to maintain at a self-sustaining discharge at frequencies below 150 W. Subsequently, pure air propellant ignition and maintenance will be further achieved by optimizing the magnetic nozzle configuration, such as different positions and magnetic field intensities, increasing the firing frequency point and RF power, etc.