RF Helicon Plasma Thruster for an Atmosphere-Breathing Electric Propulsion System (ABEP)

Published 2 Jul 2026 in physics.plasm-ph | (2607.02635v1)

Abstract: This dissertation focuses on Atmosphere-Breathing Electric Propulsion (ABEP) for propellant-less continuous orbiting in very low Earth orbits (VLEO) and very low Mars orbits (VLMO). ABEP utilizes an intake to collect atmospheric particles as propellant for an electric thruster, offsetting aerodynamic drag. An analytical system-level analysis assesses collectible mass flow, drag, and power requirements, using GOCE as a case study. Three intake designs are developed based on gas-surface interactions. Two diffuse-reflection designs yield collection efficiencies $η_c < 0.5$, while a specular-reflection design achieves $η_c < 0.95$. The specular intake demonstrates superior robustness against flow misalignment, maintaining high efficiency at large angles. The proposed ABEP thruster features a contactless, neutralizer-free design using a helicon plasma discharge, preventing erosion from aggressive species like atomic oxygen. Numerical simulations via the HELIC code determined key design parameters (frequency, magnetic field, plasma density), leading to the implementation of a resonant birdcage antenna with high electrical efficiency. The thruster was manufactured and experimentally validated using Ar, N_2, and O_2. Tests demonstrated successful ignition and operation across relevant mass flows with low power consumption ($P_f < 60$ W). Additionally, a B-dot probe was developed to detect helicon waves in the plasma plume. Ultimately, this work delivers verified intake designs and a novel contactless plasma thruster with high electrical efficiency (>99% in vacuum). Both components represent promising technologies for near-future ABEP applications. Future development outlooks, diagnostics, and spacecraft configurations are also discussed.

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Authors (1)

Francesco Romano

Summary

The paper demonstrates a novel contactless RF helicon plasma thruster that achieves over 99% electrical efficiency under VLEO conditions.
It details an optimized specular-reflecting intake design that reaches up to 0.95 efficiency and maintains robust performance even with angular misalignments.
Experimental results confirm stable operation on multiple propellants (Ar, N₂, O₂), meeting the drag compensation requirements for extended VLEO missions.

RF Helicon Plasma Thruster for Atmosphere-Breathing Electric Propulsion: Technical Summary and Assessment

Introduction and Context

Advances in satellite technology and the proliferation of commercial space activity have led to increased interest in very low Earth orbits (VLEO, $h < 450$  km), where spacecraft benefit from enhanced observational capabilities and reduced radiation exposure. However, atmospheric drag at these altitudes critically limits mission duration, with orbital lifetimes spanning from months to days without active drag compensation. Traditional propulsion increases system mass due to propellant storage, thereby limiting mission flexibility.

Atmosphere-Breathing Electric Propulsion (ABEP) systems circumvent this limitation by ingesting and utilizing ambient atmospheric particles as propellant, enabling extended operations in VLEO without carrying onboard propellant. The present work details the design and development of a novel ABEP system, focusing on a radio-frequency (RF) helicon-based plasma thruster and accompanying intake architecture. The principal innovation lies in a contactless, neutralizer-free helicon discharge thruster, which offers high efficiency and broad propellant compatibility, paired with optimized particle intake devices for maximized collection efficiency.

Figure 2: Schematic of an ABEP-equipped spacecraft employing the RF helicon-based plasma thruster.

ABEP System Analysis: Mission and System Dependencies

Power and Propulsion Requirements

ABEP performance is governed by the equilibrium between aerodynamic drag and provided thrust ( $D = T$ ), directly linking power demand to environmental conditions, spacecraft geometry, and collection/thruster efficiencies. Notably, required electrical power for full drag compensation scales sublinearly with atmospheric density and orbital velocity, but is also a strong function of drag coefficient ( $C_D$ ), frontal area ( $A_f$ ), and the intake ( $\eta_c$ ) and thruster ( $\eta_T$ ) efficiencies:

$P_{in} = \frac{1}{8} (C_D A_f)^2 v^3(h)\rho(h) \Bigg(\frac{1}{A_{in}\eta_c \eta_T}\Bigg)$

For VLEO operation, the principal atmospheric constituents are $N_2$ and atomic oxygen. The latter introduces aggressive chemical environments that challenge conventional thruster materials, making contactless architectures and material selection critical.

Intake Design: Gas-Surface Interaction and Geometry Optimization

Intake Architectures

Three principal intake types were designed and analyzed, leveraging free molecular flow and gas-surface interaction (GSI) models:

Diffuse-Reflecting Intakes: Utilize honeycomb ducts to maximize thermal accommodation. Achieved $\eta_c < 0.5$ . These designs are highly sensitive to alignment errors, with a $>40\%$ efficiency drop at misalignments of $D = T$ 0.
Specular-Reflecting Intake: Employs parabolic, optics-inspired geometry to redirect incoming particles. Achieves $D = T$ 1 and demonstrates robust performance to misalignment, with only $D = T$ 2 efficiency drop at $D = T$ 3 misalignment.
Hybrid Designs: Combine stages of specular and diffuse reflection for tunable pressure/flow regimes.
Figure 1: Diffuse-reflecting intake with honeycomb duct structure, designed for thermalization of incoming particles.

Figure 5: Specular-reflecting (parabolic) intake geometry for efficient particle redirection into the thruster.

Key Findings:

Specular designs offer significant efficiency and robustness advantages, particularly for compact spacecraft with limited attitude stability.
Material selection—such as HOPG and $D = T$ 4 coatings for specular walls, and Ti-alloys or gold coatings for diffusive surfaces—is pivotal for withstanding atomic oxygen erosion.

Thruster Development: RF Helicon Discharge and Birdcage Antenna

Technical Rationale

Atmospheric species in VLEO (notably atomic oxygen) rapidly erode conventional EP components (grids, channels, and neutralizers). To address this, the developed RF helicon thruster employs:

Contactless Operation: No component is in direct plasma contact, mitigating AO-induced erosion and extending operational life.
Propellant Flexibility: Capable of stable operation on $D = T$ 5, $D = T$ 6, and $D = T$ 7 at varying mass flow rates typical of VLEO intake conditions.
No Neutralizer Requirement: The device inherently emits a quasi-neutral plasma jet.

Birdcage Antenna: Circuit and Plasma Coupling

The innovative application of a birdcage antenna—common in MRI applications—provides:

Resonant, Impedance-Matched Excitation: Ensures minimal reflected power ( $D = T$ 8 dB), yielding $D = T$ 9 electrical efficiency under vacuum conditions.
Homogenous Transverse EM Field: Effective helicon-mode excitation, confirmed in simulation and experiment.
Tunable Power Coupling: Plasma resistance ( $C_D$ 0) and wave coupling are optimized via adjustable static magnetic field ( $C_D$ 1), as predicted by the HELIC code.
Figure 7: Conceptual diagram of the helicon plasma thruster with birdcage antenna, solenoid, and discharge channel.

Experimental Results:

Sustained operation on all tested propellants ( $C_D$ 2, $C_D$ 3, $C_D$ 4) with $C_D$ 5 W and minimal reflected power. Stable ignition and operation was demonstrated across the ABEP-relevant flow envelope.
High thruster electrical efficiency, exceeding $C_D$ 6 under matched conditions (in the absence of plasma back-reaction).
Output exhaust velocities ( $C_D$ 7) and propellant utilization match or exceed analytical requirements for full drag compensation at target altitudes for both Earth and Mars.

Intake–Thruster Integration and System Implications

Numerical and DSMC simulation, validated via prototype builds, confirm that:

High intake efficiency, particularly in the specular design, translates directly to reductions in required electrical power and system mass.
The contactless helicon thruster architecture maintains reliable operation across the spatial and compositional variability of VLEO, with no need for auxiliary neutralizing hardware.

Implications, Future Directions, and Theoretical Considerations

Practical Implications

The demonstrated ABEP system architecture enables:

Extended orbital lifetimes in VLEO with no onboard propellant storage, directly addressing end-of-life and debris mitigation concerns.
Application flexibility beyond Earth, to Mars (VLMO), Venus, Titan, and potentially gas giant atmospheres—provided sufficient electrical power.
System scalability: intake and thruster modules can be clustered or distributed for larger spacecraft, or miniaturized for cubesat-class vehicles.

Theoretical and Technological Challenges

Material selection for AO-resistant, high-fidelity GSI properties remains an open area requiring further in-orbit demonstration.
Further plasma diagnostics (Langmuir, Faraday, three-axis B-dot probe) are required to conclusively quantify $C_D$ 8, $C_D$ 9, and helicon wave presence in the exhaust.
Active impedance-matching and thermal management may be required for flight units to maintain performance over mission durations.

Conclusion

The presented RF helicon-based ABEP system substantially progresses atmosphere-breathing propulsion for VLEO applications by delivering:

High intake collection efficiency ( $A_f$ 0 up to $A_f$ 1 for specular), with low angular sensitivity;
Contactless, highly efficient quasi-neutral plasma production via a resonant birdcage antenna ( $A_f$ 2 vacuum, $A_f$ 3 W for ignition and sustained operation on all atmospheric species);
Demonstrated feeder, antenna, and matching network design supporting robust operation with variable atmospheric density and composition;
System modeling showing required $A_f$ 4 and power output align with the thrust needs for drag compensation across a broad range of altitudes and planetary atmospheres.

These findings establish a solid foundation for future ABEP spaceflight demonstration missions and address most of the critical failure modes identified in earlier ground and conceptual studies. Further work will be directed toward comprehensive plasma diagnostics, AO-material interaction testing, and integrated intake-thruster unit optimization for sustained on-orbit operation.