Development steps to a Novel Semiconductor based Microwave Ion Beam Thruster Model

Development steps to a Novel Semiconductor based Microwave Ion Beam Thruster Model IEPC-2025-027 Presented at the 39th International Electric Propulsion Conference, Imperial College London, London, United Kingdom 14-19 September 2025 Alexander Spethmann∗ , Thomas Trottenberg† , and Holger Kersten‡ Institute of Experimental and Applied Physics, University of Kiel, Germany We present ongoing development of a novel microwave ion thruster concept that employs semiconductor-based microwave technology to generate a dense plasma by electron cyclotron resonance (ECR). In contrast to conventional gridded ion thrusters, the magnetic field configuration not only enables ECR heating but also confines electrons sufficiently to eliminate the need for a screen grid. Ion acceleration is performed electrostatically, which requires an external cathode and biasing the plasma to a high potential relative to the spacecraft ground. Building on results presented at the previous IEPC, galvanic isolation of the plasma from the microwave electronics has now been achieved, which is a prerequisite for stable operation at high acceleration voltages. Ion extraction current measurements confirm the feasibility of the concept. Nomenclature B= magnetic flux density e= elementary charge I= collected ion current M= magnetization of the ferrite material me= electron mass P= microwave power p= background pressure Q= argon gas flow U= bias voltage of the plasma source ωce = electron cyclotron frequency ∗Senior Researcher and Project Leader, Project MiPlaT, Plasma Technology Group, [email protected]. †Senior Researcher and Lecturer, Plasma Technology Group, [email protected]. ‡Professor, Head of Group Plasma Technology, k[email protected]. The 39th International Electric Propulsion Conference, Imperial College London London, United Kingdom 14–19 September 2025 Copyright 2025 by the Electric Rocket Propulsion Society. All rights reserved. Page 1 I. Introduction Today, small transmitters and receivers are suitable for the mass market in many applications for telecommunications. The rapid development in the field of mobile communications has significantly contributed to advancements in microwave semiconductor technology, particularly with respect to miniaturization and cost reduction. It is therefore reasonable to use microwaves for plasma generation with the help of modern semiconductor technology, similar to frequency generators in the megahertz range for Radio Frequency Ion Thrusters (RIT).1 Traditional microwave sources such as klystrons, magnetrons, and traveling wave tubes, along with devices of lower efficiency like Gunn diodes, have long dominated the field. For instance, the Japanese Hayabusa mission2, 3 successfully employed traveling wave tubes to operate its gridded ion engines. While traveling wave tubes provide high efficiencies, their complex mechanical design implies higher costs. Magnetrons, often used in industrial plasma sources,4are robust and powerful but require cooling, which is straightforward on the ground but problematic in spacecraft applications. Building on the advances in semiconductor microwave technology, we recently introduced the concept of a microwave ion thruster driven by semiconductor devices at the previous IEPC.5The aim is to generate, by means of electron cyclotron resonance (ECR), a comparatively compact yet dense plasma from which ions can be extracted and electrostatically accelerated. The magnetic field required for the ECR process not only enables efficient heating but also confines electrons and reduces electron losses, thereby eliminating the need for a screen grid as in conventional gridded ion thrusters. In contrast to magnetic nozzle approaches,6–8 ion acceleration in our concept is performed electrostatically, requiring an external cathode and plasma biasing to a high potential relative to spacecraft ground. Since that first report, progress has been made in our experimental implementation. In particular, galvanic isolation of the plasma from the microwave electronics has been achieved, which is a prerequisite for stable operation at high acceleration voltages. First tests with varying acceleration voltages, microwave powers, and gas flow rates have been conducted, and the corresponding extracted ion currents were measured. The present paper reports on these results and discusses the next development steps. II. Experimental Setup of the Plasma Source The general layout of the plasma source follows the design reported previously.5, 9–11 For clarity, the main features are summarized here, with emphasis on the recent modifications. Magnetic configuration The source employs a stack of four longitudinally magnetized ferrite ring magnets to generate a predominantly axial magnetic field. This configuration provides a magnetic flux density of about B≈87 mT in the central region, which fulfills the electron cyclotron resonance (ECR) condition for microwave frequencies near 2.45 GHz. In this geometry, the field lines are largely parallel to the thruster axis, enabling efficient ECR heating of electrons. Toward the ends of the magnet stack, the field develops cusp-like features that reduce electron losses along the axis and thereby improve plasma confinement. Since the exact magnetization of the ferrite material was not specified by the manufacturer, we first characterized a single ring magnet experimentally. For this purpose, the magnetic flux density was measured with a Hall probe at several accessible points around the magnet. A finite element simulation of the single magnet was performed with an assumed magnetization M. Then, the value of Mwas adjusted to reproduce the measurements (the field strength scales linearly with the magnetization). The resulting magnetization was M≈300 kA/m. This value was then used in finite element simulations of the magnetic field distribution for a stack of two or more magnets. Based on the simulation results, the required number of stacked rings to achieve the ECR condition could be determined. In particular, four rings yield a central field strength slightly above B= 87 mT, which corresponds to the electron cyclotron resonance frequency ωce =e me B , (1) or ωce/2π= 2.44 GHz, where edenotes the elementary charge and methe electron mass. The 39th International Electric Propulsion Conference, Imperial College London London, United Kingdom 14–19 September 2025 Copyright 2025 by the Electric Rocket Propulsion Society. All rights reserved. Page 2 Figure 1. Geometry and magnetic configuration of the plasma source. (a) Three-dimensional view along the axis into the interior of the permanent magnet stack. (b) Longitudinal section with ceramic insert (yellow) guiding the propellant gas into the region where the electron cyclotron resonance (ECR) condition is fulfilled. The plasma source is biased via a DC block that galvanically isolates the microwave feed from the generator, allowing the source chamber to be placed at acceleration potential. (c) Finite element simulation of the magnetic flux density with contour lines for 30, 50, 75, and 87 mT. (d) Corresponding magnetic field lines starting from the interior of the magnet stack. The calculation assumes a ferrite magnetization of M= 300 kA/m. The configuration provides an axial field in the central region suitable for ECR at 2.45 GHz and cusp-like features near the ends that help confine electrons. The magnetic field created by the stack of ring magnets has another advantage. The numerical field simulation shown in Fig. 1 (c) and, in particular, (d) reveals that near the end planes of the stack, the field lines acquire an increasing radial component and even reverse direction. The field lines coming from the interior of the stack end on the end faces of the magnets, primarily at smaller radii. The magnetic field lines therefore describe an arc outside the stack and have a strong radial component, which efficiently impedes the movement of electrons in the axial direction. In addition, the flux density increases significantly in front of the end faces of the magnets due to converging field lines. This creates a magnetic cusp, and due to the magnetic mirror effect it further weakens plasma recombination at the surface where the field lines end.12 The outwardly curved, returning, and focused field lines thus act like a screen grid in gridded thrusters: They confine the electrons and enable the application of an accelerating field that would otherwise be screened by the plasma. The magnetic field predominantly affects the electrons due to their much smaller mass and gyration radii. In contrast, ions have gyration radii that are large compared to the system length, and therefore ions are not confined by the magnetic field. Figure 2(a) shows a photograph of the demonstration model of the plasma source, while Fig. 2(c) depicts it in operation. The 39th International Electric Propulsion Conference, Imperial College London London, United Kingdom 14–19 September 2025 Copyright 2025 by the Electric Rocket Propulsion Society. All rights reserved. Page 3 Electrode arrangement and gas injection Microwaves are coupled into the discharge via coaxial electrodes. The inner conductor of the microwave feed extends slightly beyond the outer conductor and acts as the powered electrode, while the outer conductor and a surrounding cylinder form the counter-electrode. In this way, a radial electric field is established that is oriented perpendicular to the axial magnetic field, which is favorable for ECR heating. A ceramic insert covers the protruding part of the inner conductor and guides the propellant gas into the resonance zone, see Fig. 1 (a) and (b). The insert is equipped with fine holes that release the gas where the electric field strength is highest, ensuring efficient use of the propellant. In this configuration, sufficient neutral gas density can be maintained in the discharge region while preserving the ECR condition. Recent modification: galvanic isolation In earlier experiments, the microwave feed was directly connected to the generator without any galvanic isolation. As a consequence, the plasma source remained at ground potential and no acceleration voltage could be applied. For testing purposes, we therefore biased the cathode negatively instead,5which is not a viable solution for spacecraft application. In the present setup, a DC block has been introduced into the coaxial feed, providing galvanic isolation of both the inner and outer conductors from the microwave generator for direct current while maintaining high-frequency coupling. This modification enables the outer conductor, and thus the entire plasma source chamber, to be biased to the acceleration potential. As a result, the plasma source operates at a high potential relative to the spacecraft ground and effectively acts as an anode in direct contact with the plasma. This step is essential for implementing electrostatic ion acceleration while keeping the microwave generator at ground potential. Preparations for further diagnostics have been made, too. Figure 2 shows photographs of the experimental setup and the collector plate with integrated diagnostics. Besides the plain plate used for the measurements reported in the following section, interchangeable collector plates can accommodate either a force probe or a retarding potential analyzer (RPA). These tools will allow direct momentum flux measurements and ion energy analysis in upcoming experiments. The photographs also illustrate intermediate configurations with additional ring electrodes inside the glass cylinder, which were used in preliminary tests to study the shaping of potential in the acceleration zone (see also our previous report5). Such modifications may help to increase extraction currents and reduce recombination losses at the walls. Results from these experiments will be reported in a forthcoming publication. III. Ion extraction experiments With the introduction of galvanic isolation via a DC block, the plasma source can be biased to a high potential and serve as an anode in direct contact with the plasma. To evaluate the feasibility of ion extraction in this configuration, a collector plate was placed a few centimeters downstream of the plasma source (Fig. 2). The plate was grounded through a picoammeter, thus attracting and collecting the ions. Figure 3 shows first current–voltage characteristics obtained at low argon flow (2 sccm) and microwave powers of 40 and 50 W. At acceleration voltages up to 1.5 kV, ion currents of up to 4 mA were measured. Although these values are lower than the maximum currents previously observed with the reversed configuration (grounded anode, negatively biased cathode), where up to 12 mA were obtained at 50 W microwave power,5the comparable order of magnitude indicates that the new configuration is viable. The earlier results also give confidence that with further optimization of electrode geometry and plasma parameters, higher extracted currents can be achieved. These first measurements therefore demonstrate the feasibility of ion extraction and acceleration from the ECR plasma with a galvanically isolated plasma source biased to high potentials. Measurements with a force probe13, 14 and an RPA15, 16 have already been prepared but could not be conducted before the conference. The photographs in Fig. 2(b) show the collector plates modified with square apertures and these diagnostics mounted. The left collector plate had already been exposed to the ion beam for several hours; a clearly sputtered spot with approximately the diameter of the glass cylinder is visible. This provides at least qualitative evidence that energetic ions reached the plate. The 39th International Electric Propulsion Conference, Imperial College London London, United Kingdom 14–19 September 2025 Copyright 2025 by the Electric Rocket Propulsion Society. All rights reserved. Page 4 Figure 2. Photographs of the experimental setup and diagnostics prepared for upcoming tests. (a) Plasma source with magnet stack, glass cylinder, and ring electrode inside the vacuum chamber. (b) Collector plate equipped with diagnostics: force probe and retarding potential analyzer (RPA). (c) Plasma discharge in operation with two ring electrodes inside the glass cylinder. (d) Top view of the collector plate with force probe. (e) Top view of the collector plate with RPA. IV. Conclusion and next steps A microwave ion thruster concept based on electron cyclotron resonance (ECR) has been further developed and tested. The key modification compared to earlier work is the galvanic isolation of the plasma source from the microwave generator by means of a DC block, which allows the entire source chamber to be biased to a high potential and act as an anode. First ion extraction measurements in this configuration demonstrated stable operation, achieving more than 4 mA of ion current at 50 W microwave power and 1.5 kV acceleration voltage. These results are in the same order of magnitude as those obtained previously with the reversed configuration,5where up to 12 mA were measured, and they confirm the feasibility of electrostatic ion acceleration from the ECR plasma source. In the next development steps, ion energy analysis with an RPA15 and direct momentum flux measurements using a force probe13, 14, 17 will be carried out with the already prepared setups. These diagnostics The 39th International Electric Propulsion Conference, Imperial College London London, United Kingdom 14–19 September 2025 Copyright 2025 by the Electric Rocket Propulsion Society. All rights reserved. Page 5 Figure 3. Ion extraction from the plasma source with galvanic isolation. (a) Schematic of the experimental setup with the plasma source biased via a DC block and a grounded collector plate positioned a few centimeters downstream. (b) Measured ion current Ias a function of anode voltage Uat an argon flow of 2 sccm and microwave powers of 40–50 W. Currents up to 4 mA were obtained at acceleration voltages of up to 1.5 kV, demonstrating the feasibility of ion extraction and acceleration in the new configuration. will provide the quantitative data needed to assess performance and to guide further optimization. Based on these results, we will aim to increase the extracted ion current by optimizing the gas flow, acceleration voltage, and microwave power. In particular, the present collector plate will be replaced by a neutralizer and, potentially, by an additional electrode at ground potential, corresponding to the satellite ground potential. This will support the advancement of the concept toward a compact, semiconductor-driven microwave ion thruster. Acknowledgments This work is supported by the German Aerospace Center DLR, Project No. 50 RS 2204. References 1K. Holste, P. Dietz, S. Scharmann, K. Keil, T. Henning, D. Zsch¨atzsch, M. Reitemeyer, B. Nausch¨utt, F. Kiefer, F. Kunze, J. Zorn, C. Heiliger, N. Joshi, U. Probst, R. Th¨uringer, C. Volkmar, D. Packan, S. Peterschmitt, K.-T. Brinkmann, H.-G. Zaunick, M. H. Thoma, M. Kretschmer, H. J. Leiter, S. Schippers, K. Hannemann, and P. J. Klar. Ion thrusters for electric propulsion: Scientific issues developing a niche technology into a game changer. Rev. Sci. 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