The DEEP PPU project aims to integrate the Radio Frequency Generator (RFG) directly into the spacecraft platform – an ambitious step that reduces cost, mass, and system complexity. However, this architectural shift introduces new technical challenges, particularly in managing high-frequency behaviour over long harnesses. This article explains how the RFG works, the complexities introduced by the harness, and the filtering solution developed to ensure stable operation.
How the Gridded Ion Thruster Works
The thruster used in this project is a gridded ion thruster, consisting of:
- A chamber containing xenon propellant
- A coil surrounding the chamber
- Two high-voltage grids that accelerate ions
- A neutraliser to maintain charge balance
Ionisation is achieved by driving a high-frequency sinusoidal current through the external coil. This current generates a magnetic field that ionises the xenon. The RFG must therefore deliver up to 1 kW of high-frequency power with high efficiency.
Why Integrate the RFG Into the PPU?
Traditionally, the RFG is installed near the thruster, while the main power electronics remain several metres away. This keeps the harness carrying only DC signals, which are easier to manage.
In DEEP PPU, the decision was made to place the RFG next to the power electronics, on the platform side. This offers several advantages:
- Improved thermal management
- Reduced overall mass
- Lower system cost
- More integrated and compact design
However, this introduces a major difficulty:
the harness now carries AC at high frequency, which significantly complicates both harness and RFG design.
Modelling the Thruster as a Variable Load
From the perspective of the RFG, the thruster behaves like:
- An inductor (L) in series with
- A resistor (R)
This is similar to the primary of a transformer, where the plasma acts like a secondary. The RFG uses:
- A resonant capacitor in series
- A square-wave inverter to excite the resonant LC network
- A closed-loop controller to adjust switching frequency
Because the inductance of the thruster changes during operation, the RFG must adapt its switching frequency dynamically.
How the Harness Creates Harmonics and Instability
Placing the harness after the RFG introduces a new problem.
The harness behaves like an RLC transmission line with its own resonances.
This means:
- The harness adds parasitic capacitance
- The RFG sees additional unwanted resonant peaks
- High-frequency harmonics from the square-wave inverter excite these resonances
- Extra currents circulate through the harness
- Losses increase
- The control loop becomes unstable
Measurements showed that:
- The sinusoidal output current becomes distorted at the RFG input
- Harmonics multiply as harness length increases (especially at 11 m)
- The closed-loop controller begins reacting to the wrong current waveform
This made it impossible to maintain stable operation at full power.
Developing a Filtering Solution
To suppress the unwanted resonances, UPM designed a two-path damping filter:
1. A series LC (one-path) filter
- Passes the fundamental resonant current to the thruster
- Blocks high-frequency harmonics
- But alone cannot dampen the harness resonance
2. A damping path with a resistor
- Absorbs the high-frequency harmonic energy
- But must avoid absorbing fundamental current, which would create large losses
3. Adding a capacitor in series with the resistor
This innovation was key:
- At low frequency → capacitor blocks current → almost no fundamental current flows to the resistor
- At high frequency → capacitor behaves as a short → resistor efficiently damps harmonics
This hybrid filter dramatically reduces losses: From 24 W down to only 8 W
Filter Performance and Validation
Prototype testing showed:
- Harmonic peaks are strongly reduced
- The RFG can now operate over its full frequency range
- The closed-loop control remains stable even with the 6 m and 11 m harness
- Simulation and prototype results match closely, with only minor differences due to higher-order harness resonances
The final design successfully eliminates the instability that originally prevented integration of the RFG before the harness.
Conclusion
Integrating the Radio Frequency Generator into the DEEP PPU is a major engineering achievement. The primary challenge – managing high-frequency behaviour introduced by long AC harnesses – was solved through the development of a specialised damping filter that preserves efficiency while ensuring stable operation.
This innovation:
- Enables full integration of the RFG with the PPU
- Reduces mass, cost, and system complexity
- Allows the thruster to operate reliably across all required frequencies
- Demonstrates the viability of this architectural approach for future platforms
The work by UPM provides a critical enabling technology for next-generation electric propulsion systems, paving the way for more efficient, compact, and cost-effective spacecraft architectures.