The DEEP PPU project introduces a major engineering challenge: designing a harness capable of reliably transmitting high-voltage, high-frequency power between the power processing unit (PPU) and the thruster. Axon Cable is responsible for developing this critical link, ensuring that the harness meets demanding electrical, thermal, and mechanical requirements while remaining compatible with space-grade standards and routing constraints.
Engineering a Harness for the Most Extreme Conditions
The harness must carry two power lines—positive and return—while withstanding harsh space conditions. These include:
- Very high voltage levels (2000Vpp)
- Elevated temperatures (>200°C)
- High-frequency operation (several MHz)
- Strict mechanical and flexibility requirements
- Compatibility with space-qualified connections and assembly rules
One of the biggest challenges is combining high voltage and high frequency in the same cable while maintaining tight limits on impedance, capacitance, inductance, and resistance. The target electrical characteristics include:
- Maximum capacitance: 150 pF/m
- Ultra-low inductance: <150nH/m
- High-frequency resistance below 50 mΩ/m
Achieving these values requires designing the harness not as a standard wire bundle but as a transmission line working in high frequency.
Managing Skin Effect, Proximity Effect, and Transmission Line Constraints
At high frequencies, electrical behavior drastically changes due to:
- Skin effect – current moves toward the surface of the conductor
- Proximity effect – conductors influence each other’s current distribution
To minimize these effects and keep resistance extremely low, Axon evaluated 28 prototype designs, testing configurations such as:
- Round wires bundles
- Flat flexible structures with round and flat conductors
- Coaxial cables assembly
- Looms (woven micro-wires)
Key findings:
- Standard round-wire bundles produced excessive proximity effect
- Flat assemblies’ structures reduced proximity effect significantly
- Loom constructions showed excellent high-frequency performance
This led to the choice of flat looms—woven assemblies of very fine conductors.
Material Selection: Why PFA Was Chosen
The cable insulation had to meet numerous criteria:
- Low dielectric constant
- High thermal stability
- Flexibility
- High-voltage resistance
- Low outgassing
- Radiation resistance
- Long lifetime
Among fluoropolymers, PFA (Perfluoroalkoxy) emerged as the best candidate. Compared to PTFE, it offers superior:
- Radiation resistance
- High-voltage durability
- High-temperature endurance
The Final Harness Architecture
The prototype harness uses:
- 128 conductors per polarity
- AWG36 gauge wires (127 μm diameter)
- Skin depth at 700 kHz ≈ 86 μm → minimal high-frequency resistance impact
- Flat loom structures for both polarities
- Low-density expanded PFTE (celloflon®) dielectric layers between looms
- An external shield braid for EMC protection
This layered, transmission-line architecture enables tight control over impedance, capacitance, and inductance.
Manufacturing is currently manual, with carefully sequenced steps for laying conductors, adding dielectric layers, and applying shielding. Each harness undergoes:
- Insulation tests
- Continuity tests
- Impedance characterization
- Partial discharge tests (passed up to 4000 V peak-to-peak at 50 Hz)
Prototype lengths delivered include 1 m, 5 m, and 6 m, all performing well in initial tests at UPM.
Designing a Custom High-Voltage Connector
Because no standard connector could accommodate the new harness design, Axon Cable developed a customized Micro-D-based connector with:
- Standard Micro-D contacts Twist Pin technology (crimped per ECSS rules)
- Special inserts for high-voltage insulation
- Increased creepage distances with segregated paths for positive and return conductors
- Compatibility with both thruster and PPU sides
The family includes:
- Bracket-mount connectors
- Panel-mount connectors
- PCB-mount versions with direct loom bonding
Prototype manufacturing is expected in the next project phase.
Conclusion and Next Steps
Axon Cable has successfully developed a promising high-frequency, high-voltage harness prototype tailored for the DEEP PPU–thruster interface. Early testing shows strong performance in terms of electrical behavior, insulation, and manufacturability.
The next steps include:
- Continued optimization of capacitance and inductance
- Thermal environment testing
- Finalization and prototyping of the custom connectors
- Further integration testing with PPU and thruster models
With ongoing refinement, this harness is poised to become a key enabling technology for the next generation of electric propulsion systems.